Structure and Bonding - Ionic

Ionic bonding occurs when electrons are transferred from a metal to a non-metal, forming positive and negative ions.

• When the metal atom loses electrons, a cation is formed. Cations are positively charged ions.

• When the non-metal atom gains electrons, an anion is formed. Anions are negatively charged ions.

• Strong electrostatic forces of attraction hold together these oppositely charged ions.

• Hence, by definition, ionic bonding is the electrostatic interaction between oppositely charged ions.

Predicting the charge on a simple ion 

You can predict the charge on a simple ion by using its group number in the Periodic Table. The group number gives the number of electrons on an atom’s outermost shell. In order to achieve a more stable full outer shell, atoms will lose or gain electrons.

Once the atoms become ions, their electronic configurations are the same as a noble gas, which is the most stable configuration.

Group Number	e' lost/gained?	Charge of the ion	Example
1	Lose 1	1+
2	Lose 2	2+
3	Lose 3	3+
5	Gain 3	3-
6	Gain 2	2-
7	Gain 1	1-

• Group 4 elements can form 2+,4+, or 4- ions.

Transition metal ions

Transition metals form ions with variable oxidation states. Rather than predicting the charges using the periodic table, the oxidation state is given in Roman numerals in parentheses e.g. iron

Compound ions

A compound (or polyatomic ion) consists of two or more atoms chemically bonded together containing a net charge.

Oppositely charged ions combine in a ratio that leads to a compound with no overall charge.

A giant ionic lattice is a repeating three-dimensional structure of ions held by ionic bonds.

Compounds that are bonded ionically contain ions that are arranged in a regular 3D lattice. Each positive ion is surrounded by negative ions and vice versa.

The ions are held together by strong ionic bonds in all directions.

Physical properties of ionic compounds

Ionic Compounds have high melting and boiling points:

• Lots of energy is required to overcome the many strong electrostatic forces of attraction between the oppositely charged ions in the lattice.

• The greater the charge and the smaller the size of the ion the stronger the electrostatic forces of attraction are and the higher the melting point.

Electrical conductivity:

• Ionic compounds do not conduct electricity in the solid state but do in the molten state.

• This is because in the solid state, the ions are in a fixed lattice whereas in the molten/ dissolved state, the ions are free to move and can carry charge.

Solubility in water

• Many ionic compounds dissolve in water

• Water is a polar solvent which stabilises the separated ions

• The solubility of an ionic compound depends on the strength of the ionic bond, the stronger the ionic bond the greater the electrostatic force between the positively and negatively charged ions and more energy is required to break these bonds apart. These compounds are less soluble as it takes a significant amount of energy to break these ions apart and dissolve the compound.