Binary Ionic Compounds (SL IB Chemistry)

What is a binary ionic compound?

• A binary ionic compound is composed 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  

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

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

• 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. O^2–
• Elements in group 15 gain 3 electrons so have a 3- charge, eg. N^3– 

Common charges of elements on the Periodic Table

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