Ions and Ionic Bonding

Metal atoms lose outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, each reaching a full outer shell. An ionic bond is the strong electrostatic attraction between oppositely charged ions. Ionic compounds form giant lattices, so they have high melting points, and they conduct electricity only when molten or dissolved, because then the ions can move.

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Section 1 of 14

Learning objectives

Explain how ions form and predict their charges from group numbers.
Draw dot-and-cross diagrams for ionic compounds.
Work out ionic formulae.
Explain the properties of ionic compounds.

Pause and think

Solid salt doesn't conduct electricity, but salty water does. What changes?

Section 2 of 14

Core idea

Start hereA swap that makes salt

A sodium atom has one lonely electron in its outer shell; a chlorine atom is one short of a full shell. When they meet, the electron jumps across and two oppositely charged ions cling together with enormous force.

The big idea

Metals lose electrons to become positive ions; non-metals gain electrons to become negative ions. The ionic bond is the electrostatic attraction between them, repeated through a giant lattice.

1. Forming ions

Group I → $1+$ , group II → $2+$ , group III → $3+$ .
Group V → $3-$ , group VI → $2-$ , group VII → $1-$ .
Each ion has the electronic configuration of a noble gas.

Worked exampleMagnesium oxide

Describe how magnesium ( $2,8,2$ ) and oxygen ( $2,6$ ) form magnesium oxide.

Goal: Electron transfer and formula.
Strategy: Find how many electrons each atom loses or gains.

$\mathrm{Mg}\rightarrow\mathrm{Mg^{2+}}+2e^-$
Mg loses its two outer electrons.
$\mathrm{O}+2e^-\rightarrow\mathrm{O^{2-}}$
O gains two to fill its outer shell.
$\mathrm{MgO}$
One of each balances the charges.

AnswerTwo electrons transfer from Mg to O, forming

\mathrm{Mg^{2+}}

and

\mathrm{O^{2-}}

\mathrm{MgO}

✓ CheckBoth ions now have configuration

2,8

✓.

2. Ionic formulae

Worked exampleAluminium sulfate

Ions: $\mathrm{Al^{3+}}$ and $\mathrm{SO_4^{2-}}$ .

Goal: Formula.
Strategy: Find the lowest common multiple of the charges.

$\mathrm{LCM}(3,2)=6$
Total positive and negative charge must each be 6.
$2\,\mathrm{Al^{3+}},\ 3\,\mathrm{SO_4^{2-}}$
$2\times3=6$ , $3\times2=6$ .

Answer

\mathrm{Al_2(SO_4)_3}

✓ Check

+6-6=0

✓.

3. Properties

Why ionic compounds behave as they do

Drawing diagram…

Structure explains properties.

Common misconceptionIons, not electrons Molten or dissolved ionic compounds conduct because ions move, not electrons.

Pause and think

Why does magnesium oxide melt at a much higher temperature than sodium chloride?

Summary

Electron transfer, metal → non-metal.
Charges balance in the formula.
Giant lattice: high mp; conducts when molten or aqueous.

Ion: A charged particle formed when an atom loses or gains electrons.
Ionic bond: Electrostatic attraction between oppositely charged ions.
Lattice: A regular, repeating 3D arrangement of particles.

HistoryDavy and electrolysis. In 1807 Humphry Davy passed electricity through molten potash and isolated potassium for the first time, showing that compounds are held together by electrical forces.

Fun factRock salt cubes. Halite crystals grow as near-perfect cubes, because sodium and chloride ions stack in a cubic lattice all the way up to the size you can see.

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