NCEA1 Chemistry Basics

CA 2005

Element Structure and the

Periodic Table

Chemical Bonding

The smallest whole particles are ATOMS

An atom consists of a central nucleus

surrounded by much smaller electrons

There are over 100 different types of atom. Each one is

called an ELEMENT.

nucleus

electron

The electrons are arranged in layers

called shells.

shells

These elements are arranged into the PERIODIC TABLE

The horizontal rows are called the PERIODS

Period 4

Elements in the same period all have the

same number of electron

shells

So elements in Period 4 all have four shells of electrons

These elements are arranged into the PERIODIC TABLE

The Vertical columns are called GROUPSGroup 18

Elements in the same group all have the same number of electrons in their outer shell so they all have similar properties.

For NCEA 1 you only have to know how the rules apply for the first 20 elements (Hydrogen to Calcium)

Inside the nucleus of the atom are subatomic particles called protons and neutrons.

Protons have a mass of 1 and a charge of +1

Neutrons have a mass of 1 but no charge.

Electrons have no mass but a charge of -1

Between the electron and the nucleus is a vacuum

(nothing!)

Vacuum

+

+

++

+

Proton

Neutron

In an element there is always the same number of

protons and electrons.

Every different element has a unique number of these.

This element has 5 protons and 5 electrons. The 5th element in the periodic

table is Boron

B5

This is called the element’s Atomic Number

Remember: In the periodic table all the elements in a horizontal period have the same number of electron

shells.

H1

Hydrogen

He2

Helium

The first electron shell can only hold 2 electrons. So Period 1 only

contains two elements.

Period 2 elements all have two shells. The first one contains two electrons but the second

one can have up to eight.

Li3

Lithium is the smallest element of period 2 with just

one electron in shell 2

Ne10

Neon is the largest with a full second shell of 8 electrons.

Period 3 and the first elements of Period 4 follow the same rules with up to 8 electrons in their outer shell.

Electron Arrangements.

The full electron arrangements in their shells for the first 20 elements are shown on the next slide.

shell 1 shell 2 shell 3 shell 4

Hydrogen 1Helium 2

Boron 2 3

Lithium 2 1Berylium 2 2

Carbon 2 4

Oxygen 2 6Nitrogen 2 5

Fluorine 2 7Neon 2 8

Aluminium 2 8 3

Sodium 2 8 1Magnesium 2 8 2

Silicon 2 8 4

Sulphur 2 8 6

Phosphorus 2 8 5

Chlorine 2 8 7Argon 2 8 8Potassium 2 8 8 1Calcium 2 8 8 2

So the electron arrangement for Sulphur is normally written:

S: 2, 8, 6

How and Why Elements Bond Together

H2

H2SO4

PO3-

Mg2+

(NH4)2CO3

H2O

CO2

AgK Na

Al Fe

O2

Cl2

H2

N2

CO2

HCl

H2SO4

Al(HCO3)3

These are all atoms of different ELEMENTS

The symbol for an element may be a single capital letter (eg K) or a

capital followed by a lower case letter (eg Na).

These are all MOLECULES. They all have more than one atom in them joined together

chemically.

These MOLECULES contain atoms from just one

element

These molecules all contain atoms

from more than one element.

They are COMPOUNDS

Elements join together chemically to form molecules because their outermost shell of electrons is incomplete.

The only elements that never form molecules are in Group 18

This is because they already have a full outer shell.

He2

Eg Helium

To get a complete outer shell atoms can bond with others in two ways:

1. They can share some of their electrons.

This is called COVALENT bonding.

or

2. They can exchange electrons.

This is called IONIC bonding.

Covalent Bonding.

An oxygen atom has 2 electrons in its first shell and 6 electrons in

its outer one.O

8oxygen

This leaves it with 2 gaps to fill in its outer shell.

By sharing 2 of their electrons each these two atoms of oxygen now have a full 8 in their outer shell each

This produces a covalent molecule of

oxygen gas: O2

Non-metals often form covalent compounds with other non-metals, but the commonest ones you will

come across are the gases.

Oxygen gas = O2 Hydrogen gas = H2

Nitrogen gas = N2Chlorine gas = Cl2

Fluorine = F2

Bromine = Br2

You will need to remember when you are balancing equations that these gases exist as molecules of two

atoms each!

Elements always have the same number of protons and electrons.

But many of them can gain or lose electrons from their outer shell to become charged particles called ions

Elements that lose electrons become positive ions.

This is what the metals do.

Elements that gain electrons become negative ions.

This is what the non-metals do.

Ionic Bonding

Positive ions attract negative ions to form ionic compounds

For instance:

Magnesium is in Group 2. It has 2 electrons in its outer shell. It can lose these to become a 2+ positive ion

Mg Mg2+ + 2e-

Oxygen is in Group 16. It has 6 electrons in its outer shell. It can gain 2 more to become a 2- ion

O + 2e- O2-

Positive and negative ions attract each other to form ionic compounds

Mg2+ + O2- MgO

You will always be given a table of common ions in the exam

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Fe 2+

Zn 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Fe 2+

Zn 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

Group 1 elements form 1+ ions

Group 2 form 2+ ions

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Fe 2+

Zn 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

A few metals can form two different ions

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Zn 2+

Fe 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

Ammonium is the only polyatomic positive ion.

( polyatomic ions contain atoms from more than one element )

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Zn 2+

Fe 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

Group 17 elements all have one space in their outer shell. They gain one electron to become 1- ions

Positive Ions

H+

Li +

Na +

K +

NH4 +

(ammonium)

Be2+

Mg 2+

Ca 2+

Cu 2+

Zn 2+

Fe 2+

Al3+

Cu3+

Fe 3+

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

Group 16

Negative Ions

F-

Cl -

Br -

I -

NO3 -

HCO3 -

O2-

S 2-

SO4 2-

CO3 2-

PO43-

hydrogencarbonate

nitrate

carbonate

sulphate

phosphate

The negative polyatomic ions

Group 18 don’t form ions. They already have a full outer shell.

Group 14 don’t form ions either. They have 4 electrons in their outer shell - so it is neither easier to gain or lose them!

It is important to balance the formula for compounds properly.

Fortunately the rules for doing this are quite simple.

Potassium can lose an electron to become a 1+ ion : K+

Chlorine can gain an electron to become a 1- ion: Cl-

These charges are equal and opposite so they cancel each other out to form a molecule of potassium chloride : KCl

Magnesium can lose 2 electrons and forms a 2+ ion : Mg2+

Sulphur can gain 2 electrons to become a 2- ion : S2-

The charges cancel each other out to form a molecule of magnesium sulphide : MgS

So if the ions’ charges are equal and opposite the compound’s formula is easy to work out!

Swap and DropYou use this rule when the charges are not the same

Mg2+

This means magnesium has to

give away 2 electrons

Cl-

But chlorine can only take 1 electron when it becomes a chloride

ion.

21

You “swap and drop” the charges from above one ion to below the other

This gives you the compound formula:

MgCl2

In other words, you need two chloride ions to

balance one magnesium one.

Aluminium oxide

Al3+ O2-

The ions:

32

Swap and dropThe compound:

Al2O3

Iron III bromide

The ions: Swap and drop The compound:

Fe3+ Br-1 3

FeBr3

Using Polyatomic IonsThese look difficult, but they work exactly the same way.

Sodium hydroxideThe ions:

Na+OH-

The compound:

NaOH

Magnesium hydroxideThe ions: The compound:

Mg2+OH-

21Mg(OH)2

MgSO4

One molecule of magnesium sulphate

2MgSO4

One magnesium ion

One sulphate ion

Two molecules of magnesium sulphate

(NH4)3PO4

3 ammonium ions 1 phosphate ion

1 molecule of ammonium phosphate

4Fe2(SO4)3

4 molecules of iron III sulphate

2 iron III ions 3 sulphate ions

Increasing the Rate of Reactions.

Reactions occur when molecules collide.

Reaction Product

A

B

Reactants

There are various ways to increase the rate at which this occurs.

1. Increase the concentration of the reactants.

A

A

AA

A

A

A

A

B B

BB

B

B

B

Reaction

Product

Product

Product

Product

More reactants = More collisions = More product!