Ionic BondingEdexcel GCSE Chemistry: Revision notes
Section 1
What is an ion and how do ionic bonds form?
An ion is an atom, or group of atoms, with a positive or negative charge, formed when electrons are lost or gained.
Ionic bonds are formed by the transfer of electrons between atoms:
- Metal atoms lose electrons to form positively charged ions, called cations
- Non-metal atoms gain those electrons to form negatively charged ions, called anions
Dot and cross diagrams show this transfer: the electron(s) lost by the metal atom are shown moving into the outer shell of the non-metal atom.
State clearly which atom loses electrons and which gains them — mark schemes reward this explicitly, not just 'electrons transfer'.
Section 2
How is ion formation limited to certain groups?
Ion formation for GCSE is limited to elements in Groups 1, 2, 6 and 7:
- Group 1 metals lose 1 electron to form 1+ ions (e.g. Na+)
- Group 2 metals lose 2 electrons to form 2+ ions (e.g. Mg2+)
- Group 6 non-metals gain 2 electrons to form 2− ions (e.g. O2−)
- Group 7 non-metals gain 1 electron to form 1− ions (e.g. Cl−)
Calculating protons, neutrons and electrons in an ion: protons and neutrons are unchanged from the atom, but the number of electrons changes according to the charge (subtract electrons for a positive ion, add electrons for a negative ion).
Mg2+ has atomic number 12, so 12 protons. It has lost 2 electrons, so it has 12 − 2 = 10 electrons.
Section 3
How are ionic compound names and formulae worked out?
The ending –ide is used when a compound contains only two elements (e.g. sodium chloride, magnesium oxide). The ending –ate is used when the compound contains oxygen as well as another element (e.g. sodium sulfate, calcium carbonate).
To deduce a formula, the total positive charge must balance the total negative charge:
- Write down the charge of each ion
- Find the lowest common multiple of the charges
- Use ratios so the charges cancel out
Common ions to know: hydroxide (OH−), nitrate (NO3−), carbonate (CO3 2−), sulfate (SO4 2−).
Magnesium (2+) and chloride (1−) combine as MgCl2, so the charges balance: +2 and 2×(−1).
Section 4
What is the structure of an ionic compound?
An ionic compound has a giant ionic lattice structure — a regular, repeating arrangement of oppositely charged ions extending in all directions. The ions are held together by strong electrostatic forces of attraction (ionic bonds) between the oppositely charged ions.
Must Know
- Ionic bonds form by the transfer of electrons: metals lose electrons to form cations, non-metals gain electrons to form anions
- An ion is an atom or group of atoms with a positive or negative charge
- Ion formation covers Groups 1, 2, 6 and 7 only
- '–ide' = two elements only; '–ate' = compound also contains oxygen
- Formulae are deduced by balancing total positive and negative charge to zero
- Ionic compounds form a giant lattice held together by strong electrostatic forces between oppositely charged ions
That's the notes covered.
Carry on to the next subtopic.