Forming ions

An electrically charged atom is called an ion. Ions are formed when atoms lose or gain negatively charged electrons.

The number of electrons lost or gained determines the charge of the ion:

  • the number lost is the same as the number of positive charges
  • the number gained is the same as the number of negative charges

The number of protons and neutrons in an ion is the same as in the atom, and is worked out in the same way.

Forming positive ions

Positive ions are called cations.

Sodium is in group 1. A sodium atom has one electron in its outer shell. The atom is more stable if it has a full outer shell.

A sodium atom can lose its outer electron. It will still have 11 positive protons but only 10 negative electrons. So, the overall charge is +1.

A positive sign is added to the symbol for sodium, Na+. Ions with greater charge include a number in their symbol, for example Al3+ (which has three positive charges).

How a sodium atom becomes a sodium ion when it loses an electronA sodium atom loses one electron to form a sodium ion
Question

Magnesium is in group 2.

  • How many electrons are in the outer shell of a magnesium atom?
  • What is the charge of a magnesium ion and what is its symbol?
  • a magnesium atom has 2 electrons in its outer shell
  • a magnesium atom can lose both outer electrons, so a magnesium ion has a charge of +2 and its symbol is Mg2+
Question

The atomic number of lithium is 3. Its mass number is 7. How many protons, neutrons and electrons are there in a lithium ion?

  • The number of protons = atomic number. So the number of protons is 3.
  • The number of neutrons = mass number - atomic number. So the number of neutrons is 7 - 3 = 4.
  • Lithium has 1 electron in its outer shell because it is in group 1. Or you can work out its electron arrangement, which is 2.1 (number of electrons = atomic number). This means that it forms an ion by losing one electron. An Li+ ion has 2 electrons.

Explaining reactivity in group 1

Going down group 1, the outer electron is further away from the positive nucleus. It is also shielded from the nucleus by more electron shells. Because of this, it becomes easier to lose the outer electron, so the elements get more reactive moving down the group.

Table showing electronic configurations of group 1 elements, lithium, sodium and potassium. Group 1 elements have similar properties and reactions as they all have one electron in their outer shell.

The reactivity depends on the number of electrons and therefore the atomic number of the element.