Formation of positive and negative ions

An ion is an atom or a group of atoms with an electrical charge because it has gained or lost electrons. Protons have a positive charge and electrons have a negative charge. In a neutral atom or group of atoms, these charges balance.

Losing electrons: positive ions

When a neutral atom or group loses electrons, it has more protons than electrons. The remaining positive charge is greater than the negative charge, so a positive ion, or cation, forms.

For example, a neutral sodium atom has 11 protons and 11 electrons. If it loses one electron, it has 11 protons but only 10 electrons. It becomes a sodium ion, $$\ce{Na+}$$, with a charge of $$+1$$.

Gaining electrons: negative ions

When a neutral atom or group gains electrons, it has more electrons than protons. The negative charge is now greater than the positive charge, so a negative ion, or anion, forms.

For example, a neutral oxygen atom has 8 protons and 8 electrons. If it gains two electrons, it has 8 protons and 10 electrons. It becomes an oxide ion, $$\ce{O^2-}$$, with a charge of $$-2$$.

For an initially neutral atom or group, the number of electrons lost or gained determines the size of the charge: losing two electrons gives a charge of $$+2$$, while gaining two gives a charge of $$-2$$.

Ionic bonding between Group I and Group VII elements

A Group I atom has one electron in its outer shell. A Group VII atom has seven electrons in its outer shell. When an electron is transferred from the Group I atom to the Group VII atom, both form ions with full outer electron shells.

The Group I atom loses one negatively charged electron, so it becomes an ion with a $$+1$$ charge. The Group VII atom gains that electron, so it becomes an ion with a $$-1$$ charge.

The oppositely charged ions attract each other. This strong electrostatic attraction is the ionic bond. The electron transfer forms the ions; it is the attraction between the ions, not the transfer itself, that is the bond.

Dot-and-cross diagram

A dot-and-cross diagram uses different symbols to show which atom each electron originally came from. For example, sodium is in Group I and chlorine is in Group VII. The diagram below uses a cross for sodium’s outer electron and dots for chlorine’s outer electrons.

The cross moves from sodium to chlorine. The resulting sodium ion has a $$+1$$ charge, and the chloride ion has a $$-1$$ charge. The brackets and charges show that both particles are ions.

The diagram shows electrons from the atoms’ original outer shells. Sodium’s new outer shell is full, but its electrons are not shown here. The chloride ion’s outer shell contains eight electrons: seven dots and the transferred cross.

Melting, boiling and electrical conductivity of ionic compounds

An ionic compound contains positive and negative ions arranged in a regular structure called an ionic lattice. Strong electrostatic attractions between oppositely charged ions hold the lattice together.

Melting and boiling points

To melt or boil an ionic compound, energy must be supplied to overcome the attractions between its ions. These attractions are strong, so a large amount of energy is needed. Ionic compounds therefore generally have high melting and boiling points.

Electrical conductivity

Electrical conduction requires charged particles that can move and carry charge through a substance. Ionic compounds contain charged ions, but whether they conduct depends on whether those ions are free to move.

• Solid: The ions are fixed in position in the lattice. They cannot carry charge through the solid, so it conducts poorly.
• Molten: The lattice has broken down and the ions can move. They carry charge through the liquid, so it conducts electricity.
• Dissolved in water: The ions separate and can move through the solution. They carry charge through the solution, so it conducts electricity.