Binary Ionic Compounds (DP IB Chemistry): Revision Note
Binary ionic compounds
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
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:
Lithium + sulfur
Calcium + nitrogen
Sodium + hydrogen
Answer 1:
Metal: lithium
Nonmetal: sulfur becomes sulfide when bonded to a metal
Name: lithium sulfide
Answer 2:
Metal: calcium
Nonmetal: nitrogen becomes nitride
Name: calcium nitride
Answer 3:
Metal: sodium
Nonmetal: hydrogen becomes hydride
Name: sodium hydride
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
Some non-metals also form positive ions such as ammonium, NH4+, and hydrogen, H+
Group 1, 2, and 13 metals form ions with charges of +1, +2, and +3, respectively
Transition elements can form ions with variable charges
Their charge is shown using Roman numerals in names
This is called Stock notation, named after Alfred Stock
Roman numerals indicate the oxidation state of a transition metal in a compound
In copper(II) oxide, Cu has a 2+ charge
In copper(I) nitrate, Cu has a 1+ charge
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
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:
Magnesium chloride
Aluminium oxide
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
Formula: 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
Formula: 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
Formula: (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.
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