Formulae of Ions [KEYWORDS]
ION: An atom or group of atoms in which the total number of elctrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. When an atom loses or gains an electron it becomes an ion.
CATION: A positively charged ion. Most cations are metal ions, e.g. Na+.
ANION: A negatively charged ion. Anions usually contain a full outer orbital of electrons. Non-metals form anions but complex ions can also be anions.
+ The Common Cations +
1+ 2+ 3+
H+ Hydrogen Mg2+ Magnesium Al3+ Aluminium
Li+ Lithium Fe2+ Iron (ll) Fe3+ Iron (lll)
Na+ Sodium Ba2+ Barium
K+ Potassium Cu2+ Copper (ll)
Cu+ Copper (l) Ca2+ Calcium
Ag+ Silver Pb2+ Lead
NH4+ Ammonium Zn2+ Zinc
- The Common Anions -
OH- Hydroxide O2- Oxide
HCO3- Hydrogencarbonate CO32- Carbonate
Cl- Chlorine SO42- Sulphate
Br‑ Bromine S2- Sulphide
[Notable: HNO3- Nitric Acid]
Working out the Formula
Working out the formula for common compounds is easy, It's all about balancing!
If we take a compound such as Iron(ll) nitrate;
- We know Iron (ll) ion has a charge of 2+ and nitrate ion has a charge of 1-.
- So to balance this up with have to include an additional nitrate ion to match Iron(ll)'s charge of 2.
- This means the compound is written as Fe (NO3)2 the reason we include brackets is that we need 2 nitrates and if we wrote the compound without brackets it would look like 32 and people might get mixed up.(NB: The numbers should be written in subscript)
Lets do another, Aluminium hydrogencarbonate;
- Aluminium has a charge of 3 + and hydrogencarbonate has a charge of 1-.
- So we need 3 hydrogencarbonate ions to balance this up.
- The compound is written as Al (HCO3)3
Can you do these? barium hydroxide, lithium sulphate, calcium nitrate