CHEMICAL FORMULAS

Write the correct formula.

Use ion charges, valencies or prefixes. For ionic compounds, use the smallest whole-number ratio.

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Question 1

Ionic compound

Sodium chloride

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Type ordinary numbers as subscripts, for example Ca(NO3)2. Capital letters matter.

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Formula guide

Main-group ion charges

1+1 2+2 13+3 15−3 16−2 17−1

Transition-metal charges are given by Roman numerals in the compound name.

Common polyatomic ions

AmmoniumNH4+HydroxideOH−
NitrateNO3−CarbonateCO32−
SulfateSO42−PhosphatePO43−

Use brackets when more than one copy of a polyatomic ion is needed: Ca(NO3)2.

Covalent prefixes

mono 1di 2tri 3 tetra 4penta 5hexa 6 hepta 7octa 8nona 9 deca 10

Do not reduce subscripts in a molecular formula when prefixes specify the numbers.