Names of compounds:

For compounds consisting of 2 atoms:

• If one is a metal and the other a nonmetal, then the name of the metal atom comes first and the ending of the second atom is replaced by adding -ide

  • Eg. NaCl which contains sodium and chlorine thus becomes sodium chloride

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• If both atoms are nonmetals and one of those is hydrogen, then hydrogen comes first

  • Eg. hydrogen and chlorine combined is called hydrogen chloride

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For other combinations of nonmetals as a general rule, the element that has a lower Group number comes first in the name

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• Eg. carbon and oxygen combine to form CO₂ which is carbon dioxide since carbon is in Group 4 and oxygen in Group 6

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 For compounds that contain certain groups of atoms:

​There are common groups of atoms which occur regularly in chemistry:

• Examples include the carbonate ion(CO₃^2-), sulfate ion (SO₄^2-), hydroxide ion (OH^-) and the nitrate ion (NO₃^-)

• When these ions form a compound with a metal atom, the name of the metal comes first

• Eg. KOH is potassium hydroxide, CaCO₃ is calcium carbonate

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• The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)

• The empirical formula tells you the simplest whole-number ratio of atoms in a compound

• The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H₂ has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom

Chemical formulae

​Valency

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• The concept of valency is used to wok out the formulae of compounds

• Valency or combing power tells you how many bonds an atom can make with another atom

• Eg. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds

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• We can use the combining power of each atom to work out a formula

• Example: what is the formula of aluminium sulfide?

  • Write out the symbols of each element and write their combining powers underneath:

  • Al         S

    3          2

• The formula is then calculated by cross multiplying each atom with the number opposite, hence the formula for aluminium sulfide is Al₂S₃

​Deducing Formulae of Ionic Compounds:

The formulae of these compounds can be calculated if you know the charge on the ions.

Balance the charges by multiplying them out:

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Al³⁺ x 2 = +6

SO₄^2- x 3 = -6

so,

+6 - 6 = 0

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• So the formula is Al₂(SO₄)₃

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• For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0

• Example: what is the formula of aluminium sulphate?

  • Write out the formulae of each ion, including their charges

  • Al³⁺ SO₄^2-