Question

What is atomic radius?Discuss its trend in 3rd row element

Answer 1

Well as we face the Periodic Table....

Explanation:

Atomic radius is reasonably the radius of the valence electron as it whizzes round the atom.

Atomic radius DECREASES across a Period, from LEFT TO RIGHT, as we face the Table. The third Period starts with sodium metal...which has a radius of `180xx10^-12*m`...it finishes with argon, which has a radius of `71xx10^-12*m`

See this site for data..

Two factors are important here: (i) atomic charge, which contracts the electronic radius; and (ii) shielding by other electrons. Which leads us to an important Periodic consequence. Incomplete electronic shells shield the nuclear charge VERY imperfectly. And thus atomic radius DECREASES across the Period from left to right as we face the Table, BUT INCREASES DOWN a Group.

And so if I were you, I would seek a site or a text that LISTS the atomic radii of the third row atoms and quote that data....

Answer 2

Atomic Radius is a term used to define the size of the atom. Although, the radius is not very definite because the atoms are constantly in motion, but, the answer below shall give you an idea.

Explanation:

Atomic radius is defined as the distance between the nuclei of two atoms that are just touching each other. This gives the diameter of the sphere which divided by `2` gives the radius.

In other words, Atomic radius is the distance from the center of the nucleus to the outermost shell containing electrons. It is the distance from the center of the nucleus to the point up to which the density of the electron cloud is maximum.

Reference

ATOMIC RADII IN THE 3RD PERIOD (ROW)
The atomic radius goes on decreasing in the 3rd row from Sodium to Argon. This is because:-

The no. of protons in the nucleus increase, so, the nuclear charge also increases. As we progress through the 3rd period, the no. of electrons entering the outermost orbit, increases by 1 each time, but, the increased charge on the nucleus does not allow the electrons to shield themselves from the attractive force. This shielding does not happen as the electrons are entering the same energy orbit (each orbit has a definite energy level).

Thus, the force of attraction increases as we progress through the 3rd period and the atomic radius decreases.

See this document here for more clarity on the radii in 3rd period and their trends