Edexcel IGCSE Chemistry · Spec 1.37-1.43
Ionic Bonding
How ionic bonds form, ionic structures, and the properties of ionic compounds.
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Ionic Bonding
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Explained
Ionic bonding, and why the word transfer does all the work
Ionic bonding happens between a metal and a non metal. Electrons are transferred from the metal atom to the non metal atom, and the oppositely charged ions that result attract each other.
Transferred, not shared. That single word separates this whole topic from covalent bonding, and a mark scheme will not let it pass. If you write shared anywhere in an ionic bonding answer, you have described the other kind of bond.
Why the electrons move
A metal atom has few electrons in its outer shell, so the quickest route to a full outer shell is to lose them. Sodium has one, loses it, and becomes Na plus. Magnesium has two, loses both, and becomes Mg two plus.
A non metal atom has an outer shell that is nearly full, so the quickest route is to gain. Chlorine has seven, gains one, and becomes Cl minus. Oxygen has six, gains two, and becomes O two minus.
The charge on the ion is therefore predictable from the group number. Group 1 gives 1 plus, group 2 gives 2 plus, group 6 gives 2 minus, group 7 gives 1 minus. Learn the pattern rather than the individual ions.
The numbers have to balance, because the compound overall is neutral. One magnesium ion carrying 2 plus needs two chloride ions carrying 1 minus each, giving MgCl2. Two sodium ions are needed for one oxide ion, giving Na2O.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks candidates to describe what happens to the electrons when sodium oxide forms, for two marks. The first is for saying that sodium loses electrons and oxygen gains electrons. The second is for the numbers: two atoms of sodium each lose one electron, and oxygen gains two.
Two notes sit beside it. Correctly labelled diagrams are accepted, so a dot and cross drawing can earn the marks without a sentence. And then: mention of sharing electrons scores zero.
Not zero for that clause. Zero for the question. An answer that correctly says sodium loses electrons and then adds that they are shared with oxygen has thrown away a mark it had already earned.
Notice also what the second mark is really testing. It is not asking you to recite a formula; it is asking you to account for every electron. Two sodium atoms give up one each, oxygen takes both, and nothing is left over. That accounting is why the formula is Na2O and not NaO.
The structure
An ionic compound is not made of molecules. The ions arrange themselves into a giant ionic lattice, a regular three dimensional arrangement in which every positive ion is surrounded by negative ions and every negative ion by positive ones.
The bonding is described as strong electrostatic attraction between oppositely charged ions, and that phrase is worth learning word for word. Electrostatic says what kind of attraction it is. Oppositely charged says between what. Strong is what the properties depend on.
The attraction acts in every direction throughout the lattice, so no ion belongs to any one partner. The formula NaCl gives the ratio of ions in the lattice rather than the contents of a separate particle, and the correct name for it is a formula unit.
Explaining the properties
Every property here comes from one of two facts: the bonds are strong, or the ions cannot move.
High melting and boiling points, because a great deal of energy is needed to overcome the strong electrostatic attraction between the ions throughout the lattice.
Solid ionic compounds do not conduct electricity, because the ions are held in fixed positions and cannot move to carry charge. The charges exist; they are simply stuck.
Molten or dissolved, they do conduct, because melting or dissolving breaks the lattice apart and the ions become free to move.
Brittle, because a blow can shift one layer of the lattice so that ions of the same charge line up, and they then repel and split the crystal apart.
Two questions get asked more than any other, and both are conduction. Notice they have the same answer given from two sides: it is always about whether the ions are free to move, never about electrons.
Comparing two ionic compounds
Where a question asks why one ionic compound melts at a higher temperature than another, the answer is the size of the charges.
Magnesium oxide is built from Mg two plus and O two minus, while sodium chloride is built from Na plus and Cl minus. Higher charges attract more strongly, so more energy is needed to separate them, so magnesium oxide melts at a far higher temperature.
Spec 1.37-1.43
What you need to know
- Explain how ions form by transferring electrons
- Describe the giant ionic lattice
- Explain the properties of ionic compounds
Active recall
Quick check
Answer each question before opening the answer.
What is ionic bonding?
Electrons are transferred from a metal to a non-metal, forming oppositely charged ions held together by strong electrostatic attraction.
Why do ionic compounds only conduct electricity when molten or dissolved?
The ions are then free to move and carry charge; in the solid they are held in fixed positions.
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