The atomic radii of the elements Na , Mg , Al , and Si are given in...

The given elements $\mathrm{Na}$, $\mathrm{Mg}$, $\mathrm{Al}$, and $\mathrm{Si}$ belong to the third period of the periodic table.

As we move from left to right across a period, the effective nuclear charge increases, which pulls the valence electrons closer to the nucleus. Consequently, the atomic radius decreases.

The order of atomic radii for these elements is $\mathrm{Na} > \mathrm{Mg} > \mathrm{Al} > \mathrm{Si}$.

Arranging the given values in decreasing order, we get $186\text{ pm} > 160\text{ pm} > 143\text{ pm} > 118\text{ pm}$.

Matching the elements with their corresponding radii:
$\mathrm{Na} = 186\text{ pm}$
$\mathrm{Mg} = 160\text{ pm}$
$\mathrm{Al} = 143\text{ pm}$
$\mathrm{Si} = 118\text{ pm}$

The atomic radius of $\mathrm{Al}$ is $143\text{ pm}$.

Answer: $143\text{ pm}$