IB DP Chemistry: SL复习笔记4.1.3 Formulae & Names of Ionic Compounds

Formulae & Names of Ionic Compounds

  • Ionic compounds are formed from a metal and a nonmetal bonded together
  • Ionic compounds are electrically neutral; the positive charges equal the negative charges

Charges on positive ions

  • All metals form positive ions
    • There are some non-metal positive ions such as ammonium, NH4+, and hydrogen, H+
  • The metals in Group 1, Group 2 and Group 13 have a charge of 1+ and 2+ and 3+ respectively
  • The charge on the ions of the transition elements can vary which is why Roman numerals are often used to indicate their charge
  • This is known as Stock notation after the German chemist Alfred Stock
  • Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions
    • Eg. in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper (I) nitrate, the copper has a charge of 1+

Non-metal ions

  • The non-metals in group 15 to 17 have a negative charge and have the suffix ‘ide
    • Eg. nitride, chloride, bromide, iodide
  • Elements in group 17 gain 1 electron so have a 1- charge, eg. Br-
  • Elements in group 16 gain 2 electrons so have a 2- charge, eg. O2-
  • Elements in group 15 gain 3 electrons so have a 3- charge, eg. N3-
  • There are also more polyatomic or compound negative ions, which are negative ions made up of more than one type of atom

1.2-Atoms-Molecules-Stoichiometry-Ionic-ChargesThe charges of simple ions depend on their position in the Periodic Table

  • There are seven polyatomic ions you need to know for IB Chemistry:

Formulae of Polyatomic Ions Table


Worked Example

Determine the formulae of the following ionic compounds

  1. magnesium chloride
  2. aluminium oxide
  3. ammonium sulfate


Answer 1: Magnesium chloride

    • Magnesium is in group 2 so has a charge of 2+
    • Chlorine is in group 17 so has a charge of 1-
    • Magnesium needs two chlorine atoms for each magnesium atom to be balanced so the formula is MgCl2

Answer 2: Aluminium oxide

    • Aluminum is in group 13 so the ion has a charge of 3+
    • Oxygen is in group 16 so has a charge of 2-
    • The charges need to be equal so 2 aluminium to 3 oxygen atoms will balance electrically, so the formula is Al2O3

Answer 3: Ammonium sulfate

    • Ammonium is a polyatomic ion with a charge of 1+
    • Sulfate is a polyatomic ion and has a charge of 2-
    • The polyatomic ion needs to be placed in a bracket if more than 1 is needed
    • The formula of ammonium nitrate is (NH4)2SO4

Exam Tip

Remember: polyatomic ions are ions that contain more than one type of element, such as OH-