GCSE Combined Science (AQA)

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Ionic Formulae

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Bonding, structure, and the properties of matter Chemical bonds, ionic, covalent and metallic

Ionic Formulae

4:01 Ionic Compounds and Formulae
Spec 4.1.1.1
Previous Properties of Ionic Compounds Next Advanced Ionic Formulae
  • Ionic compounds can be represented by chemical formulae using the symbols of the atoms from which they were formed.
  • Sodium and chlorine form an ionic compound known as sodium chloride, represented as NaCl.
  • Ionic compounds are neutral, meaning they have an overall charge of zero.
  • The charges of the ions in the compound must equal zero for the compound to be neutral.
  • Sodium ion has a one plus charge, and chloride ion has a one minus charge, balancing each other in NaCl.
  • To determine the formula of an ionic compound, know the charges of the ions involved.
  • Calcium chloride consists of calcium and chloride ions; calcium ion has a charge of two plus, and chloride ion has a charge of one minus.
  • To balance charges in calcium chloride, two chloride ions are needed for each calcium ion, resulting in the formula CaCl2.
  • Sodium oxide consists of sodium and oxygen ions; sodium ion has a charge of one plus, and oxide ion has a charge of two minus.
  • To balance charges in sodium oxide, two sodium ions are needed for each oxide ion, resulting in the formula Na2O.
  • The swap and drop method is a shorthand way to determine the formula of ionic compounds.
  • Example: Potassium sulfide is formed by potassium (K) and sulfur (S).
  • Potassium is in group one with a charge of one plus, and sulfur is in group six with a charge of two minus.
  • Swap the charges to determine the formula, resulting in K2S for potassium sulfide.

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