bonding

Bonding in Chemical

Compounds

Goals – Know about:• Ions, anions, cations, polyatomic ions • “Eight singing horses”• Bonds: ionic, covalent, metallic• Oxidation numbers • Chemical formulas• Lewis structures (e- dot diagrams)• Molecular geometry and bond angles

What is an ion?• An ion is a charged atom or

a charged group of atoms (polyatomic ion).

• Ions acquire a charge because they either gain or lose one or more electrons.

What is an ion?• They are positive if they lose

negatively charged electrons. (cations)

• They are negative if they gain negatively charged electrons. (anions)

“Cat-ions”

Cats don’t have owners…

… they have staff.

“Cat-ions”

Therefore, cations are positive ions!

Cats are positive that

they are in charge.

Cation Formation• Cations form when one or more

electrons are lost.

• Energy is usually added.

• A positive ion is formed.

• The resulting cation often has a more stable electron arrangement, (like that of an inert gas).

Anion = a negative ion

Anions are called that because they are attracted to the anode (the positive electrode) in an electrolytic cell.

Anions must be negative.

Anion Formation• Anions form when one or more

electrons are added.

• Energy is sometimes released.

• A negative ion is formed.

• The resulting anion has a more stable electron arrangement, (like that of an inert gas).

What are “eight singing horses”?When an atom gains or loses

electrons, and …… it has the same electron

arrangement as an inert gas,we say that it has a …

Stable Octet

Consider an atom of chlorine.

ClIt has 7 valence electrons.

It would be more stable if it had eight electrons, just like an inert gas.

Consider an atom of chlorine.

ClNow it has eight electrons, just like an inert gas.

An additional electron is added, making Cl- ion.

It has a “stable octet”!

Chemical Bonding

There are two kinds of chemical bonds:

Keywords for each are…

Ionic Bond = transfer

Covalent Bond= share

Bonding - Review

Ionic bonds - transfer electrons to get more stable arrangements.

Covalent bonds - share electrons to get more stable arrangements.

When is a bond ionic and when is it covalent?

It is ionic when electrons have been transferred.

Some bonds between metals and nonmetals are clearly ionic, where the metal transfers an electron completely to the nonmetal so that each element has a stable octet.

Na + F [Na]+ + [ F ] -1

It is covalent when electrons are shared

Some bonds are clearly covalent, where electrons must be shared so that each element has an octet of electrons.

Cl C + 4 Cl Cl C Cl Cl

% Bond Character

100% 50% 0%Covalent Covalent Covalent0 1.7 3.3 4

Electronegativity Difference

0% 50% 100%Ionic Ionic Ionic

There is a third kind of bond…

No, not the James Bond

There is a third kind of bond…

The metallic bond

Array of positive metal ions surrounded by a sea of

mobile electrons.

Metallic Bonding• Found only in metals

• Atoms in a regular arrangement

• Bonding electrons are free to move from atom to atom throughout the solid

• A “sea of mobile electrons”

• Accounts for metallic properties

Ionic Compounds

Ionic Compounds contain ionic bonds

An ionic bond is the electrostatic attraction between a positive ion

and a negative ion.

Consider this reaction:

Na(s) + Cl(g) Na+ + Cl-

NaStart with a sodium atom and it’s one electron.

Cl+

Then add a chlorine atom.The sodium donates it’s electron to chlorine.

ClNa +

Makes Na+ ions and Cl- ions

+ -

An ionic compound: NaCl

We can use the valence electrons to predict the

ions that are formed when atoms react.

Example: When Al and F react, why will one Al3+ ion and three F- ions form?

FAl

F

FStart with the ions:

FAl

F

F

FAl

… produces one Al3+ ion and 3 F- ions. -- AlF3

+ 33+ -

The reaction between aluminum and fluorine …

Predict the ions that will be formed, and the formula of the compound.

1. Potassium (K) and chlorine (Cl)

2. Calcium (Ca) and fluorine (F)

3. Cesium (Cs) and oxygen (O)

4. Scandium (Sc) and sulfur (S)

5. Barium (Ba) and nitrogen (N)

Ionic CompoundsAnswers

1. K+, Cl-

2. Ca2+, F-

3. Cs+, O2-

4. Sc3+ , S2-

5. Ba2+, N3-

KCl

CaF2

Cs2O

Sc2S3

Ba3N2

Ionic Charge vsOxidation Number

Oxidation number is an arbitrary number assigned to an element …

It is useful for determining the formulas of compounds.

It is equal to the ionic charge on an element.


Ionic charges have the number written first, followed by the sign:

Na+ Ca2+ Al3+ Ti4+ Mn7+

Cl- O2- P3-

The oxidation number has the sign first followed by the number.


The oxidation number of aluminum ion Al3+ is +3.

The oxidation number of sulfur in K2S is –2.

See the handout on oxidation number for more information.


What is the oxidation number of each element in the following:

1. H2O2. FeCl3

3. As2S3

H = +1, O= -2Fe = +3, Cl = -1As = +3, S = -2

Quick and dirty formula writing!

Use the “criss-cross”method.

Start with: Ax+ and By-

The formula is AyBx

The Crisscross Method

•Simple but effective,

•…most of the time.

•Be aware of the potential problems with this method.

Suppose you are writing the formula for copper & chlorine

First, write down First, write down the symbols.the symbols.

CuCu ClCl

The metal is first – positive

oxidation number

Then write the oxidation numbers for copper & chlorine

CuCu ClCl-1-1

The most common The most common oxidation number oxidation number of copper of copper is +2is +2 +2+2

Get the Get the oxidation oxidation

number of number of chlorine chlorine from the from the

periodic table.periodic table.

Crisscross the numbers for the formula of copper & chlorine

CuCu ClCl+2+2 -1-1

2211

When you crisscross, When you crisscross, ignore the signs.ignore the signs.

Clean up the subscripts in the formula of copper & chlorine

CuCu ClCl+2+2 -1-1

22

Subscripts of 1 are invisible. Subscripts of 1 are invisible. (don’t even put the 1)(don’t even put the 1)

11

Now you have the formula for a compound of copper

and chlorine

CuCu ClCl22

Write formulas for these ionic compounds:

1. Potassium (K) and iodine (I)

2. Aluminum (Al) and sulfur (S)

3. Oxygen (O) and magnesium (Mg)

4. Iron (Fe) and bromine (Br)

5. Gold (Au) and arsenic (As)

Remember: The positive ion is written first, and there may be more than one compound.

Answers1. KI

2. Al2S3

3. MgO

4. FeBr2 and FeBr3

5. Au3As and AuAs

Properties of Ionic Compounds

As ionic compounds form …

1. The chemical and physical properties of the compound will differ from the properties of the elements.

2. The charges of the ions depend upon the electron arrangement of the element.

3. The radii of the ions differ from the radii of the atoms.

More changes…4. Energy is released during bond

formation. Forming a bond produces a more stable arrangement. Energy is released as the compound goes to a lower energy state (exothermic).

5. There are no “molecules” of ionic compounds. Use “formula units” for ionic compounds.

Properties of Ionic Compounds

• High melting points• High boiling points• Hard and brittle• Good conductors in molten state• Good conductors when dissolved

in water

Covalent Compounds

Covalent compounds contain covalent bonds

• Covalent bonds = sharing electrons

• Covalent bonds usually form between nonmetals.

• Covalent bonds can involve multiple pairs of electrons: single, double, triple bonds.

More on covalent compounds:

• Covalent compounds generally have low melting and boiling points.

• Covalent compounds are usually soft.

• Covalent compounds are poor conductors.

Covalent BondingCovalent bonds form by sharing

electrons between atoms …

… so that each atom appears to have an octet of electrons.

Diatomic elements are good examples of covalent bonding.

The Diatomic Elements are:

H2 N2 O2 F2 Cl2 Br2 I2

Known worldwide as the “hairogens”:

H, N & O, halogens

N and O = ??? air

H air ogens …hence, the

Bonding in the Halogens

F

F + F F2 F

Formation of a F-F bond

Bonding in the Halogens

F

F + F F2

The overlap of two p-orbitals creates the single bond between fluorine atoms.

F

F - F

See how a double bond occurs in an

oxygen molecule and a triple bond occurs in a nitrogen molecule.

Bonding in Oxygen

O

O

O + O O2

The overlap of four p-orbitals creates the double bond between oxygen atoms. O = O

Bonding in Nitrogen

N

N

N + N N2

The overlap of six p-orbitals creates the triple bond between nitrogen atoms. N N

Comparison of single, double and triple bonds:

Bond length: s--i--n--g--l--e > d o u b l e > triple

A B A B A B

Bond strength:

single < double < triple

Covalent bonds result from the overlap of orbitals.

Points to remember when writing formulas

of Covalent Compounds

1. Write the symbol of each element. Write the symbol of the

element with the positive oxidation number first.

+4 -2

C O

2. Change the subscript so that the sum of the oxidation numbers is zero.

+4 -2

C O

+4 -4

2

= 0

In summary …

The sum of the oxidation numbers of all of the elements is zero.

The symbol of the element with the positive oxidation number is written first.

In summary … The element with lower

electronegativity is assigned the positive oxidation number.

The element with the greater electronegativity gets the negative oxidation number.

Write the formula(s) for the following covalent compounds:

1. Boron (B) and nitrogen (N)

2. Fluorine (F) and arsenic (As)

3. Sulfur (S) and chlorine (Cl)

4. Nitrogen (N) and oxygen (O)

5. Bromine (Br) and selenium (Se)

Answers

1. B & N = BN

2. F & As = AsF3 and AsF5

3. S & Cl = SCl2, SCl4 and SCl6

4. N & O = N2O, NO, NO2, N2O3, and N2O5

5. Br & Se = SeBr2, SeBr4, SeBr6

Bond Polarity

Polar Bonds

Q. Where would you be most like to find polar bonds?

a. Nome, Alaskab. McMurdo Sound, Antarcticac. Sitka, Siberiad. Water molecule

More on Polar Bonds

• A polar bond is where one atom has a greater pull on the bonding electrons than the other.

• Ionic bonds are polar, but what about covalent bonds?

• Polar covalent bonds occur when the electronegativity difference is greater than zero.

Consider a C-Cl bond• The electronegativity difference (EN)

between chlorine and carbon is 0.5.

EN = (3.0 – 2.5) = 0.5

• Since the EN > 0, the covalent bond is polar. The chlorine exerts a slightly greater pull on the bonding electrons, pulling them slightly closer to Cl.

• If EN = 0 the bond would be nonpolar.

So what is the answer?• You would be most likely to find a

polar bond in a water molecule.

• The electronegativity of oxygen is greater than hydrogen, so the pull on the bonding electrons by the oxygen is greater, giving a polar bond.

• Because of the polar bonds and its geometry water is a polar molecule.

Gilbert N. LewisA renown American chemist who lived and worked around the turn of the century. He developed an explanation of the reaction between acids and bases. To explain his theory, he developed “electron dot diagrams” which are still in use today to explain molecular structure as well as acids and bases.

Writing Lewis Structures1. Add up all of the valence electrons

2. Decide on a central atom. It has the lowest EN. H is never a central atom; halogens rarely are.

3. Draw the skeleton of the molecule and connect each symbol with a dash to indicate a bonding pair of electrons

Writing Lewis Structures4. Complete the octet of the terminal

atoms, add all the electrons and compare to step #1.

5. Add any additional electrons to the central atom, even if it means having more than 8.

6. If there are not enough electrons to give every element an octet, consider multiple bonds.

Writing Lewis StructuresSome things to remember:

• Hydrogen can only have two electrons around it, not an octet.

• The central atom is frequently the one that there is only 1 of.

• Halogens are almost never the central atom and they never have double or triple bonds!

Write the Lewis structures for the following compounds:

1. H2O

2. CH4

3. OF2

4. PCl3

5. HCN

6. CO

7. CO2

8. SCl4

9. PCl5

10. XeCl4

Hydrogen Bonding

A hydrogen bond is not a molecular bond, but rather a very strong intermolecular attraction.

It exists in molecules containing hydrogen and certain small, highly electronegative nonmetals, like oxygen, nitrogen and fluorine.

Hydrogen bonding exists in water

The length of a hydrogen bond is about 180 pm

oxygen

hydrogen

Two lone pairs of electrons

~100 pm bond length

In which of the following might you expect to see hydrogen bonding?

1. HF

2. NH3

3. CH4

4. CH3OH

5. CH3OCH3

6. H2S

YesYesNoYesNoNo

Molecular Geometry

Molecular geometry deals with the three-dimensional

shapes of molecules.

Molecular Geometry …… depends on the arrangement of

atoms around the central atom

… depends on the number of bonding and nonbonding pairs of electrons around the central atom

… depends on the repulsive property of electrons, electron pairs get as far apart as possible. VSEPR

Theory

VSEPR Theory

Valence Shell Electron Pair Repulsion

VSEPR Theory can be used to predict the 3-dimensional shape of molecules based on the number of bonding and nonbonding pairs of electrons on the central atom.

Molecular Geometry Electron

pairsMax angle

2 ?

3 ?

4 ?

5 ?

6 ?

VSEPR & Electron Pair Geometry

e- pairs Electron pair geometry Angle

2

3

4

5

6



2 Linear 180

3

4

5

6



2 Linear 180

3 Trigonal planar 120

4

5

6



2 Linear 180


4 Tetrahedral 109.5

5

6



2 Linear 180


4 Tetrahedral 109.5

5 Trigonal bipyramidal 90 & 120

6



2 Linear 180


4 Tetrahedral 109.5

5 Trigonal bipyramidal 90 & 120

6 Octahedral 90

e- pair geometry

Molecular geometry

Bond angle

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle

2 2 0

3 3 0

3 2 1

4 4 0

4 3 1

4 2 2

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle

2 2 0 Linear Linear 180

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle


3 3 0 Trigonal planar Trigonal planar 120

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle



3 2 1 Trigonal planar Bent 120

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle




4 4 0 Tetrahedral Tetrahedral 109.5

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle





4 3 1 Tetrahedral Trigonal pyramidal

109.5

Ele

ctro

ns

Bon

ding

Non

bond

.

e- pair geometry

Molecular geometry

Bond angle





4 3 1 Tetrahedral Trigonal pyramidal

109.5

4 2 2 Tetrahedral Bent 109.5

Ele

ctro

ns

Bon

ding

Non

bond

.

Linear Molecular Geometry

180 degrees

Two bonding pairs, no nonbonding pairsTwo electron pairs

Linear electron-pair geometry

Trigonal Planar Molecular Geometry

Three bonding pairs, no nonbonding pairsThree electron pairs

Trigonal planar electron-pair geometry

120 degrees

Bent Molecular Geometry

Two bonding pairs, one nonbonding pairThree electron pairs

Trigonal planar electron-pair geometry

120 degrees

Tetrahedral Molecular GeometryFour electron pairs

Tetrahedral electron-pair geometry

109.5 degrees

Four bonding pairs, no nonbonding pairs

Four electron pairs

Tetrahedral electron-pair geometry 109.5 degrees

Three bonding pairs, one nonbonding pair

Trigonal Pyramidal Molecular Geometry

107 degrees in ammonia

Four electron pairs

Tetrahedral electron-pair geometry 109.5 degrees

Two bonding pairs, two nonbonding pairs

Bent Molecular Geometry

105 degrees in water

bonding

Science

ionic compoundsanswers1

positive ions

negative ion

f ions form

covalent bonds

octetof electrons

electrons havebeen

ionsan ionic compound