Question

Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why it is so?

Answer 1

Hint: Atomic size is generally called the distance which is present between the centre of the atom nucleus and the outermost shell. Whereas the atomic radius is defined as the half of the distance present between the adjacent atoms of the same elements in the molecule. There are three factors on which the atomic size of the atom depends and that is the number of protons in the nucleus, the shielding effect and the number of shells.

Complete step-by-step answer:So the atomic size is the distance which is present between the centre of the nucleus to the boundary of the surrounding shell. We can estimate the size when the identical atoms tend to combine together then we can check the distance between the nuclei of those two identical atoms. The unit used for measuring the atomic size is Angstroms\[{{A}^{o}}\].
\[1{{A}^{o}}={{10}^{-10}}m\]
 In the modern periodic table when we move from left to right in a period we see that the increase in the electrons in the valence shells takes place. Due to the addition of electrons the electrons are more attracted towards the nucleus which makes the size of the atom smaller. This increased nuclear charge tends to overcome the repulsion between the electrons which are added in the valence shells. So the size of the atoms decreases from left to right in the period.
When we go top to bottom in the group the atomic number of the elements increases as the extra shells of the electrons are being added in the atom so the size of the atom increases and the attraction between the nucleus and the electron decreases. So the size of the atom increases down the group.

Note:Atomic radius of the atom is usually measured by the spectroscopy. So when we move across the period the atomic size decreases because the effective nuclear charge increases and the shielding of electrons remains constant. While we move down the group the atomic size increases so the number of shells increases by 1 down the group.