Ionic size

When an atom loses an electron to form a cation, or when an atom gains an electron to form an anion, the size of the ion is not the same as the original atom. In this section, we will take a look at how the size of:

• Metals
• Nonmetals
• Isoelectronic series

Metals

Compare the size of the atom with its cation

Metal atoms tend to lose their valence electrons to form positive ions called cations. Within a group of elements, ion sizes increase with atomic number.

Nonmetals

Compare the size of the atom with its anion

Nonmetal atoms tend to gain electrons to form negative anions. Within a group of elements, ion sizes increase with atomic number.

Isoelectronic series

Isoelectronic ions are ions that have the same number of electrons. There are many ions that are isoelectronic.

1. Na⁺ and F^- each has 10 electrons.
2. S^2-, Cl^-, K⁺, Ca⁺² each has 18 electrons.

We expect that Na⁺ has a smaller ionic radius than F^- because Na⁺, with its greater nuclear charge (11 protons) will have a greater attraction of its electrons than F^- (9 protons).

We expect the Ca⁺², with the greatest nuclear charge (20 protons) will have a greatest attraction of its electrons. S^-2 (16 protons) has the smallest nuclear charge will not have as much pull of its electrons toward the nucleus. Therefore, the ionic size:

Ca⁺² (smallest) < K⁺ < Cl^- < S^-2 (largest)

Content suitability

BCIT courses: CHEM 0011