Question

Which is larger,cl or al? And why

Answer 1

Aluminium Al and Chlorine Cl have the atomic numbers 13 and 17, respectively . They are both situated in the third period of the Periodic Table of Elements, in the columns 13 and 17 for Al and Cl, respectively.

The trend of atomic size across the period is that the atomic size decreases, so Cl will be smaller than Al. This happens because the number of the protons in the nucleus increases and the electrons in the valence shell feel a higher effective nuclear charge ( the sum of the charges of the protons in the nucleus and the charges of the inner, core electrons). Therefore, they are held more strongly and the atom contracts.

N.B. The valence electrons are the electrons situated in the outermost electronic shell of the atom. The core electrons are the electrons situated in the inner electron shells of the atom.