SQA National 5 Chemistry Formulae
This page covers the following topics:
1. Predicting formulae
There are many compounds that are regularly used in chemistry, so their chemical formula and name are presumed to be common knowledge. Some of them are provided in the list below:
H₂O is water,
NH₃ - ammonia,
CH₄ - methane,
CO₂ - carbon dioxide,
SiO₂ - sand.
The formulae of ionic compounds can be predicted by cross-multiplying the charges of the anion and cation and dividing them by a common factor. The name for ionic compounds is derived by combining the names of the two ions, using a suitable suffix for the anion; “ide” for single-atom ions like sulphide, “ate”/“ite” for group ions like sulphate.
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 such compound make a fixed number of covalent bonds with some of them being a part of a double or a triple bond.
What is the molecular formula of a covalent compound that boron and fluorine would form?
What is the name of LiF?
Work out the formula of aluminium sulphate. Sulphate ion has a formula SO₄²⁻.
Provide the molecular formula of water.
Work out the formula of calcium oxide.
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