On moving along the period , the atomic radii decreases. Explain.

Explanation

To explain why the atomic radii decrease as we move along a period from left to right, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Atomic Radius**:
- The atomic radius is defined as the distance from the nucleus of an atom to the outermost shell of electrons. It gives an idea of the size of the atom.

2. **Movement Along a Period**:
- As we move from left to right across a period in the periodic table, we are observing elements that have increasing atomic numbers.

3. **Increase in Atomic Number**:
- The atomic number of an element is the number of protons in its nucleus. As we move from left to right, the atomic number increases, meaning there are more protons in the nucleus.

4. **Increase in Nuclear Charge**:
- With the increase in the number of protons, the positive charge of the nucleus increases. This is referred to as an increase in nuclear charge.

5. **Addition of Electrons**:
- Along with the increase in protons, electrons are also added to the same energy level (or shell). However, these electrons do not shield each other effectively from the increased nuclear charge.

6. **Effective Nuclear Charge**:
- The effective nuclear charge is the net positive charge experienced by electrons in the outermost shell. As the nuclear charge increases and the shielding effect remains relatively constant, the effective nuclear charge felt by the outer electrons increases.

7. **Attraction of Electrons**:
- The increased effective nuclear charge means that the nucleus exerts a stronger attractive force on the outermost electrons. This pulls the electrons closer to the nucleus.

8. **Resulting Decrease in Atomic Radius**:
- As a result of the stronger attraction between the nucleus and the electrons, the atomic radius decreases. The outermost electrons are drawn closer to the nucleus, leading to a smaller atomic size.

### Conclusion:
- Thus, the decrease in atomic radii as we move from left to right across a period can be attributed to the increase in nuclear charge and the corresponding increase in effective nuclear charge, which pulls the electrons closer to the nucleus.

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