IVL 4 IVL

Periodicity of Atomic Radius

Atomic radii for elements in Periods 2 and 3

Atomic radii can be classified into three categories:

• Covalent radius:
• Metallic radius
• Van der Waals radius

Effecting factors of the atomic radius:

• Screening effect of the inner shell electrons: negatively-charged shells repel one another and are being pushed further away from the nucleus; screening effect increase; and size of the atoms increase.
• Nuclear charge (number of protons in the nucleus) that pulls all the electrons closer to the nucleus: The higher the nuclear charge; the stronger the attraction between nucleus and the electron cloud; and the size of the atom decrease.
• Effective nuclear charge = No. of protons – No. of inner electrons

A) Atomic radius across a period

Example: Period 2 (Li, Be, B, C, N, O, F, Ne) and Period 3 (Na, Mg, Al, Si, P, S, Cl, Ar)

Across the period:

• Number of protons increase by one.
• Number of electrons increase by one.
• Screening effect does not affect much (same quantum shell).
• Nuclear charge increase (stronger attraction between nucleus and electron cloud).
• Size of the atoms decrease.

B) Atomic radius down a group

Example: Group 2 (Be, Mg, Ca, Sr, Ba)

Down the group:

• Screening effect increase.
• Nuclear charge increase.
• Effective nuclear charge decrease.
• Size of the atoms increase (the increase in the screening effect is larger than the increase in the nuclear charge).

C) Ionic radius (radius of a cation or or an anion) across Period 3

Isoelectronic – species have the same number of electrons and the same electronic configuration.

When given number of electrons (Na⁺, Mg²⁺, Al³⁺) or (P^3-, S^2-, Cl^-)

• higher the nuclear charge,
• higher the force of attraction
• smaller the atomic size or ionic size.

When given nuclear charge,

• larger the number of electrons in an atom or an ion,
• greater the repulsion between electrons
• larger the atomic or ionic size.

Conclusion:

• Cationic size decreases (increasing proton number).
• Anionic size decreases (increasing proton number).

D) Ionic radius down a group

Example: Group 2 (Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) & Group 17 (F^-, Cl^-, Br^-, I^-)

Going down the Group 2 and Group 17:

• Each successive ion has one additional shell filled with electrons.
• Screening effect increase
• Ionic size increase.