The atomic radius decreases as we move across a period because

Options

A. atomic mass increases
B. atomic number increases
C. effective nuclear charge increases
D. electrons are added removed.

Explanation

To understand why the atomic radius decreases as we move across a period in the periodic table, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Atomic Structure**:
- Each atom consists of a nucleus (containing protons and neutrons) and electrons that orbit around the nucleus in shells.

2. **Atomic Number and Protons**:
- The atomic number of an element is defined as the number of protons in its nucleus. As we move across a period from left to right, the atomic number increases.

3. **Increase in Positive Charge**:
- With the increase in atomic number, the number of protons in the nucleus increases. This results in a greater positive charge in the nucleus.

4. **Electron Shells**:
- While moving across a period, the electrons are added to the same principal energy level (or shell). This means that the number of electron shells does not increase.

5. **Effective Nuclear Charge (Z-effective)**:
- The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. As the number of protons increases while the number of electron shells remains the same, the effective nuclear charge felt by the outermost electrons increases.

6. **Attraction Between Nucleus and Electrons**:
- A higher effective nuclear charge means that the attraction between the positively charged nucleus and the negatively charged electrons becomes stronger. This stronger attraction pulls the electrons closer to the nucleus.

7. **Decrease in Atomic Radius**:
- As the electrons are pulled closer to the nucleus due to the increased effective nuclear charge, the overall size of the atom decreases. Therefore, the atomic radius decreases as we move across a period.

### Conclusion:
The atomic radius decreases across a period because the effective nuclear charge increases, leading to a stronger attraction between the nucleus and the electrons, which pulls the electrons closer to the nucleus and reduces the size of the atom.

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