Understanding the Periodic Trends of Size of Ions: Cations vs. Anions
The size of ions differs significantly from that of their neutral atoms. Whether an atom loses or gains electrons directly affects its number of shells, electron-electron repulsion, and nuclear attraction—all of which determine ionic radius.
Let’s break this down by looking at cations and anions.
Cations: Smaller Than Neutral Atoms
Cations are positively charged ions formed when an atom loses one or more electrons. This electron loss leads to significant changes in atomic structure.
Example 1: Potassium (K) → Potassium ion (K⁺)
K (neutral atom):
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Has 4 electron shells -19 protons and 19 electrons.
K⁺ (cation):
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶
Now has 3 shells only
19 protons but only 18 electrons
Example 2: Magnesium (Mg) → Magnesium ion (Mg²⁺)
Mg: 3 shells, 12 protons, 12 electrons
Mg²⁺: 2 shells, 12 protons, 10 electrons
Key Observations for Size of Cations:
- Fewer electron shells after electron loss
- More protons per electron, increasing nuclear pull
- Higher effective nuclear charge
- Result: Cations are smaller than their parent atoms because the electrons are pulled closer to the nucleus and there’s less shielding.
Anions: Larger Than Neutral Atoms
Anions are negatively charged ions, formed when an atom gains one or more electrons. This addition increases electron-electron repulsion and affects the atom’s size.
Example 1: Chlorine (Cl) → Chloride ion (Cl⁻)
- Cl (neutral):
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁵. It has 3 shells, 17 protons, 17 electrons - Cl⁻ (anion):
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶. It still 3 shells, but 17 protons and 18 electrons
Example 2: Oxygen (O) → Oxide ion (O²⁻)
- O: 2 shells, 8 protons, 8 electrons
- O²⁻: 2 shells, 8 protons, 10 electrons
Key Observations for Size Of Anions:
- No change in the number of shells
- More electrons than protons, reducing effective nuclear pull
- Greater inter-electron repulsion, causing shell expansion
- Lower effective nuclear charge
Result: Anions are larger than their neutral atoms because increased electron repulsion pushes the outermost electrons farther from the nucleus.
Summary: of Periodic Trends of Size of Ions
- Cations → Smaller than neutral atoms
- Anions → Larger than neutral atoms
This trend plays a crucial role in ionic bonding, lattice energy, and chemical reactivity.
