Ionic bondingIB MYP Chemistry: Revision notes

Atoms are neutral because they have equal numbers of protons (positive) and electrons (negative). Most atoms are more stable with a full outer shell of electrons, like a noble gas. They can reach this by losing or gaining electrons.

An atom that loses or gains electrons becomes an ion, a charged particle:

• Losing electrons makes a positive ion (a cation), because there are then more protons than electrons.
• Gaining electrons makes a negative ion (an anion), because there are then more electrons than protons.

Metals (on the left of the periodic table) tend to lose electrons. Non-metals (on the right) tend to gain them.

Ionic bonding happens when a metal reacts with a non-metal. Electrons are transferred from the metal atoms to the non-metal atoms.

• Sodium (2,8,1) loses its one outer electron to become Na⁺ (2,8).
• Chlorine (2,8,7) gains one electron to become Cl⁻ (2,8,8).
• Magnesium (2,8,2) loses two electrons to become Mg²⁺, and oxygen (2,6) gains two to become O²⁻.

Both ions now have the full outer shell of a noble gas. The number of electrons lost by the metal must equal the number gained by the non-metal.

A dot-and-cross diagram shows only the outer-shell electrons. The metal's electrons are drawn as dots and the non-metal's as crosses, so you can see where each electron came from.

To draw one for an ionic compound:

1. Draw the outer shell of each atom, showing the dots and crosses.
2. Show the electrons moving from the metal to the non-metal.
3. Draw each ion in square brackets with its charge at the top right.
4. Show the metal ion with no outer electrons (its next shell is full) and the non-metal ion with eight.

Worked example: magnesium chloride. Magnesium has 2 outer electrons and each chlorine has 7. Magnesium gives one electron to each of two chlorine atoms, so the diagram has one [Mg]²⁺ and two [Cl]⁻ ions, each Cl⁻ with eight outer electrons. The formula is MgCl₂ because 2+ is balanced by 2 × 1−.

An ionic compound has no overall charge, so the total positive charge equals the total negative charge.

• Sodium chloride: Na⁺ and Cl⁻, one of each, so NaCl.
• Magnesium oxide: Mg²⁺ and O²⁻, one of each, so MgO.
• Calcium chloride: Ca²⁺ and Cl⁻, so two Cl⁻ for each Ca²⁺, giving CaCl₂.
• Sodium oxide: Na⁺ and O²⁻, so two Na⁺ for each O²⁻, giving Na₂O.

Oppositely charged ions attract each other. This strong electrostatic attraction acts in all directions and is the ionic bond.

Because attraction acts in all directions, an ionic compound does not form separate molecules. Instead, huge numbers of ions pack into a giant ionic lattice: a regular, repeating 3D arrangement in which positive and negative ions alternate. Each ion is held by strong attractions to its oppositely charged neighbours.

The regular lattice explains why ionic crystals have regular shapes, such as the cubes of sodium chloride, and why they break along flat faces.