Chemical symbols are used to represent an atom or elements. E.g. The chemical symbol for hydrogen is H.
Symbols For Common Ions
| Symbol | Ion | Valency | Symbol | Ion | Valency |
|---|---|---|---|---|---|
| $\text{H}^{+}$ | Hydrogen | 1 | $\text{F}^{-}$ | Fluoride | 1 |
| $\text{NH}_{4}^{+}$ | Ammonium | 1 | $\text{Cl}^{-}$ | Chloride | 1 |
| $\text{Ag}^{+}$ | Silver | 1 | $\text{Br}^{-}$ | Bromide | 1 |
| $\text{Na}^{+}$ | Sodium | 1 | $\text{I}^{-}$ | Iodide | 1 |
| $\text{K}^{+}$ | Potassium | 1 | $\text{OH}^{-}$ | Hydroxide | 1 |
| $\text{Ba}^{2+}$ | Barium | 2 | $\text{NO}_{2}^{-}$ | Nitride | 1 |
| $\text{Ca}^{2+}$ | Calcium | 2 | $\text{NO}_{3}^{-}$ | Nitrate | 1 |
| $\text{Cu}^{2+}$ | Copper (II) | 2 | $\text{HCO}_{3}^{-}$ | Hydrogen carbonate | 1 |
| $\text{Fe}^{2+}$ | Iron (II) | 2 | $\text{CH}_{3}\text{COO}^{-}$ | Ethanoate | 1 |
| $\text{Mg}^{2+}$ | Magnesium | 2 | $\text{S}^{2-}$ | Sulphide | 2 |
| $\text{Pb}^{2+}$ | Lead (II) | 2 | $\text{O}^{2-}$ | Oxide | 2 |
| $\text{Sn}^{2+}$ | Tin (II) | 2 | $\text{SO}_{3}^{2-}$ | Sulphite | 2 |
| $\text{Zn}^{2+}$ | Zinc | 2 | $\text{SO}_{4}^{2-}$ | Sulphate | 2 |
| $\text{Ni}^{2+}$ | Nickel (II) | 2 | $\text{CO}_{3}^{2-}$ | Carbonate | 2 |
| $\text{Al}^{3+}$ | Aluminium | 3 | $\text{CrO}_{4}^{2-}$ | Chromate (VI) | 2 |
| $\text{Cr}^{3+}$ | Chromium (III) | 3 | $\text{Cr}_{2}\text{O}_{7}^{2-}$ | Dichromate (VI) | 2 |
| $\text{Fe}^{3+}$ | Iron (III) | 3 | $\text{PO}_{4}^{3-}$ | Phosphate | 3 |
The valency of an element is needed to be able to write the formulae of a compound. Recall that valency of an element is the number of valence electron the element has. In ionic compounds, the valency of an ion is equal to its charge.
Writing Formula For Ionic Compounds
Ionic compounds are made up of positive and negative ions. In an ionic compound, the total positive charges must equal the total negative charges $\rightarrow$ the total charge on the ionic compound must be zero.
Make use of the table of symbols for common ions to write out the formula for ionic compounds
Examples:
Sodium chloride
- Relevant ions are: $\text{Na}^{+}$ and $\text{Cl}^{-}$
- Simplest ratio to obtain zero total charge: 1 : 1
- Formula: $\text{NaCl}$
Calcium Nitrate
- Relevant ions are: $\text{Ca}^{2+}$ and $\text{NO}_{3}^{-}$
- Simplest ratio to obtain zero total charge: 1:2 $\rightarrow$ this means 1 of calcium ion and 2 of nitrate ions to form calcium nitrate
- Formula: $\text{Ca}\left(\text{NO}_{3}\right)_{2}$
- Note: The brackets are used to indicate that there are two $\text{NO}_{3}$ ions in the ionic compound
Sodium Sulphate
- Relevant ions are: $\text{Na}^{+}$ and $\text{SO}_{4}^{2-}$
- Simplest ratio to obtain zero total charge: 2:1
- Formula: $\text{Na}_{2}\text{SO}_{4}$
Writing Formula For Covalent Compounds
The way to write formula for covalent compounds is slightly different from that of ionic compounds. You will need to know the number of valence electrons that each of the elements have.
Examples:
Hydrogen Chloride
- Relevant elements: H and Cl
- Valence electrons: 1 for H and 7 for Cl
- 1 covalent bond is formed
- Formula: HCl
Water
- Relevant elements: H and O
- Valence electrons: 1 for H and 6 for O
- O requires 2 electrons to achieve stable noble gas configuration. H provides 1 electron.
- Hence, 2 H will be required for O to achieve stable noble gas configuration
- 2 covalent bonds are formed
- Formula: $\text{H}_{2}\text{O}$
Carbon Dioxide
- Relevant elements: C and O
- Valence electrons: 4 for C and 6 for O
- C provides 4 electrons but O only requires 2 electrons to achieve stable noble gas configuration
- Hence, 2 O will be required for C to achieve stable noble gas configuration
- 2 covalent bonds are formed
- Formula: $\text{CO}_{2}$
