Atomic Radius - Chemistry

1. Fundamental Concepts

Atomic Radius is defined as the distance from the nucleus of an atom to the outermost valence electron shell. It is a measure of atomic size, typically expressed in picometers (pm, 1 pm = 10⁻¹² m).

Key measurement types:
- Covalent Radius: Half the distance between the nuclei of two covalently bonded identical atoms (e.g., Cl₂).
- Metallic Radius: Half the distance between the nuclei of two adjacent atoms in a metallic crystal (e.g., Na metal).

Across a Period (Left to Right): Atomic radius decreases.
- Reason: Increasing nuclear charge (more protons) pulls valence electrons closer to the nucleus; electron shielding remains constant.
Down a Group (Top to Bottom): Atomic radius increases.
- Reason: Addition of new electron shells (higher principal quantum number n); electron shielding outweighs increased nuclear charge.

Noble gases have larger atomic radii than adjacent halogens because they are measured by van der Waals radius (weaker intermolecular forces).
Transition metals show slight variation in atomic radius across periods due to d-electron shielding.

Atomic radius ____________ (increases/decreases) as you move down Group 1 (alkali metals).

Answer: increases

In Period 3, the atom with the largest atomic radius is ____________ (Na/Mg/Al/Si/P/S/Cl/Ar).

Answer: Na

Arrange the following atoms in order of increasing atomic radius: O, S, Se, F ____________.

Answer: F < O < S < Se

Trend Application:
- First, identify the period and group of each atom using the periodic table.
- Compare across periods (nuclear charge) and down groups (electron shells) separately.
Exception Handling:
- Exclude noble gases when comparing with adjacent elements in the same period (due to van der Waals radius).
Stepwise Comparison for Multiple Atoms:
- Group atoms by the same period or group first.
- Use a "chain comparison" (e.g., A < B in Period 2, B < C in Group 16 → A < B < C).
Unit Awareness:
- Ensure consistency in units (pm is standard) when comparing numerical values of atomic radii.