What is the atomic radius of chlorine?

Question

I'm trying to study for chemistry and there's a question in my book about identifying an element based on its atomic radius. But when I looked up the value for chlorine, I found different answers from different sources:

• 79 pm

• 175 pm (from the Princeton Review)

• 0.97 Å

• 175 pm (van der Waals radius?)

• 79 pm

What is the correct value?

Answer 1

There is no such thing as unified atomic radius. An atomic radius is a class consisting of van der Waals radii $R_\mathrm{vdW}$ (steric interactions), covalent radii $R_\mathrm{cov}$, and ionic radii $R_\mathrm{i}$ (and some other as well).

From the recent edition of CRC Handbook [1, p. 9-57]:

$$ \begin{array}{llrr} \hline \text{Element} & \text{Symbol} & R_\mathrm{vdW}~(Å) & R_\mathrm{cov}~(Å)\\ \hline \text{Chlorine} & \ce{Cl} & 1.75 & 1.00\\ \hline \end{array} $$

Ionic radii are given for the crystallographic data separately [1, p. 12-12]:

$$ \begin{array}{llr} \hline \text{Ion} & \text{C.N.} & R_\mathrm{i}~(Å)\\ \hline \ce{Cl^{+5}} & 3~\text{(pyramidal)} & 0.12\\ \ce{Cl^{+7}} & 4 & 0.08\\ \hline \end{array} $$

Reference

1. Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data.; 2017; Vol. 97. ISBN 978-1-4987-5429-3.