Which of the following is generally true regarding effective nuclear charge `(Z_(eff))`:

Options

A. It increases on moving left to right in a period.
B. It remains almost constant on moving top to bottom in a group.
C. For isoelectronic species, as `Z` increases, `Z_(eff)` decreases.
D. Both `(A)` and `(B)`

Explanation

To determine which statements are generally true regarding effective nuclear charge (Z_eff), we will analyze the concept of effective nuclear charge and the provided statements step by step.

### Step 1: Understand Effective Nuclear Charge (Z_eff)
Effective nuclear charge (Z_eff) is defined as the net positive charge experienced by an electron in a multi-electron atom. It can be calculated using the formula:
\[ Z_{\text{eff}} = Z - \sigma \]
where:
- \( Z \) is the atomic number (the total number of protons in the nucleus),
- \( \sigma \) is the shielding constant, which accounts for the repulsion from other electrons that reduces the full nuclear charge felt by an individual electron.

### Step 2: Analyze Statement A
**Statement A:** "Z_eff increases on moving left to right in a period."
- As we move from left to right across a period, the atomic number \( Z \) increases because we are adding protons to the nucleus.
- The number of electrons also increases, but they are added to the same principal energy level, which does not significantly increase the shielding effect.
- Therefore, as \( Z \) increases and \( \sigma \) remains relatively constant, \( Z_{\text{eff}} \) increases.
- **Conclusion:** This statement is **true**.

### Step 3: Analyze Statement B
**Statement B:** "Z_eff remains almost constant on moving from top to bottom in a group."
- When moving down a group, the atomic number \( Z \) increases, but the number of electron shells also increases, leading to a larger atomic size.
- The increased distance between the nucleus and the outermost electrons, along with increased shielding from inner electrons, means that the effective nuclear charge felt by the outermost electrons does not significantly change.
- Therefore, \( Z_{\text{eff}} \) remains relatively constant.
- **Conclusion:** This statement is **true**.

### Step 4: Analyze Statement C
**Statement C:** "For isoelectronic species, Z increases so Z_eff decreases."
- Isoelectronic species have the same number of electrons but different atomic numbers.
- For isoelectronic species, as the atomic number \( Z \) increases, the effective nuclear charge \( Z_{\text{eff}} \) also increases because there are more protons in the nucleus attracting the same number of electrons.
- **Conclusion:** This statement is **false**.

### Final Conclusion
Based on the analysis:
- Statement A is true.
- Statement B is true.
- Statement C is false.

Thus, the correct answer is that both **A and B** are generally true regarding effective nuclear charge.