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Formulae for 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:

Hโ‚‚O is water,
NHโ‚ƒ - ammonia,
CHโ‚„ - methane,
COโ‚‚ - carbon dioxide,
SiOโ‚‚ - 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; โ€œideโ€ for single-atom ions like sulphide, โ€œateโ€/โ€œiteโ€ 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.

Predicting formulae

1

What is the molecular formula of a covalent compound that boron and fluorine would form?

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.

Boron has 3 unpaired electrons in its outer shell; thus, its valency is 3. Fluorine has 1 unpaired electron in its outer shell; therefore, its valency is 1. Cross-multiplying the valencies of boron and fluorine gives BFโ‚ƒ. Since the subscripts have a common factor of 1, they cannot be simplified.

BFโ‚ƒ

What is the molecular formula of a covalent compound that boron and fluorine would form?

2

What is the name of LiF?

The name of ionic compounds is derived by combining the names of the two ions, using a suitable suffix for the anion; โ€œideโ€ for single-atom ions like fluoride.

lithium fluoride

What is the name of LiF?

3

Work out the formula of aluminium sulphate. Sulphate ion has a formula SOโ‚„ยฒโป.

Aluminium is in group 3, thus its atoms each have 3 electrons in their outer shells. Since aluminium is a metal we have to add a symbol โ€œ+โ€ to the number of valence electrons to get its most probable ionic charge 3+. Sulphate (SOโ‚„ยฒโป) has a charge of 2โˆ’.

Cross-multiplying the charges of the aluminium ion and the sulphate ion gives Alโ‚‚(SOโ‚„)โ‚ƒ. Since the subscripts have a common factor of 1, they cannot be simplified.

Alโ‚‚(SOโ‚„)โ‚ƒ

Work out the formula of aluminium sulphate. Sulphate ion has a formula SOโ‚„ยฒโป.

4

Provide the molecular formula of water.

The molecular formula of water is Hโ‚‚O.

Hโ‚‚O

Provide the molecular formula of water.

5

Work out the formula of calcium oxide.

Calcium is in group 2, thus its atoms each have 2 electrons in their outer shells. Since calcium is a metal we have to add a symbol โ€œ+โ€ to the number of valence electrons to get its most probable ionic charge 2+.

Oxygen is in group 6, thus its atoms each have 6 electrons in their outer shells. Since oxygen is a nonmetal we have to subtract 8 from the number of valence electrons to get its most probable ionic charge: 6 โˆ’ 8 = 2โˆ’.

Cross-multiplying the charges of the calcium ion and the oxygen ion gives Caโ‚‚Oโ‚‚. Since the subscripts have a common factor of 2, they both can be divided to give a formula of CaO.

CaO

Work out the formula of calcium oxide.

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