Ionisation potential increases in a period from left to right because

Options

A. atomic radius increases and nuclear charge increases.
B. atomic radius decreases and nuclear charge decreases.
C. atomic radius increases and nuclear charge decreases.
D. atomic radius decreases and nuclear charge increases.

Explanation

To answer the question "Ionisation potential increases in a period from left to right because," we will analyze the factors affecting ionization potential and determine the correct option.

### Step-by-Step Solution:

1. **Understanding Ionization Potential**:
- Ionization potential (or ionization energy) is defined as the energy required to remove an electron from the outermost shell of an isolated gaseous atom.

2. **Trends in a Period**:
- As we move from left to right across a period in the periodic table, several changes occur:
- The number of electrons in the outermost shell increases.
- The number of protons (nuclear charge) in the nucleus also increases.

3. **Effect of Nuclear Charge**:
- An increase in nuclear charge means that there are more protons in the nucleus, which leads to a stronger positive charge attracting the negatively charged electrons.
- This increased attraction makes it more difficult to remove an electron, thus increasing the ionization potential.

4. **Effect on Atomic Radius**:
- As we move from left to right, the atomic radius decreases. This is because the increased nuclear charge pulls the electrons closer to the nucleus, reducing the size of the atom.
- A smaller atomic radius means that the outermost electrons are closer to the nucleus, which also contributes to the increased ionization potential.

5. **Conclusion**:
- Therefore, the correct explanation for the increase in ionization potential from left to right across a period is that the atomic radius decreases and the nuclear charge increases.
- Thus, the correct option is: "atomic radius decreases and nuclear charge increases."

### Final Answer:
The ionization potential increases in a period from left to right because atomic radius decreases and nuclear charge increases.

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