Question

Of the following, the one with the largest size is?

• A. Cl^-
• B. Ar
• C. K⁺
• D. Ca²⁺

Answer 1

The correct option is A

Cl^-

Let's compare these elements using their electronic configuration and number of protons.

$C{l}^{-}$ - 1s²2s²2p⁶3s²3p⁶
It has 18 electrons and 17 protons

Ar - 1s²2s²2p⁶3s²3p⁶
It has 18 electrons and 18 protons

$K^{+}$ - 1s²2s²2p⁶3s²3p⁶
It has 18 electrons and 19 protons

$C{a}^{2+}$ - 1s²2s²2p⁶3s²3p⁶
It has 18 electrons and 20 protons

From this information, we can easily conclude that $C{l}^{-}$ would be the largest because it has a higher number of electrons as compared to protons and so the 'effective nuclear charge' would be lower on each electron.