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Writing Ionic Formulae

Key Concepts

  • Positively charged ions are called cations

  • Negatively charged ions are called anions

  • The formula of an ionic compound represents the simplest whole number ratio of ions present.

  • The net charge on an ionic compound is zero, so the sum of the positive charges equals the sum of the negative charges.

  • A subscript number written to the right of an element's symbol tells us how many of those ions are present in the formula.
    for example: potassium sulfide has the formula K2S, 2 potassium ions (K+) and 1 sulfide ion (S2-) are present.
    If no number is given, then only one of that ion is present.
    for example: sodium hydride has the formula NaH, 1 sodium ion (Na+) and 1 hydride ion (H+) are present.

  • If more than one polyatomic ion (ion having more that one element in its formula) is present, round brackets enclose the formula of the polyatomic ion and a subscript number written to the right of the final bracket tells us how many of that polyatomic ion are present in the formula.
    for example, the nitrate ion is a polyatomic ion, NO3-, it is an ion containing nitrogen and oxygen atoms. Calcium nitrate has the formula Ca(NO3)2 and it contains 1 calcium ion (Ca2+) and 2 nitrate ions (NO3-).
Type of Ion Charge Examples
CATIONS +1 Group 1 (IA) ions (Li+, Na+, K+, Rb+, Cs+)
Ag+, H+, NH4+[amminium], Cu+[copper (I)], Hg22+[mercury(I)]

+2 Group 2 (IIA) ions (Be2+, Mg2+, Ca2+, Sr2+, Ba2+)
Zn2+, Cu2+[copper(II)], Fe2+[iron(II)], Pb2+[lead(II)], Sn2+[tin(II)], Hg2+[mercury(II)]

+3 Al3+, Fe3+[iron(III)]

+4 Pb4+[lead(IV)], Sn4+[tin(IV)]
 
ANIONS -1 Group 17 (VIIA) ions (F-[fluoride], Cl-[chloride], Br-[bromide], I-[iodide])
H-[hydride], OH-[hydroxide], NO3-[nitrate], NO2-[nitrite]

-2 Group 16 (VIA) ions (O2-[oxide], S2-[sulfide])
SO42-[sulfate], SO32-[sulfite], CO32-[carbonate]

-3 N3-[nitride], P3-[phosphide], PO43-[phosphate]

Examples

Charge on Cation EQUALS Charge on Anion

magnesium oxide
  • CATION: magnesium (Group 2) charge is +2: Mg2+
  • ANION: oxide ion (Group 16) charge is -2: O2-
  • 1 Mg2+ cation and 1 O2- anion combine to give a compound with zero net charge,
        +2 + -2 = 0
  • Formula of magnesium oxide is MgO
ammonium hydroxide
  • CATION: ammonium ion, charge is +1: NH4+
  • ANION: hydroxide ion, charge is -1: OH-
  • 1 NH4+ cation and 1 OH- anion combine to give a compound with zero net charge,
        +1 + -1 =0
  • Formula of ammonium hydroxide is NH4OH

Charge on Cation DOES NOT EQUAL Charge on Anion

aluminium chloride
  • CATION: aluminium (Group 13), charge is +3: Al3+
  • ANION: chloride (Group 17), charge is -1: Cl-
  • 1 Al3+ cation and 3 Cl- anions combine to give a compound with zero net charge:
        +3 + (3 x -1) = 0
  • Formula of aluminium chloride is AlCl3
sodium oxide
  • CATION: sodium (Group 1), charge is +1: Na+
  • ANION: oxide (Group 16), charge is -2: O2-
  • 2 Na+ cations and 1 O2- anion combine to give a compound with zero net charge:
        (2 x -1) + -2 = 0
  • Formula of sodium oxide is Na2O
ammonium sulfate
  • CATION: ammonium, charge is +1: NH4+
  • ANION: sulfate, charge is -2: SO42-
  • 2 NH4+ cations and 1 SO42- anion combine to give a compound with zero net charge:
        (2 x -1) + -2 = 0
  • Formula of ammonium sufate is (NH4)2SO4
barium hydroxide
  • CATION: barium (Group 2), charge is +2: Ba2+
  • ANION: hydroxide, charge is -1: OH-
  • 1 Ba2+ cation and 2 OH- anions combine to give a compound with zero net charge:
        +2 + (2 x -1) = 0
  • Formula of barium hydroxide is Ba(OH)2


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