Binary Ionic Compounds | DP IB Chemistry Revision Notes 2023 (2025)

Binary Ionic Compounds

What is a binary ionic compound?

  • A binary ionic compound iscomposed of ions of two different elements

    • They consist of a metal cation and a non-metal anion

  • For example, sodium and chlorine react together to form the binary ionic compound, sodium chloride

Sodium and chlorine atoms react to form sodium chloride

Binary Ionic Compounds | DP IB Chemistry Revision Notes 2023 (1)

Cations and anions bond together using strong electrostatic forces, which require a lot of energy to overcome

What is ionic bonding?

  • One definition of ionic bonding is:

'the force of attraction between oppositely charged species / ions'

  • Cations and anions are oppositely charged and therefore attracted to each other

  • Electrostatic attractions are formed between the oppositely charged ions to form ionic compounds

  • This form of attraction is very strong and requires a lot of energy to overcome

    • This causes high melting points in ionic compounds

Naming Ionic Compounds

Nomenclature of binary ionic compounds

  • Binary ionic compounds are named with the cation first, followed by the anion

    • The anion adopts the suffix “ide”

  • For example, when sodium reacts with iodine:

    • The name of the binary ionic compound starts with the metal, sodium

    • The name of the binary ionic compound ends with the nonmetal, including the "ide" suffix

      • Iodine becomes iodide

    • So, the binary ionic compound formed when sodium reacts with iodine is sodium iodide

Worked Example

Give the IUPAC names of the binary ionic compounds formed in the following reactions:

  1. Lithium + sulfur

  2. Calcium + nitrogen

  3. Sodium + hydrogen

Answer 1:

  • The metal is lithium

  • The nonmetal is sulfur, which becomes sulfide when it is bonded to a metal

  • Therefore, the name of the binary ionic compound is lithium sulfide

Answer 2:

  • The metal is calcium

  • The nonmetal is nitrogen, which becomes nitride when it is bonded to a metal

  • Therefore, the name of the binary ionic compound is calcium nitride

Answer 3:

  • The metal is sodium

  • The nonmetal is hydrogen, which becomes hydride when it is bonded to a metal

  • Therefore, the name of the binary ionic compound is sodium hydride

  • The following is a list of binary ionic compounds, because they contain a metal cation and a nonmetal anion:

    • Lithium fluoride

    • Sodium chloride

    • Potassium bromide

    • Magnesium sulfide

    • Calcium oxide

What is the charge of an ionic compound?

  • Ionic compounds are formed from a metal and a non-metal bonded together

  • Ionic compounds are electrically neutral; the positive charges equal the negative charges

  • This means that the overall charge of an ionic compound is 0

    • They are neutral

Charges on positive metal 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+

Charges on negative nonmetal ions

  • The non-metals in groups 15 to 17 have a negative charge and 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–

Common charges of elements on the Periodic Table

Binary Ionic Compounds | DP IB Chemistry Revision Notes 2023 (2)

The charges of simple ions depend on their position in the Periodic Table

What are polyatomic ions?

  • Polyatomic ions are sometimes called compound negative ions

  • They are ions that are made up of more than one type of atom

    • There are generally negative ions, although there are some positive ones such as the ammonium ion

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

Formulae of Polyatomic Ions Table

Ion

Formula and charge

Ammonium

NH4+

Hydroxide

OH

Nitrate

NO3

Hydrogencarbonate

HCO3

Carbonate

CO32–

Sulfate

SO42–

Phosphate

PO43–

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-

  • Each magnesium atom needs two chlorine atoms to balance the charges

  • So, the formula is MgCl2

Answer 2: Aluminium oxide

  • Aluminium 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, which means that 2 aluminium atoms require 3 oxygen atoms to balance electronically

  • 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-

  • To balance the charges, 2 ammonium ions are needed for each sulfate ion

  • Careful: The polyatomic ion needs to be placed in a bracket if more than 1 is needed

  • So, the formula of ammonium sulfate is (NH4)2SO4

Examiner Tips and Tricks

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

If more than one polyatomic ion is needed in a chemical formula, then it is placed inside a bracket with the number of them outside the bracket, e.g. Ca(NO3)2.

Binary Ionic Compounds | DP IB Chemistry Revision Notes 2023 (2025)
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