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Formulae

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The molecular formula denotes all the atoms of each element present in a compound. The empirical formula represents the smallest ratio of elements in a compound, the molecular formula is divided by the greatest common factor.

There are many compounds that are regularly used in chemistry, so their chemical formula and name are presumed to be common knowledge.

An ion is an atom/molecule which has either a positive or negative charge, due to the loss or gain of electrons. The charge of an ion can be predicted from where it appears in the periodic table. Metals lose electrons and gain the same charge as their group number. Non-metals gain electrons, their charge is their group number – 8. This allows the atoms to follow the octet rule and have a full outer shell of electrons.

You can predict the formulae of ionic salts by balancing the charges of the anion and cation. The name is then derived by combining the names of the two ions, using a suitable suffix for the anion (eg. -ide, -ate). When predicting the formulae of covalent molecules, the valency of each atom must be considered. The number of bonds an atom can make depends on the number of unpaired electrons present in the outer shell. Each atom in the compound must make the correct number of covalent bonds. Remember, some atoms can form double or triple bonds.