1

Recap Atomic Structure

• Nucleus contains p+ and n.

• The number of p+ defines the element.

• Mass of atom = Σ (p+ + n)

• Electrons occupy orbits of defined energy.

• Electron configuration related to position in Periodic Table

Ne10

20

2

Ions

• Neutral atoms always have the same number of electrons as the number of protons in the nucleus.

• Many atoms may gain or lose electrons to form ions, which have a charge.

 

3

• Gain of electrons gives an anion.

e.g. F + e- F-

Ions

• Loss of electrons gives a cation.

e.g. Li Li+ + e-

9 e- 10 e-

3 e- 2 e-

4

Ions

• More than one electron may be gained/lost but >3 electrons not common.

O O2- N N3-

Mg Mg2+

Al Al3+

Note: charge is always shown at the top right

the sign comes after the number.

Eg.

5

Why do ions form?

Note on Noble Gases:

• Last column of periodic table.

• 8 electrons in outermost shell (2 for

He).

• Stable electronic structure.

• Do not gain/lose electrons.

• Occur as isolated atoms.

6

Noble Gases

• He: 2 electrons therefore n = 1 totally full.

• Ne: 10 electrons therefore n = 1 and 2 totally full.

7

Predicting which ions form?

• Atoms gain or lose electrons to become isoelectronic as the nearest Noble gas.

• Non-metals tend to form anions

• Metals tend to form cations

8

Ions

1+2+ 2- 1-

1+, 2+ or 3+

9

Ions

Examples:

Li Li+

+

NeHe

Li Li+ + e-

F F+

-

F + e- F-

IonsExamples: O, Mg

O + O

2-

Ne

O + 2 e- O2-10

Mg + Mg

2+

Mg Mg2+ + 2 e-

11

Na + Cl Na+ + Cl-

ClNa +

+Na Cl

-+

12

Ionic Bonding• The electrostatic attraction of cations

and anions results in ionic bonds being formed.

Cl

-

Na

+

Crystal structure of sodium chloride, NaCl

13

Ionic Bonding

• There must always be metal atom (to lose electrons) and a non-metal atom (to gain them).

• Production of ions always results from complete transfer of electrons between bonding atoms.

14

Mg + O Mg2+ + O2-

O

2-

Mg

2+

+

OMg +

One Mg2+ for every one O2- so compound is MgO

15

2Na + O 2Na+ + O2-

O

2-

Na

+

Na

+

+ +

ONa + Na

+

Two Na+ for every one O2- so compound is Na2O

16

2Al + 3O 2Al3+ + 3O2-

O OO

Al Al

O

2-

Al

3+

O

2-

O

2-

Al

3+

Two Al3+ for every three O2- so compound is Al2O3

17

Ionic Bonding

• Produces a solid - crystal lattice.

• Cations and anions are packed so as to maximise the attractive forces and minimise repulsion.

• Ratio of cations to anions ensures ZERO overall charge on the compound.

• Cation-anion attraction does not depend on direction: non-directional bonding.

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Ionic Bonding

Different ionic compounds have different crystal structure

SodiumChlorideNaCl

ZincSulfideZnS

Learning Outcomes:

• By the end of this lecture, you should:– be able to work out the number of electrons an

ion has from its symbol  – recognize that most ions have a noble gas

configuration  – predict whether an element will form a cation or

an anion– predict the charge on the cation or anion an

element will form– be able to balance the charges– explain the characteristics of ionic bonding

– be able to complete the worksheet (if you haven’t already done so….) 19

Questions to complete for the next lecture:

20

• Predict the formula of the ionic compound formed between the following pairs:

a) Li and Brb) Li and Sc) Li and Nd) Mg and Bre) Mg and Sf) Mg and Ng) Al and Fh) Al and Oi) Al and N