1.21–1.24Ionic bonding
When a metal atom reacts with a non-metal atom, electrons in the outer shell of the metal atom are transferred. Metal atoms lose electrons to become positively charged ions. Non-metal atoms gain electrons to become negatively charged ions.
The ions produced by metals in Groups 1 and 2, and by non-metals in Groups 6 and 7, have the electronic structure of a noble gas (Group 0). That is why the transfer happens at all: it is the shortest route to a full outer shell for both atoms.
The charge on the ions produced by metals in Groups 1 and 2, and by non-metals in Groups 6 and 7, relates to the group number of the element in the periodic table.
You should be able to work out the charge on the ions of metals and non-metals from the group number, limited to metals in Groups 1 and 2 and non-metals in Groups 6 and 7.
Worked example
Draw a dot and cross diagram for magnesium oxide, and give the formula.
- Magnesium is in Group 2, so a magnesium atom has two outer electrons to lose.
- Oxygen is in Group 6, so an oxygen atom needs two electrons to fill its outer shell.
- Both electrons transfer from the one magnesium atom to the one oxygen atom, so the ions pair up one to one.
- Draw the magnesium ion with an empty outer shell and a 2+ charge, and the oxide ion with eight outer electrons — six dots of its own and two crosses from the magnesium — and a 2− charge. Put square brackets and the charge around each ion.
Answer: Mg²⁺ with an empty outer shell and O²⁻ with a full outer shell; the formula is MgO.
You should be able to draw dot and cross diagrams for ionic compounds formed by metals in Groups 1 and 2 with non-metals in Groups 6 and 7. Two conventions matter: use dots for one element's electrons and crosses for the other's, so the transfer can be seen; and put square brackets with the charge around each ion, because an ion is not an atom.
Common mistake
In an ionic dot and cross diagram the positive ion usually has nothing in its outer shell — it has lost those electrons entirely. Drawing the metal ion with its electrons still there is the commonest error, and it contradicts the charge written beside it.
- Ion
- a charged particle formed when an atom loses or gains electrons; metal atoms lose electrons to become positive ions and non-metal atoms gain electrons to become negative ions
1.25–1.26Naming compounds and working out formulae
The name of a compound tells you what is in it, once you know two endings.
A name ending in –ide means the compound contains just the two elements named. Sodium chloride is sodium and chlorine, and nothing else. Magnesium oxide is magnesium and oxygen.
A name ending in –ate means the compound contains those elements and oxygen as well. Copper sulfate contains copper, sulfur and oxygen. Calcium carbonate contains calcium, carbon and oxygen. The –ite ending works the same way but with less oxygen — a sulfite has fewer oxygen atoms than a sulfate.
Working out the formula of an ionic compound is arithmetic, not memory. The compound has no overall charge, so the positive and negative charges must cancel exactly.
Write down the two ions with their charges. Then take as many of each as you need to make the charges balance, and write the numbers as subscripts. Magnesium is 2+ and chloride is 1−, so one magnesium needs two chlorides: MgCl₂. Sodium is 1+ and oxide is 2−, so two sodiums are needed for one oxide: Na₂O.
When a compound ion such as nitrate, carbonate or sulfate is needed more than once, put brackets round it before the number: calcium nitrate is Ca(NO₃)₂, not CaNO₃₂.
Worked example
Write the formula of aluminium sulfate.
- Aluminium forms Al³⁺. Sulfate is SO₄²⁻.
- The charges are 3 and 2, so the smallest number that both divide into is 6.
- Two aluminium ions give 6+; three sulfate ions give 6−.
- The sulfate is needed three times, so it goes in brackets.
Answer: Al₂(SO₄)₃
Common mistake
The subscript belongs to the ion in front of it, not the whole formula. Writing CaNO₃₂ for calcium nitrate says there are 32 oxygen atoms; Ca(NO₃)₂ says there are two nitrate ions, which is what you meant.
1.27Ionic compounds
An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice, and this is called ionic bonding.
"In all directions" is the phrase that does the work. A sodium ion is not bonded to one particular chloride ion — it is attracted to every chloride ion around it, and the structure repeats until the crystal ends.
You should be able to:
• deduce that a compound is ionic from a diagram of its structure in one of the specified forms • describe the limitations of using dot and cross, ball and stick, two-dimensional and three-dimensional diagrams to represent a giant ionic structure • work out the empirical formula of an ionic compound from a given model or diagram that shows the ions in the structure.
You should be familiar with the structure of sodium chloride, but you do not need to know the structures of other ionic compounds.
Worked example
A diagram of a lattice shows 4 X ions and 8 Y ions in the repeating block. Work out the empirical formula.
- The empirical formula is the simplest whole-number ratio of the ions.
- The ratio in the block is 4 X to 8 Y.
- Divide both by the smaller number: 4 ÷ 4 = 1 and 8 ÷ 4 = 2.
- Write the ratio as a formula, cation first.
Answer: XY₂
- Ionic compound
- a giant structure of ions held together by strong electrostatic forces of attraction acting in all directions between oppositely charged ions
- Giant ionic lattice
- the regular structure formed by the ions in an ionic compound
Before you practise
- An ionic bond forms when electrons transfer from a metal atom to a non-metal atom.
- The metal loses electrons to form a positive ion (a cation); the non-metal gains them to form a negative ion (an anion).
- Both ions end up with a full outer shell, which is why the arrangement is stable.
- –ide means two elements only; –ate means those elements plus oxygen.
- Get a formula by balancing the charges to zero, and bracket a compound ion before multiplying it.
- An ionic compound is a giant lattice of alternating ions held by strong electrostatic forces in every direction.