recap – ions and ionic bonding

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Recap – Ions and Ionic Bonding

• Cations are positively charged and formed from metals.

• Anions are negatively charged and formed from non-metals.

• Cations and anions are formed at the same time by electron transfer.

• An ionic bond is the electrostatic attraction between cations and anions.

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Ionic BondingCl

-ClNa + Na

+

• It takes energy to remove an electron from an atom.

• Electrostatic attraction between cations and anions supplies energy.

• Generally do not get very highly charges ions as too much energy is required to form them.

C C+ C2+ C3+ C4+ e- e- e- e-

2 360 kJ

1 090 kJ

4 630 kJ

6 230 kJ

Sharing of Electrons• In ionic compounds a Noble gas electron

configuration is obtained by transfer of electrons.

• In covalent compounds a Noble gas electron configuration is obtained by sharing of electrons.

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F F F F

Sharing of ElectronsH2

4

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Reason for Covalent Bonding

• Electrons are shared to achieve a Noble gas configuration for each atom.

• Electron density between the two atoms: the bond is directional.

• Only electrons from the outermost shell can be involved in bonding – valence electrons.

• Electrons tend to form pairs.

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Covalent Bonding: C and H

H

H

H

HC

• Carbon has 4 electrons in outer shell – needs 8

• Hydrogen has 1 electron in outer shell – needs 2

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Covalent Bonding - methane

CH4

H H

H

H

C

H

CH H

H

8

OH H

Covalent Bonding - water• Oxygen has 6 electrons in outer shell – needs

8• Hydrogen has 1 electron in outer shell – needs

2

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Covalent Bonding – H2O• We can identify bond pairs and

lone pairs (or non-bonding pairs) of electrons

OH H

OH H

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Covalent Bonding: N and H XXXX

XX

X

NXH

XXXX

XX

X

N

XH

X HXH

• Show only valence electrons

H N

H

H

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Valence• Valence electrons are used in bonding.• The valence of an atom is the size of the charge if

it forms an ion or the number of bonds it forms in a neutral molecule.

1 2 3 4 3 2 1

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• The diatomic elements (eg H2, N2, O2, F2) all have a similar formula but different bonding requirements.

Diatomic Molecules

X HXH

H H

F2Single Bond

H2Single Bond

XXXX

XF

XX

XX

XXXX

X F

XX

XX

F F

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• Double bonds occur where two electron pairs are shared between atoms.

Bonding Requirements

XXO

X

XXXO

X

X XXO

X

XXXO

X

X

O OO2

Double Bond

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• Triple bonds occur where three electron pairs are shared between atoms.

Bonding Requirements

XXN X

X

XXNX

XX X

X

X X

XXXN

X

XXX

XX NX

XX

X

N N

XX

N2

Triple Bond

• By the end of this lecture, you should:− understand the difference between ionic and

covalent bonding− be able to draw diagrams representing the

bonding in simple molecules− know the difference between a bond pair and

lone pair of electrons− know the valence of elements in groups 1, 2, 13

– 17− be able to predict the formula of simple covalent

compounds− appreciate the difference between a single,

double & triple bond

− be able to complete the worksheet (if you haven’t already done so…)

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Learning Outcomes:

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Questions to complete for next lecture:

1. Would the following pairs of elements form ionic or covalent bonds? H and O, Na and O, N and O, S and O.

2. Draw a diagram representing the molecule HCN.3. How many lone pairs does each of the following

molecules possess? H2, Cl2, H2S, HBr.4. Oxygen has a valence of 2. Give an example of

an ionic compound and a covalent compound containing oxygen that illustrates this concept.

5. Predict the formula of the simplest compounds formed between sulfur and fluorine, phosphorus and chlorine, magnesium and iodine.