=The atomic radius decrease with increase on atomic number on moving from left to right across to same shell, no new shell formed Li to F.




=decrease across a period and increases down a group

Atomic Radius Variation Across a Period:
The atomic radius refers to the size of an atom, which is determined by the distance between the nucleus and the outermost electron. The atomic radius generally decreases across a period from left to right in the periodic table. This can be explained by the following factors:

1. Nuclear Charge: As we move across a period, the number of protons in the nucleus increases, leading to a stronger positive charge in the nucleus. This increased nuclear charge pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

2. Effective Nuclear Charge: The effective nuclear charge is the net positive charge experienced by the outermost electrons. As we move across a period, even though the number of electrons increases, the number of valence electrons remains the same. Therefore, the increased nuclear charge has a greater effect on the outermost electrons, causing them to be more strongly attracted to the nucleus and reducing the atomic radius.

3. Electron Shielding: Electron shielding refers to the repulsion between electrons in the inner energy levels and the outermost electrons. As we move across a period, the number of inner electrons remains constant, but the nuclear charge increases. This leads to a stronger pull on the outermost electrons, resulting in a smaller atomic radius.

Atomic Radius Variation Down a Group:
The atomic radius generally increases down a group in the periodic table. This can be explained by the following factors:

1. Principal Energy Levels: As we move down a group, new energy levels are added. These additional energy levels increase the distance between the nucleus and the outermost electrons, resulting in an increase in atomic radius.

2. Electron Shielding: Down a group, the number of inner energy levels increases. This increased electron shielding reduces the attractive force between the nucleus and the outermost electrons, causing the atomic radius to increase.

3. Effective Nuclear Charge: Although the nuclear charge increases down a group, the effect of this increase is relatively small compared to the addition of new energy levels and electron shielding. Therefore, the increase in atomic radius down a group is primarily due to these two factors.

In summary, the atomic radius generally decreases across a period due to increased nuclear charge, effective nuclear charge, and electron shielding. On the other hand, the atomic radius increases down a group due to the addition of new energy levels and increased electron shielding.