5.4 Writing formulae

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Writing chemical formulae

The formulae of covalent compounds

To work out the formulae of covalent compounds, we need to use the valency of the atoms in the compound. This is because the valency tells us how many bonds each atom can form. This in turn can help to work out how many atoms of each element are in the compound, and therefore what its formula is. The following are some examples where this information is used to write the chemical formula of a compound.

Write the chemical formula for water

Write down the elements that make up the compound.
A molecule of water contains the elements hydrogen and oxygen .
Determine the valency of each element
The valency of hydrogen is 1 and the valency of oxygen is 2. This means that oxygen can form two bonds with other elements and each of the hydrogen atoms can form one.
Write the chemical formula
Using the valencies of hydrogen and oxygen, we know that in a single water molecule, two hydrogen atoms will combine with one oxygen atom. The chemical formula for water is therefore:

$H_{2} O$ .

Write the chemical formula for magnesium oxide

Write down the elements that make up the compound.
A molecule of magnesium oxide contains the elements magnesium and oxygen .
Determine the valency of each element
The valency of magnesium is 2, while the valency of oxygen is also 2. In a molecule of magnesium oxide, one atom of magnesium will combine with one atomof oxygen.
Write the chemical formula
The chemical formula for magnesium oxide is therefore:

$MgO$

Write the chemical formula for copper (II) chloride.

Write down the elements that make up the compound.
A molecule of copper (II) chloride contains the elements copper and chlorine .
Determine the valency of each element
The valency of copper is 2, while the valency of chlorine is 1. In a molecule of copper (II) chloride, two atoms of chlorine will combine with one atom of copper.
Write the chemical formula
The chemical formula for copper (II) chloride is therefore:

$CuCl_{2}$

The formulae of ionic compounds

The overall charge of an ionic compound will always be zero and so the negative and positive charge must be the same size. We can use this information to work out what the chemical formula of an ionic compound is if we know the charge on the individual ions. In the case of $NaCl$ for example, the charge on the sodium is $+ 1$ and the charge on the chlorine is $- 1$ . The charges balance ( $+ 1 - 1 = 0$ ) and therefore the ionic compound is neutral. In $MgO$ , magnesium has a charge of $+ 2$ and oxygen has a charge of $- 2$ . Again, the charges balance and the compound is neutral. Positive ions are called cations and negative ions are called anions .

Some ions are made up of groups of atoms, and these are called compound ions . It is a good idea to learn the compound ions that are shown in [link]

Table showing common compound ions and their formulae

Name of compound ion	formula	Name of compound ion	formula
Carbonate	$CO_{3}^{2 -}$	Nitrite	${NO}_{2}^{-}$
Sulphate	${SO}_{4}^{2 -}$	Hydrogen sulphite	${HSO}_{3}^{-}$
Hydroxide	${OH}^{-}$	Hydrogen sulphate	${HSO}_{4}^{-}$
Ammonium	${NH}_{4}^{+}$	Dihydrogen phosphate	$H_{2} {PO}_{4}^{-}$
Nitrate	${NO}_{3}^{-}$	Hypochlorite	${ClO}^{-}$
Hydrogen carbonate	${HCO}_{3}^{-}$	Acetate (ethanoate)	${CH}_{3} {COO}^{-}$
Phosphate	${PO}_{4}^{3 -}$	Oxalate	$C_{2} O_{4}^{2 -}$
Chlorate	${ClO}_{3}^{-}$	Oxide	$O^{2 -}$
Cyanide	${CN}^{-}$	Peroxide	$O_{2}^{2 -}$
Chromate	${CrO}_{4}^{2 -}$	Sulphide	$S^{2 -}$
Permanganate	${MnO}_{4}^{-}$	Sulphite	${SO}_{3}^{2 -}$
Thiosulphate	$S_{2} O_{3}^{2 -}$	Manganate	${MnO}_{4}^{2 -}$
Phosphide	$P^{3 -}$	Hydrogen phosphate	${HPO}_{4}^{3 -}$

Questions & Answers

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Every time someone flushes a toilet in the apartment building, the person begins to jumb back automatically after hearing the flush, before the water temperature changes. Identify the types of learning, if it is classical conditioning identify the NS, UCS, CS and CR. If it is operant conditioning, identify the type of consequence positive reinforcement, negative reinforcement or punishment

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Source: OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7

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