3.6 CATIONS AND ANIONS

Cations

Cations are positively charged ions that form when an atom loses one or more electrons. Cations are usually formed from metal atoms that tend to lose electrons to achieve a stable electron configuration similar to a noble gas. When an atom loses one or more electrons, it forms a cation. The resulting cation has the electronic configuration of a noble gas. Neutral atoms have an equal number of protons and electrons. When an atom loses one or more electrons, the number of protons becomes greater than the number of electrons, resulting in a positive charge.

Example 3.1: Describing the Formation of Cations

Describe the formation of Na+ and Mg2+ cations.

Problem Solving Strategy:

  1. Sodium belongs to Group IA on the periodic table. It has only one electron in the valence shell. Sodium atom loses its valence electron and is left with an octet. Represent this by drawing the complete electronic configuration or using an electron dot structure.
  2. Magnesium belongs to Group IIA in the periodic table. It has two valence electrons. Magnesium atom loses these electrons to achieve noble gas configuration. Represent this by drawing the complete electronic configuration or using an electron dot structure. This number also corresponds to the Group number in the periodic table.

Solution:

Formation of Na+ ion:

Na: 1s² 2s² 2p⁶ 3s¹ → Na+: 1s² 2s² 2p⁶

You can also represent this by the following electron dot structure:

Formation of Mg2+ ion:

Mg: 1s² 2s² 2p⁶ 3s² → Mg2+: 1s² 2s² 2p⁶

You can also represent this by the following electron dot structure:

CONCEPT ASSESSMENT EXERCISE

Describe the formation of cations for the following metal atoms:

  • (a) Li (atomic number 3)
  • (b) Al (atomic number 13)

Anions

Anions are negatively charged ions that form when an atom gains one or more electrons. This process usually occurs when an atom has a relatively high electron affinity, meaning that it can easily attract and capture more electrons to achieve a stable electron configuration similar to a noble gas. When an atom gains one or more electrons, the number of electrons becomes greater than the number of protons, so it acquires a negative charge.

Example 3.1: Describing the Formation of Anions

Describe the formation of anions for the following non-metal atoms:

Problem Solving Strategy:

  1. Write the electronic configuration or dot structure.
  2. Find the number of electrons needed to acquire an octet electron configuration.
  3. Represent the addition of electrons.

Solution:

Formation of anion by oxygen atom:

Oxygen belongs to Group VIA on the periodic table. It has six electrons in its valence shell. It needs two electrons to achieve a noble gas configuration.

O: 1s² 2s² 2p⁴ + 2e⁻ → O2-: 1s² 2s² 2p⁶

You can also represent this by the following electron dot structure:

Formation of anion by fluorine atom:

Fluorine belongs to Group VIIA on the periodic table. It has seven electrons in the valence shell. A fluorine atom therefore requires only one electron to complete its octet.

F: 1s² 2s² 2p⁵ + e⁻ → F-: 1s² 2s² 2p⁶

You can also represent this by the following electron dot structure:

CONCEPT ASSESSMENT EXERCISE 3.3

Describe the formation of anions by the following non-metals:

  • (a) Sulphur (atomic number 16)
  • (b) Chlorine (atomic number 17)