Question

In modern periodic table, arrange the third-row elements $Na,Mg,Al$ and $Si$ in the increasing order of their atomic size.
(A) $Na > Mg > Al > Si$
(B) $Al > Si > Na > Mg$
(C) $Si > Al > Mg > Na$
(D) $Na > Al > Si > Mg$

Answer 1

The distance between the Centre of the nucleus of an atom to its outermost shell is known as the atomic radius of that atom. In a molecule of the same element, the atomic radius is half the distance between the adjacent atoms. The properties of the elements belonging to the third period change gradually from metallic to non-metallic.$Na,Mg,Al$ are metals while $Si$ is a metalloid.

Complete answer: We know that on moving left to right across a period the atomic size of the element decreases as the valence electrons are in the same outermost shell also the atomic number increases on moving across the period which in turn also increase the effective nuclear charge and therefore increases the attractive force between the outermost shell and the nucleus of the atom thereby causing a decrease in the atomic size of the element.
On the other hand, the atomic size of the element increases on moving down a group in the periodic table because one more shell is added to the element which thus decreases the effective nuclear charge on the element. This will weaken the attractive force present between the nucleus and the outermost shell and will therefore increase the atomic size of the element.
Hence the correct order of increasing atomic radius is $Na > Mg > Al > Si$

So, the correct answer will be Option (A).

Note: To solve questions like these we must always remember the atomic number of the elements in order to determine the group and the period in which they belong. By knowing the atomic number, we can easily write the electronic configuration and by that figure out the number of valence electrons. Thus, valence electrons will tell us the group the element will belong to.