1fIonic bonding

Atoms react in order to reach a full outer shell. There are two ways to get there, and which one an atom takes depends on whether it is a metal or a non-metal.

Metals lose, non-metals gain

A metal has few outer electrons, so losing them is the shorter route. A non-metal has nearly a full shell, so gaining the last one or two is easier.

Why losing electrons gives a positive charge

This catches people out, because it feels backwards.

A sodium atom has 11 protons (+11) and 11 electrons (−11), so it is neutral. Lose one electron and it still has 11 protons but only 10 electrons. More protons than electrons means an overall charge of 1+.

The charge is opposite to what was lost. Losing a negative leaves a positive.

The same logic runs the other way for chlorine: 17 protons, gain an electron to make 18 electrons, and the ion carries a 1− charge.

The name changes too

A chlorine atom becomes a chloride ion. An oxygen atom becomes an oxide ion. Non-metal ion names end in -ide.

Metal ions keep their element name — a sodium atom becomes a sodium ion.

What the ion looks like afterwards

When sodium (2,8,1) loses its outer electron it becomes 2,8 — the outer shell is emptied entirely, not left holding zero. Write 2,8, not 2,8,0.

That configuration is the same as neon's, which is the point: the ion has reached the stable arrangement it was reacting to obtain.

Charges you can read off the Periodic Table

For Groups 1–3 the charge is the group number. For Groups 5–7 it is the group number minus 8, which gives a negative.

Charges you have to learn

These do not follow from a group number:

Where a name carries a Roman numeral, that numeral is the charge. Iron(III) is Fe³⁺. Nothing to memorise there beyond reading the name.

Ions made of several atoms

Two patterns help. Ammonium is the only common positive ion made of more than one element. And carbonate and sulfate are both 2−, while nitrate and hydroxide are both 1− — learn them as two pairs.

Building a formula

One rule: total positive charge must equal total negative charge, so the compound is neutral overall.

Mg²⁺ with Cl⁻: one 2+ needs two 1−, giving MgCl₂.

Al³⁺ with SO₄²⁻: the lowest number that 3 and 2 both divide into is 6, so you need two Al³⁺ and three SO₄²⁻ — Al₂(SO₄)₃.

Brackets

Use brackets when a multi-atom ion appears more than once.

Ca(NO₃)₂ is right; CaNO₃₂ is not, because that subscript would apply only to the oxygen. The brackets are not decoration — they change which atoms get multiplied.

And the charges never appear in the final formula. Once the compound is neutral, they have done their job.

What the bond actually is

Ionic bonding is the strong electrostatic attraction between oppositely charged ions.

That sentence is the mark scheme. Two details in it are worth noticing:

• It is between ions, not atoms. The electron transfer happens first; the bonding is what holds the resulting ions together.
• It is electrostatic — the ordinary attraction between a positive charge and a negative one. No special force is involved.

The transfer, in order

For sodium chloride:

1. The sodium atom loses one electron → Na⁺
2. The chlorine atom gains that electron → Cl⁻
3. The two ions attract each other

A dot-and-cross model shows this with an arrow for the moving electron, and the transferred electron drawn differently from the others so you can see where it came from. Both ions are then shown in square brackets with their charges.

It is a lattice, not a pair

One sodium ion is not bonded to one particular chloride ion. Every ion is surrounded by ions of the opposite charge, in a giant lattice extending in all directions.

That is why there is no such thing as a molecule of sodium chloride, and why NaCl is a formula showing a ratio, not a count of atoms in a molecule.

High melting points

Because the attractions are strong and there are enormous numbers of them running through the whole lattice, a great deal of energy is needed to pull the ions apart. So ionic compounds are solids at room temperature with high melting and boiling points.

A complete answer names three things: the attractions are strong, they are between oppositely charged ions, and they act throughout the lattice.

Charge size changes the strength

Magnesium oxide melts at 2852 °C; sodium chloride at 801 °C.

The reason is charge. MgO contains Mg²⁺ and O²⁻; NaCl contains Na⁺ and Cl⁻. Larger charges attract more strongly, so more energy is needed to separate them.

When a question compares two ionic compounds, compare the charges first — it is almost always the answer.

Solid sodium chloride does not conduct electricity. Molten sodium chloride does. So does a solution of it in water.

That pattern is one of the standard tests for ionic bonding, and the explanation is short.

Conduction needs two things

1. Charged particles — and an ionic compound always has them.
2. Those particles free to move — and this is what changes.

In the solid, the ions are locked in fixed positions in the lattice. The charges are there, but they cannot go anywhere, so no current flows.

Melting breaks the lattice down. Dissolving separates the ions into the water. Either way the ions become free to move, and a current can flow.

The sentence that loses marks

"The solid does not conduct because it has no ions."

It has exactly the same ions as the liquid. The reason it does not conduct is that they cannot move. Say that, and the mark is safe.

What moves in each case

Worth keeping straight, because it comes back in electrolysis:

A metal conducts as a solid, because its electrons move even while the ions stay put. An ionic compound cannot manage that, because the only charged things it has are the ions themselves.