Question

The correct order of atomic size of Na, Be and Mg is:
A) $$Be>Mg>Na$$
B)$$Na>Mg>Be$$
C)$$Mg>Na>Be$$
D)$$Na=Mg>Be$$

Answer 1

Hint: The options given to us have elements from the different periods and different groups. Atomic radius is the distance between the nucleus and the last electron present in that element. If we know the general trends of atomic radius i.e. atomic size increases down the group and decreases along the period, we can find out the correct order easily.

Complete step-by-step answer:
Atomic radius depends on different factors such as electron shells, nuclear charge and shielding by electrons. The circular orbitals around the nucleus in which electrons are revolving are called energy shells. As the principal quantum number increases, atomic radius also increases while moving down the group as the distance between nucleus and the last electron increases.
The electrons and protons are oppositely charged species and have attractive force acting between the two. This is called the nuclear charge. So, as we along the period, a new electron enters the same shell and the nuclear charge increases, thereby decreasing the atomic radius. When outer electrons feel repulsive force from the already present inner electrons, atomic radius increases as the nuclear charge effect reduces due to poor shielding of electrons.
Let us look at the three elements given in the question.
Na: Sodium belongs to the third period and first group of the periodic table. As per general trend which we discussed, sodium will have maximum atomic size in its respective period and decreases along the period as sodium is the first element.
Be: Beryllium belongs to the second period and second group of the periodic table. As per general trend, it has minimum atomic radius in its group and the size increases as ewe go down the group as it is the first element of that group.
Mg: Magnesium belongs to the third period and second group of the periodic table. It is placed just after the atom in that period. As per general trend, it has second highest atomic radius in its period after sodium and the size decreases as we go along the period. Also, it has higher atomic radius than beryllium and the size increases as we go down the group.
As per these trends, the correct order will be $$Na>Mg>Be$$ .
Hence, the correct option is (B).

Note: The atomic size of inert gases does not follow this trend because we know that inert gases have Vander Waal radius and also, they do not form covalent bonds. The Vander Waal radius is usually larger than the covalent radius and thus, the inert gases have the largest atomic radius in their respective periods whereas the trend says that it will have lease atomic radius.