Science 10 – Unit AChapter A2 – Elements &

Compounds

A2.1The Periodic Table

ElementsThere are about 90 naturally occurring elements,

and 25 synthetic elements (made in a lab, not found in nature)

The elements are organized in a table called the Periodic Table of the Elements, based on both their physical AND chemical

propertieselements can be sorted into three classes

metals metalloids non-metals

Periodic Table of the Elements song

Periodic Table of the ElementsPeriodic Table of the Elements songVersion 2

Relating the structure of the atom

In order to understand the organization of the periodic table, we must review what we know the structure of the atom

We’ll start by recalling the three types of sub-atomic particles:

Subatomic particlesParticle PROTONS NEUTRONS ELECTRONS

symbol p+ n e-

charge +1 0 -1

number in an atom

each element has its own number, it

never changes

can vary among atoms of the same

element, called isotopes

changes when an atom

becomes an ion, and develops a

net charge

location in the nucleus in the nucleusaround the

nucleus, in set energy levels

mass 1.7 x g 1.7 x g

9.1 x g (about 1/2000th that of a p+ or

n)

Subatomic particles and the periodic table

The elements in the periodic table are numbered according to the # of p+remember, this number does not change, it is

like the ID number for that elementthis is why the number of protons is also called

the atomic number

Subatomic particles and the periodic table

The elements also increase according to the mass of the elementthe mass is mostly based on its # of p+ and n○in smaller elements, the number of neutrons is

usually = to # p+since the job of the neutrons is to separate the

protons to keep them from repelling each other, it reasons that larger elements need more neutrons than protons

Subatomic particles and the periodic table

The arrangement of elements into rows corresponds to the arrangement of electrons around the nucleus

The row number is equal to the number of energy levels into which the electrons are organized

Back to the periodic tableThe three classes of elements, (metals,

metalloids and non-metals) are so divided based on their physical properties what happens to their electrons during a

chemical reaction

MetalsLocation on the periodic table

To the left of the staircase line

MetalsPhysical properties

COLOUR: most are silver or grey and shiny (lustrous)

CONDUCTIVITY: most are good conductors of electricity and heat

ABILITY TO CHANGE SHAPE: most are both malleable and ductile

STATE AT ROOM TEMP: all are solid, except mercury which is liquid

MetalsChemical properties

REACTIVITY: varies, as some are highly reactive with air and water and others are mostly inert (unreactive)

DURING A CHEMICAL REACTION: metals react by giving away “loose” electrons, which gives them a positive ionic charge

Non-metalsLocation:

To the right of the staircase line

Non-metalsNon-metals vary widely in their properties,

and are similar only in the fact that they are not metals

Physical propertiesCOLOUR: vary CONDUCTIVITY: all are non-conductiveSTATE AT ROOM TEMP: five are solid, one

(bromine) is liquid and the rest are gases

Non-metalsChemical properties

REACTIVITY: some are highly reactive and others, unreactive

FORMATION OF MOLECULES: about half the non-metals exist as groups of atoms called molecules

DURING A CHEMICAL REACTION: non-metals gain extra electrons, either by taking in the electrons lost by metals,or by sharing electrons with other non-metals

MetalloidsLocation on the periodic table

Along the staircase line

MetalloidsProperties are intermediate between metals

and non-metalsThey rarely gain or lose electrons

Hydrogen – the exceptionSome periodic tables position hydrogen

(element number 1) on the left side with the metals

Other periodic tables position it on the right with the non-metals

Recall, the elements are classified according to what happens to their electrons in a chemical reactionthe reason hydrogen can be in either spot is

because it is capable of both giving away and taking in electrons

The Periodic TablePERIOD (row)

based on the number of electron energy levels in that element’s atoms

the number of elements in each period correspond to that energy level’s capacity for electrons

GROUP or FAMILY (column)groups of elements with similar chemical and

physical properties

FamiliesGROUP 1: the alkali metals

soft, shiny and silververy reactive with waterform compounds that are white solids that dissolve

in water (e.g. table salt)examples: lithium, sodium and potassium

GROUP 2: the alkaline-earth metalsshiny and silverless reactive than Group 1produce colourful flames, which is why they are

often used in fireworksexamples: magnesium, calcium & barium

FamiliesGROUP 17: the halogens

poisonousreact easily with the alkali metals to form

saltsexamples: fluorine, chlorine and bromine

GROUP 18: the noble gasesgases that are very unreactivecan be made to glow when electricity is

passed through themexamples: helium, neon and argon

The role of neutronsRecall the expression “opposites attract” – this applies

to the protons (+) and electrons (-) We would expect, since they have all the same charge,

that the protons would all repel each other, and the nucleus would fly apart

We might also expect, since they have an opposite charge, that the electrons might come crashing into the nucleus

The job of the neutrons is to prevent either of these things from happening by separating the protons, and “diluting” the positive charge

It makes sense then, that larger elements that have more protons, also need more neutrons to keep the nucleus stable

IsotopesWhile the number of protons in an element

never changes, the number of neutrons can vary

Atoms of the same element that differ in the number of their neutrons are called isotopes

We distinguish between different isotopes by writing the name of the element followed by the mass numberthe mass number is the number of protons +

neutronse.g. carbon-12 and carbon-14 both have 6

protons but vary in their number of neutrons (carbon-12 has 6 and carbon-14 has 8)

Isotopes & atomic molar massOne of the pieces of information included on

the periodic table for each element is the atomic molar massthis can be thought of as the average mass

number of all of the isotopes of all atoms of that element

e.g. the atomic molar mass of carbon is 12.01. From this we can determine that most carbon atoms are carbon-12, however there are other isotopes that occur less frequently

Practice problem:

IsotopeMass

numberNumber of

protonsNumber of neutrons

Most common isotope? (Y/N)

oxygen-18

nitrogen-14

lithium-6

20 10

19 21

13 yes

Electrons and energy levelsEnergy levels are regions around the nucleus Different levels can hold a different number

of electrons, called the octet ruleThe lowest energy level is the one closest to

the nucleus and can hold up to 2 electronsThe next two energy levels can each hold 8

electronsBeyond that, the pattern is more complicated,

so you’re only responsible for the arrangement of the first 20 elements, not the transition metals (groups 3 – 16)

Bohr diagramsBohr diagrams are drawing to illustrate elements’

atomic structureThey use a circle to represent the nucleus, and rings to

represent each energy levelThe number of protons and neutrons are written in the

nucleusThe electrons are illustrated using dotsThe outermost level (or shell) is called the valence shell

and the electrons are valence electrons

Drawing Bohr diagramsStart by drawing a circle for the nucleus

Inside the circle, indicate the number of protons and neutrons (assume the most common isotope)

Draw rings around the nucleus – the number of rings is the same as the row number the element is in

9p+10n0

Drawing Bohr diagramsStart placing electrons in the rings

remembering the octet rule:you must begin in the first energy level and fill

one energy level before moving to the next the first energy level (closest to the nucleus) can fit 2

e- the next energy level can fit 8 e- the next energy level can fit 8 e-

9p+10n0

Drawing Bohr diagramselectrons will be drawn in pairs, except in the

valence shell where they only pair up if there’s more than four

9p+10n0

Practice problemDraw Bohr diagrams for elements 11 – 18Compare these diagrams to the ones above.

What do you notice?

Practice problem (solution)The three elements in Group 1: hydrogen,

lithium and sodium all have one valence electron

The three elements in Group 18: helium, neon and argon, all have full valence shells

Elements in the same group (family) have the same electron arrangement – this results in similar chemical properties.

Relating Bohr diagrams to chemical properties

Compare the two Bohr diagrams to the right

Fluorine has one empty space its valence level, and as a halogen, is very reactive

Neon’s valence shell is already full, and as a noble gas, is very stable (unreactive)

9p+10n0

10p+10n0

Relating Bohr diagrams to chemical properties

Is there a connection between the number of valence electrons and the reactivity of an element?

YES! Elements want to adopt the electron arrangement of the nearest noble gas because this arrangement is the most stable

9p+10n0

10p+10n0

How do non-metals become more stable?

Recall, non-metals react in a chemical reaction by taking in extra electrons

Non-metals take in as many extra valence electrons as they need to fill their valence shell and become like the nearest noble gas

9p+10n0

7p+7n0

How do metals become more stable?

Recall, metals react in a chemical reaction by giving away their “loose” electrons

Examining the Bohr diagrams of different metals explains why

Rather than try to gain seven electrons to fill their valence shell, metals give away their valence electrons to become like the nearest noble gas

3p+4n0

11p+12n0

Atoms vs. ionsAtoms have the same number of electrons as

their atomic number since # e- = # p+, the positive charges and

negative charges cancel each other outatoms therefore have a net charge of zero

Atoms vs. ionsIons are formed when atoms

gain or lose electrons to become like the nearest gasnow, # e- # p+, which means

that the positive and negative charges no longer cancel each other out

metal ions form when valence electrons are lost, so they have more protons than electrons which gives them a positive charge

3p+4n0

3p+4n0

atom of lithium3p+ and 3e-

net charge = 0

ion of lithium3p+ and 2e-

net charge = 1+

Atoms vs. ionsnon-metal ions form when

valence electrons are gained, so they have more electrons than protons which gives them a negative charge

8p+8n0

8p+8n0

atom of oxygen8p+ and 8e-

net charge = 0

ion of oxide8p+ and 10e-

net charge = 2-

Ion chargeYour periodic table will tell you the charge on

that element’s ionsSome elements do not have an ion listed –

this occurs when their electron arrangement makes forming ions unlikely (e.g. carbon) or impossible (e.g. noble gases)

Some metals have more than one ion listed – these are called multi-valent metals, and occur because these metals cannot give away enough electrons to become fully stable, but try different strategies to become more stable

Practice problem

Element

Metal or

non-metal

Number of

protons

Number of

electrons in an atom

Number of

electrons in an

ion

Number of

electrons

gained or lost

Ion charge

calcium

nitrogen

magnesium

chlorine

potassium

fluorine

oxygen

argon

Elements combine to form compounds

A chemical reaction can occur when a metal atom that wants to lose its valence electrons comes in contact with a non-metal atom that wants to gain electrons

The metal atom will transfer its electrons to the non-metal, making the metal a positively-charged ion (cation) and the non-metal a negatively-charged ion (anion)

The resulting cation and anion are bonded together in an ionic bond and form an ionic compound

Example – how salt is formedAn atom of sodium has one

valence electron it wants to give one electron away

An atom of chlorine has one empty space in its valence shell it wants to take in one extra electron

11p+12n0

17p+18n0

Example – how salt is formedIf you react sodium and

chlorine together, you make the compound sodium chloride, also known as table salt.

11p+12n0

17p+18n0

HomeworkA2.1 – Check and Reflect p.39#1-12

A2.2aNaming Ionic Compounds

IUPACThe International Union Pure and Applied

Chemists is a group of scientists responsible for the naming rules for elements and compounds

the system developed by IUPAC ensures that chemicals have the same name, regardless of language or country

Ionic compoundsIonic compounds form when electrons

transfer from one atom to anotherWhen the two kinds of ions group together,

they form an organized framework called a crystal lattice

Properties of ionic compoundsbesides forming crystal lattices, ionic

compounds have some common propertiesall are solid at room temperaturethey are very stable, which means they melt at

a very high temperatureall ionic compounds dissolve at least a little bit

in water and the solutions conduct electricity

Formula unitsA formula unit is the ratio of cations to

anionsThe chemical formula for an ionic compound

represents this ratioe.g. the formula for sodium chloride is NaCl

this indicates that you need one sodium ion for each chloride ion

this makes sense if you recall what you know about the structure of the two ions sodium has one electron to give away and the

chloride has room for one more electron

Ionic compoundsThe ratio of cations to anions is based on the

number of electrons the metal has to give away, compared to the number of electrons the non-metal has room for

e.g. calcium fluoridecalcium has two electrons to give awayfluorine only has room for one electronwe need two fluorine atoms to accommodate

the calcium’s electronsthe formula for calcium fluoride, therefore, is

CaF2

Naming ionic compoundsThe IUPAC system of naming ionic compounds is

very simple – it just requires you to name the two ions, following these rules:name the cation first by using the element’s name name the anion second by using the element’s

name, but changing the ending of the name to ide the name of the anion is written under the element’s

symbol on the periodic tablee.g. K3N potassium & nitrogen potassium

nitridee.g. MgF2 magnesium & fluorine magnesium

fluoride

Practice problemsName the following compounds:

Li2SCa3N2

MgORbIBa3P2

Practice problems (solutions)Name the following compounds:

Li2S lithium & sulfur lithium sulfideCa3N2 calcium & nitrogen calcium nitrideMgO magnesium & oxygen magnesium

oxideRbI rubidium & iodine rubidium iodideBa3P2 barium & phosphorus barium

phosphide

Practice problemsName the ionic compound formed during the

reaction of:aluminium and oxygenchlorine and yttriumnitrogen and magnesiumzirconium and sulfur

Practice problems (solutions)Name the ionic compound formed during the

reaction of:aluminium and oxygen aluminium oxidechlorine and yttrium yttrium chloridenitrogen and magnesium magnesium nitridezirconium and sulfur zirconium sulfide

Formulas for ionic compoundsAs we saw in the previous examples, the

formula for an ionic compound uses the symbols for the two ions (e.g. NaCl)

Subscripts (small numbers written at the bottom) are used when you need more than one of an ion (e.g. in MgCl2 , the 2 indicates that you need 2 chloride ions to accommodate the electrons from the magnesium)You do not need a subscript 1 when there is

only one of an ion in the formula.

Formulas for ionic compoundsLastly, the state of the compound at room

temperature is indicated(s) = solid, (l) = liquid, (g) = gas, and (aq) =

aqueous solution, which means dissolved in waterSince all ionic compounds are solid, each formula

will be followed by (s), unless they are dissolved in water, (aq)

The key to writing the correct formula is ensuring

your charges are balanced, which means the same number of electrons are lost by the cation as gained by the anion

Writing formulas for ionic compounds

Identify the ions and their chargesDetermine the total charges needed to balance

(use the lowest common multiple)Note the ratio of cations to anionsUse subscripts to write the formula, if needed

e.g. calcium nitride Ca2+ (gives away 2) and N3- (takes in 3) the lowest common multiple is 6 electrons we need 3 calciums and 2 nitrogens to balance charges Ca3N2

Practice problemsWrite the formulas for the following ionic

compoundsmagnesium chloridesodium sulfidecalcium phosphidepotassium nitridecalcium fluoride

Practice problems (solutions)Write the formulas for the following ionic

compoundsmagnesium chloride Mg2+

and Cl-

MgCl2(s)sodium sulfide Na+ and S2- Na2S(s)calcium phosphide Ca2+ and P3- Ca3P2(s)potassium nitride K+ and N3- K3N(s)calcium fluoride Ca2+ and F- CaF2(s)

Compounds with multivalent metals

Recall, some elements in the transition metals have more than one possible ion charge – these are called multivalent metalsIn the formula, you can tell which of the ions it

is based on the balanced charges e.g. copper has two ions, Cu2+ and Cu+ the copper

in CuCl2 has to be Cu2+ in order for the charges to be balance

In the name, the charge of the ion for a multivalent metal is indicated by roman numerals in brackets e.g. copper (II) chloride

Practice problemsWrite the formulas for:

iron (III) chloridelead (IV) oxidecopper (I) sulfide

Write the names for:Ni2S(s)CuF2(s)Cr2S3(s)

Practice problems (solutions)Write the formulas for:

iron (III) chloride Fe3+ and Cl- FeCl3(s)lead (IV) oxide Pb4+ and O2- PbO2(s)copper (I) sulfide Cu+ and S2- Cu2S(s)

Write the names for:Ni2S3(s) Ni3+ and S2- nickel (III) sulfideCuF2(s) Cu2+ and F- copper (II) fluorideCr2S3(s) Cr3+ and S2- chromium (III) sulfide

Binary ionic compounds vs. polyatomic ionic compounds

The examples we’ve looked at so far have been binary compounds, that is, one metal with one non-metalThese compounds only account for some of the possible

combinationsImagine these compounds are like a man and woman

getting married, and the woman changes her name (-ide)Polyatomic ions are more complex – like a man

marrying a woman who already has children – they come as a “package deal”Polyatomic ions are made up of several non-metallic

atoms joined together, which have a charge as an entire group

Polyatomic ionsSome common polyatomic ions include:

hydroxide, OH-

nitrate, NO3-

sulfate, SO42-

carbonate, CO32-

Polyatomic ions behave the same way as a simple anion – you still have to balance the charges between the cation and the anion

A list of common polyatomic ions is included above your periodic table in your data booklet

Additional rules for polyatomic ions

Writing formulas for compounds with polyatomic ionsbecause we’re treating the ion as one unit, if

you need more than one of the ion to balance charges, you must put the ion in brackets, with the subscript after the bracket e.g. calcium nitrate Ca2+ and NO3

- Ca(NO3)2(s)

if you only need one of the ion, no brackets are needed e.g. magnesium carbonate Mg2+ and CO3

2- MgCO3(s)

Additional rules for polyatomic ions

Naming compounds with polyatomic ionsthe process is exactly the same as for simple

anions except that you do NOT change the ending of the name to –ide

you simply name the metal and the polyatomic ion, as it is listed above your periodic table

e.g. NaNO3(s) sodium nitratee.g. Ca3(PO4)2(s) calcium phosphate

One exception: ammoniumIf you scan your list of polyatomic ions, you

should notice one ion that is unlike the othersammonium is the only polyatomic ion with a

positive charge, making it the only polyatomic cationthe formula for ammonium is NH4

+ (+1 charge)

the same rules still apply – you simply name the cation (ammonium) and the anione.g. NH4Cl(s) ammonium chloride

Practice problemsWrite the formulas for:

barium hydroxideiron (III) carbonatecopper (I) nitrate

Write the names for:Au(NO3)3(s)(NH4)3PO4(s)K2SO3(s)

Practice problems (solutions)Write the formulas for:

barium hydroxide Ba(OH)2(s)iron (III) carbonate Fe2(CO3)3(s)copper (I) nitrate CuNO3(s)

Write the names for:Au(NO3)3(s) gold (III) nitrate(NH4)3PO4(s) ammonium phosphateK2SO3(s) potassium sulfite

A2.2bNaming Molecular Compounds

Quick review of ionic compounds

A result of ionic bondsIonic bonds form

between metals and non-metals

Naming: the metal is always first, the non-metal second. The non-metals name is changed to have an “ide” ending

(e.g. sodium chloride)

Ionic compounds dissolve easily in water.

In solution, ionic compounds conduct electricity,

Ionic compounds tend to form crystalline solids with high melting temperatures.

Ionic BondsIn ionic bonding, valence

electrons are completely transferred from one atom to another. 

The result? Ions! Electrically charged atoms. Cations are positively charged

(Mg2+, H+, Na+)Anions are negatively charged

(O2-, Cl-)The oppositely charged ions

are attracted to each other by electrostatic forces.

Let's take a look!

What happens where there’s no metal to give up electrons?

An ionic compound can only form if there is a metal available to transfer its electrons to the non-metal

How can a non-metal become more stable of there’s no metal? The answer is: it shares electrons with other non-metals

This type of compound is called a molecular compound and is made up entirely of non-metals

Molecular compoundsConsider the atomic

structure of oxygen and fluorine atoms

The oxygen has room for two more electrons, and the fluorines each have room for one

Since there are no metals to donate electrons, the three non-metals will SHARE their valence electrons

8p+8n0

9p+10n0

9p+10n0

Molecular compoundsThe result is a

molecular compound

The oxygen and fluorines are attached by a covalent bond

The name of this compound is oxygen difluoride

8p+8n0

9p+10n0

9p+10n0

Naming molecular compoundsRecall, writing the formula for ionic

compounds involves balancing the charges, which means ensuring the total number of electrons given up by the cations is equal to the total number taken in by the anions

However in molecular compounds, there is no balancing because both compounds are non-metals with negative charges

For this reason, the names of molecular compounds have to include the number of each non-metal

ExampleIn these diagrams, carbon is

represented by the black spheres and oxygen by the red spheres

Both these molecules have carbon sharing electrons with oxygenone is harmless to humans, but

causes a major environmental problem

the other is poisonous, and can be easily produced by your car or furnace

ExampleIf these two gases have

different physical and chemical properties, they must also have different names – we can’t just name them both carbon oxidethe first: carbon dioxidethe second: carbon

monoxide

Naming molecular compoundsThe two elements are named

much the same way as ionic compounds – you name the first element, then the second, and change the ending to ide

However, you need to use prefixes to tell you how many of each elemente.g. the name carbon dioxide

indicates one carbon atom, two oxygen atoms

1 – mono2 – di3 – tri4 – tetra5 – penta6 – hexa7 – hepta8 – octa9 – nona

(ennea)10 - deca

Naming molecular compoundsone additional rule:

if there is only one of the FIRST element, you do NOT need to include the prefix mono

if there is only one of the SECOND element, you DO need the prefix e.g. SO2(g) is named sulfur dioxide and H2S(g) is

named dihydrogen monosulfide

Practice problemsName the following molecular compounds:

CO2(g)

N2O(g)

PCl3(g)

CF4(g)

SeBr2(g)

Write the formulas for:oxygen difluoride gasdinitrogen tetrasulfide gassulfur trioxide gas tetranitrogen decaoxide

Covalent bondsThe bonds of molecular compounds are called

covalent bonds – which reflects the fact that the bond is a sharing (“co”) of valence electrons (“valent”)

Each atom donates half of the electrons to be shared.

If you think of the electron energy levels as being clouds of negative charge, then the bonds occur where the clouds overlap

Molecular (Polyatomic) elements

Some non-metals are so unstable (reactive) on their own that they rarely exist as single atoms

Instead, the atoms group together and share electrons in the same way as a molecular compound

Several elements are diatomic, meaning the element exists as pairs of atoms – these are H2 O2 N2 I2 F2 Cl2 and Br2

Molecular (Polyatomic) elements

Two elements are polyatomic, meaning the element exists in groups – these are:P4 and S8

To remind you, the polyatomic elements are all listed above your periodic table in the data booklet

*prefixes are not used for elements (e.g. O2 is “oxygen” not dioxygen”)

Molecular compounds with common names

While all molecular compounds have IUPAC names (e.g. dihydrogen monoxide) some are better known by their common names (e.g. water)

Some examples you should know are:water H2O(l)

glucose C6H12O6(s)

sucrose (table sugar) C12H22O11(s)

methane CH4(g)

methanol CH3OH(l)

ethanol C2H5OH(l)

ammonia NH3(l)

A note about molecular formulas

Recall, ionic formulas are called formula units, because they describe the ratio of cation to anion, not the actual amount of each elementThis means, if it is possible, you should simplify the

formula, e.g. Mg2O2 should be written as MgO(s)

Molecular formulas are differentThey describe the number of atoms that are actually in

the moleculeThey should not be simplifiedTwo different compounds may have the same ratio, but

are different substances, e.g. ethyne C2H2(g) and benzene C6H6(l)

Summary of ionic & molecular compounds

IONIC MOLECULAR

How to recognize them

Formula starts with a metal or ammonium

(NH4+)

Formula starts with a non-metal

Type of bonding Ionic bond Covalent bond

What’s happening to the electrons

Transferred from the cation (+ ion) to

anion (- ion)

Shared between the atoms in the molecule

What the formula represents

A formula unit – the ratio of cations to

anions

The actual number of each atom in the

molecule

How do you know how many of each

element?Balance your charges Use the prefixes

HomeworkCheck and Reflect #6, 8-12 (p.50)

A2.3Properties and Classification of

Ionic & Molecular Compounds

Identifying ionic compoundsIonic compounds are easily recognizable by

formula or name because it starts with a metal

Can you recognize an ionic compound based on its properties? Yes!Even though there are over a million different

ionic compounds that are found in nature, all ionic compounds have some common properties

Explaining ionic propertiesWe’ve already seen some of the properties

that ionic compounds share, such as:Solid at room tempHigh melting/boiling pointForm crystalline structuresSoluble in waterConduct electricity in solution

These properties can all be explained if you think about the ionic bonds

Explaining ionic properties:Solid at Room Temperature

The state of a substance at room temperature (about 200C) depends on the stability of the compound and the strength of its bonds

An ionic bond, which occurs when electrons are transferred, is very strong and hard to break

This means that it takes a lot of energy, or very high temperatures to break the bonds

At 200C, the particles simply do not have enough energy to separate, as they do in a liquid or gas

Explaining ionic properties:High melting/boiling point

the strength of the ionic bonds, and the fact that it takes a lot of heat energy to break those bonds

also explains why ionic compounds have very high melting pointsSalt, pictured below on the left, melts at 8010CCompare that to sugar, pictured below on the

right, which melts at 1860C

Explaining ionic properties:Form crystalline solids

The ability to form crystals is not exclusively a property of ionic compounds, for example, sugar is a molecular compound that also forms crystals

However, ionic crystals are much harder, stronger and more durable than sugar crystals

This is again, due to the strength of the ionic bonds, and the fact that ionic compounds form from repeating units of + and – ions, which gives them an organized structure

Crystal latticesDifferent shapes of crystals are formed by

different ratios of cations to anions

1:1 ratioe.g.• CaO(s)

• NaCl(s)

• MgS(s)

1:2 ratioe.g.• CaCl2(s) • Na2O(s)

Explaining ionic properties:Solubility in waterAll ionic compounds dissolve in water to

some extent, whereas most molecular compounds are insoluble in water

To understand this, we have to first explain something about the molecular structure of water

Water: a polar moleculeAs a molecular compound, water involves a sharingHowever, the sharing of those electrons is not done

equallyimagine that the electron pair is like the rope in a tug-of-

warin some molecular compounds, the electron sharing is

like a tug-of-war between identical twins because both atoms pull on the electrons with equal strength, the electron pair sits exactly between them

in water, however, the tug-of-war is between a large, strong individual (the oxygen) and two small, weak individuals (the hydrogens), so the electrons sit closer to one end of the molecule than the other

Water: a polar moleculeThis unequal sharing of electrons

makes the oxygen end of the molecule slightly more negative and the hydrogen end slightly more positive (the symbol δ = “slightly”)

Because it has a δ- pole and a δ+ pole, we call water a polar moleculeMolecular compounds that share

electrons equally, so do not have a δ- pole and a δ+ pole are called non-polar

Water: a polar moleculePolar solvents such as water will dissolve other

polar solutes, because their +/- poles will attract each other

However, non-polar molecules such as oil, will not dissolve in water, but rather form a layer on top

Explaining ionic properties:Solubility in waterBecause water is polar, when an ionic compound dissolves in

it, the charged ions of the ionic compound are attracted to the polar ends of the water moleculesIf the attraction is strong enough, the entire solid will dissolve

easily in water and we say that the compound is highly solubleIf the attraction between ions is stronger than the attraction

between the ions and water, then the compound will only dissolve a little bit and we say that it is slightly soluble

Solubility of ionic compoundsYou can determine whether or not an ionic

compound is soluble by using a solubility table, like the one found in your data booklet

Solubility of ionic compoundsTo use the table, locate the anion (- ion) in the

headerRead downif the cation

(+ion) is listed in the top row, the ionic compound is very soluble, and you should list its state as aqueous (aq)

Solubility of ionic compounds

if the cation (+ ion) is listed in the bottom row, the ionic compound is only slightly soluble, and you should list its state as solid (s)

Practice problemsIndicate the solubility of each of these

substances by recording the state as (aq) if the compound is highly soluble in water, and (s) if the compound is slightly soluble in water(NH4)2SAgClPbSO4

Sr(OH)2

PbI4

CuS

Practice problemsState whether or not the following

compounds are very soluble or slightly soluble in water.potassium carbonateiron (II) nitratecopper (I) chloridebarium hydroxideammonium sulfitecalcium sulfidelead (IV) bromide

Explaining ionic properties:Conduct electricity in solution

Substances that conduct electricity do so because they have a free-flow of electrons

When ionic compounds dissolve in water, the compound separates out into its ionsThis means that electrons are moving from

cations to anions, and the solutions will conduct an electrical current

Ionic compounds that are highly soluble conduct a strong current and those that are slightly soluble conduct a weak current

Properties of molecular compounds

Recall, molecular compounds are composed of covalent bonds

The attraction between individual atoms in a molecule is very strong but the attraction between neighbouring molecules is weak

Properties of molecular compounds

As a result:molecular compounds can be solids, liquids or

gases at room temperaturetheir melting points are much lower than ionic

compoundssome form crystals but they are weak and do

not hold their shapewhile some molecular compounds are soluble

in water, their solutions will not conduct electricity

Special properties of waterBecause of its polarity, water has some

special properties that are unlike most molecular compoundsother molecular compounds that are similar in

size and structure to water are gases at room temperature

if water were not polar, it would boil at around -800C that would mean that liquid water would not exist on Earth

Special properties of waterThe formation of ice is also a

unique property of waterThe 3D structure of ice crystals

results from the unusually strong attraction between water particles, and means that:ice crystals and snowflakes are

six-sidedice is less dense than liquid water ice floats

HomeworkCheck and Reflect A2.3 (p. 61)

#1-6, 8-9

A2.4Acids & Bases

Acids & bases in your bodyThe human body contains several fluids that

are either acidic or basic, most of which are located in the digestive systemsaliva – slightly basic to protect your teeth

from acidic foodsstomach acid – contains hydrochloric acid to

dissolve food and speed up digestion of proteins

pancreas – located just after the stomach in the digestive tract, produces basic compounds to neutralize any acids left in the food leaving the stomach

BuffersBuffers are compounds that keep the pH of a

solution constant, despite the addition of acids or bases

Buffers are important in the body because even a change of 0.1 in the pH of the blood can be lethal

Buffers are also found in nature – in Alberta,

buffering compounds naturally found in our lakes means we don’t experience the negative effects of acid rain

pH

“pH” stands for the power of hydrogenthe lower the pH, the more hydrogen is present in the

solution, and the stronger the acida movement of one number on the pH scale

corresponds to a ten-fold increase in the strength of the acid

Practice problemsHow much more acidic is a lemon (pH 3)

compared to a tomato (pH 5)?While most soaps have a pH of 10, shampoo

is “pH balanced” to be gentler on the hair. What does the term “pH balanced” mean in terms of the shampoo’s ingredients?

Properties of acids and basesACIDS BASES

Flavour Sour Bitter

Feel Wet Slippery

Examples in your home

Citrus fruits, vinegar Soap, baking soda

Examples in the lab HCl(aq), H2SO4(aq) NaOH(aq), Mg(OH)2(aq)

Look for formulas that contain…

Hydrogen, either at the start or –COOH at

the end of the formula

A cation and hydroxide (OH) –

bases are a type of ionic compounds

Reaction with metals Bubbling, metal corrodes

No reaction

pH less than 7 more than 7

Reaction with litmus Turns RED Turns BLUE

Universal indicator

pH indicatorspH indicators are chemicals that turn colour

based on the presence and amount of acids or bases in a solution

litmus paper is used to identify a solution as acidic, neutral or basic, but it does not give you a specific pH

another pH indicator, called universal indicator, is a mixture of chemicals that turn a specific colour for a specific pH

pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Naming acidsTwo different systems exist for naming acids

classical names such as “hydrochloric acid” or “sulfuric acid” are the ones still used in labs

IUPAC names have also been developed to tell you exactly what elements are present in the acid, e.g. “aqueous hydrogen chloride” and “aqueous hydrogen sulfate”

Naming acidsThree types of acid formulas exist

hydrogen + non-metal anion (ending in “ide”) e.g. HCl(aq) - aqueous hydrogen chloride e.g. H2S(aq) – aqueous hydrogen sulfide

hydrogen + polyatomic ion ending in “ate” e.g. HClO3(aq) – aqueous hydrogen chlorate e.g. H2SO4(aq) – aqueous hydrogen sulfate e.g. HNO3(aq) – aqueous hydrogen nitrate

hydrogen + polyatomic ion ending in “ite” e.g. HNO2(aq) – aqueous hydrogen nitrite e.g. H2SO3(aq) – aqueous hydrogen sulfite

Naming acidsWhen named using the classical system, they

becomehydro_______ic acid

e.g. HCl(aq) - hydrochloric acid e.g. H2S(aq) – hydrosulfuric acid

______ic acid e.g. HClO3(aq) – chloric acid e.g. H2SO4(aq) – sulfuric acid e.g. HNO3(aq) – nitric acid

______ous acid e.g. HNO2(aq) – nitrous acid e.g. H2SO3(aq) – sulfurous acid

Practice problemsComplete the following tableIf necessary, consult the polyatomic ions

table above your periodic table, or on page 491 of your textbook

Formula IUPAC name Classical name

H2CrO4(aq) aqueous hydrogen chromate

chromic acid

HI(aq) hydroiodic acid

H3PO4(aq)

CH3COOH(aq) aqueous hydrogen acetate

chlorous acid

aqueous hydrogen carbonate

boric acid

Naming basesBecause bases are one type of ionic

compound, they are named exactly the same way as any other

Bases are harder to recognize by their formula, but many contain hydroxide ions

NeutralizationWhen an acid and a base react together, their

properties cancel each other outthis is a reaction called neutralizationthe products of this reaction are water and a

saltE.g. the reaction between hydrochloric acid

and sodium hydroxideHCl(aq) + NaOH(aq) H2O(l) +

NaCl(aq)

acid + base water + table salt

A2.5Our Chemical Society

Hazardous chemicalsThousands of chemicals are classified as

hazardous, or as “controlled substances” by the Canadian government, based ontheir environmental effects

e.g. CFCs are compounds found in aerosol containers and old fridges that destroy the ozone layer

e.g. benzene is a compound used in the manufacture of plastics, dyes, detergents, and some medicine but has to be controlled due to its carcinogenic (cancer causing) properties

health concerns level of toxicity addictive properties (e.g. alcohol, nicotine)

Chemistry-related careersMany different careers require a knowledge

of chemistry, even if the person isn’t a chemist or chemical engineer

Practice problem:With a partner, brainstorm three careers that

use chemistry and describe two chemicals used in that career.

HomeworkCheck and Reflect A2.4 (p. 69)

#1 (a, c, e…)#3 (b, d, f…)#4 (a, c, e…)#6, 8

Solutions1. (a) basic (c) acidic(e) acidic(g) acidic(h) basic(j) basic3. (a) base (c) Neither(e) Acid(g) Acid4. (a) HNO3

(c) CH3COOH

(e) HCl(g) Potassium hydroxide6. A universal indicator changes color over a range of values. Litmus changes only at pH of 78. (a) pH changes 4 units, therefore 104 = 10,000 times(b) It has become 10,000 times less basic