Question

Write the trend of atomic size along a group and a period in the modem periodic table.

Answer 1

Answer:

Atomic size increases down the group.

Atomic size decreases along a period from left to right.

Explanation:

FOR GROUP :

As we move down the group, there is an increase in new shell of an isolated atom. Thus the number of energy level increases as we move down the group as number of electrons increases.

Each subsequent energy level is further from the nucleus of an element than the previous element of the group because a new shell comes in the nucleus. The outermost electron is repelled by the inner electrons ( i.e. screening effect) and hence the effective nuclear charge to hold the shells decreases. So the shells are pulled less towards the nucleus.

Hence their atomic radii increases down the group .

Thus, their atomic size increases down the group .

Also due to this ionisation enthalpy as well as electron gain enthalpy decreases along the group.

Hence, atomic size increases down the group.

FOR PERIOD :

In a period the electrons are being added in the same shell.

Since, the nuclear charge (no. of protons) increases, the electrons experience a greater attraction towards the nucleus. To hold the increase in an electron, the effective nuclear charge increases.

Thus the shells of an isolated atom are pulled more closer to the nucleus , decreasing its atomic size.

Hence atomic radii decreases along the period from left to right.

So atomic size decreases along the period from left to right.

Also due to this ionisation enthalpy and electron gain enthalpy increases along the period from left to right.

Hence, atomic size decreases along the period from left to right.