Question

An atom of which element has the largest atomic radius?

Answer 1

Answer:

B) Be

Explanation:

Across the period (going from left to right of the periodic table), the atomic radius decreases. Beryllium is located further on the left of the periodic table compared to the other elements in the option and thus it has the largest atomic radius.

Explanation of trend:

Across the period, the number of protons and electrons increases in the atom. Electrons are added to the valence shell, hence the electrons added are not able to shield the increased nuclear charge well. (Since shielding effect is most effective when core electrons shield the valence electrons) Therefore, the effective nuclear charge (ENC) increases. As ENC increases, valence electrons are increasingly attracted to the nucleus. They are drawn closer to the nucleus, and so the atomic radius decreases across the period.

What is ENC?

ENC is the net nuclear charge acting on the valence electron (since the valence electron experiences electrostatic forces of attraction by the positively charged protons in the nucleus, and an electron-electron repulsion force by the core electrons)

➣ ENC

= nuclear charge (atomic no.) - shielding constant (no. of core electrons)

See diagram: black arrow represents the shielding effect (also known as electron-electron repulsion), while the orange arrows represent the nuclear charge (electrostatic forces of attraction)