What is the trend of atomic size on moving from left to right in periodic table?

Explanation

### Step-by-Step Solution:

1. **Understanding Atomic Size**: Atomic size refers to the distance from the nucleus to the outermost shell of electrons in an atom. It is generally measured as the atomic radius.

2. **Periodic Table Structure**: The periodic table is arranged in such a way that moving from left to right across a period, the atomic number increases. The atomic number is the number of protons in the nucleus of an atom.

3. **Increasing Atomic Number**: As we move from left to right, the atomic number increases, which means there are more protons in the nucleus. For example, moving from sodium (Na) to chlorine (Cl), the number of protons increases from 11 to 17.

4. **Nuclear Charge**: The increase in the number of protons leads to an increase in the nuclear charge. Nuclear charge is the total charge of the nucleus due to the protons. More protons mean a higher positive charge in the nucleus.

5. **Effective Nuclear Charge**: The effective nuclear charge is the net positive charge experienced by the valence (outermost) electrons. As more protons are added, the effective nuclear charge felt by the valence electrons increases because the nucleus exerts a stronger attractive force on these electrons.

6. **Electron Shielding**: While moving across a period, the number of electron shells remains the same, which means that the inner electrons do not significantly shield the outer electrons from the increasing nuclear charge. This results in a stronger attraction between the nucleus and the valence electrons.

7. **Decrease in Atomic Size**: Due to the increased effective nuclear charge, the nucleus pulls the valence electrons closer, resulting in a decrease in atomic size. Thus, the atomic radius decreases as we move from left to right across a period.

8. **Conclusion**: Therefore, the trend of atomic size on moving from left to right in the periodic table is that it decreases.