C3.05 The chemical formulae of elements and compounds

To sum up the basic structure of an element or compuond it is faster to use a chemical formula.

Ionic compounds are solid at room temperature and their formulae are simply the whole number ratio of the positive to the negative ions in the structure.

The size of the charge of an ion is a measure of its valency or combining power.

PASSAGES FOR WRITING FORMULA

• Write down the correct symbol

• Write the charge of the ions

• "Cross-over"the charges

All of this can be applied also to write formulae of compounds containing ionic groups because each of them has an overall charge.

This idea of an atom having a valency, or combining power, can also be applied to work out the formula of covalent compounds.

• Here the valency of an atom is the number of covalent bonds it cat form.

• The "crossing-over" can be applied also to covalent compounds in two different situations

  • simple molecules with a central atom (ex. water, carbon dioxide...)

  • giant covalent molecules, where the formulae is simply the whole-number ratio of the atoms present in the giant lattice (ex. silica)

For elements in Groups I-IV ---> Valency= group number

For elements in Group V-VII ---> Valency= 8- the group number

General rules for naming the compounds

• if there is a metal in the compound, it is named first

• Where the metal can form more than one ion, then the name indicates which ion is present (ex. iron (II) chloride contains the Fe2+)

• Compounds that contain only two elements have the name ending in - ide (ex. sodium chloride (NaCl))

• Compounds containing an ionic group have names ending in -ate (ex. calcium carbonate (CaCO3))

• The names of some compounds use prefixes to tell you the number of the particular atom in the molecule (ex. carbon monoxide (CO))