Which of the following triads have approximately equal size ?

Options

A. `Na^(o+), Mg^(2+), Al^(3-)` (isoelectronic)
B. `F^(ɵ), Ne, O^(2-)` (isoelectronic)
C. `Fe, Co, Ni`
D. `Mn^(1+), Fe^(2+), Cr` (isoelectronic)

Explanation

To determine which of the given triads have approximately equal sizes, we need to analyze each option based on the concept of isoelectronic species and the periodic trends in atomic and ionic sizes.

### Step-by-Step Solution:

1. **Understanding Isoelectronic Species**:
- Isoelectronic species are atoms or ions that have the same number of electrons. They can be compared in terms of size because they have similar electron configurations.

2. **Analyzing the Options**:
- **Option 1: Na+, Mg2+, Al3+**
- Na+ has 10 electrons (11 protons - 1 electron).
- Mg2+ has 10 electrons (12 protons - 2 electrons).
- Al3+ has 10 electrons (13 protons - 3 electrons).
- Although they are isoelectronic, the ionic sizes differ due to the increasing nuclear charge. Al3+ is the smallest, followed by Mg2+, and Na+ is the largest.
- **Conclusion**: Not approximately equal in size.

- **Option 2: F-, Ne, O2-**
- F- has 10 electrons (9 protons + 1 electron).
- Ne has 10 electrons (10 protons).
- O2- has 10 electrons (8 protons + 2 electrons).
- Here, O2- is the largest due to the lower nuclear charge, followed by F-, and Ne is the smallest.
- **Conclusion**: Not approximately equal in size.

- **Option 3: Fe, Co, Ni**
- Fe, Co, and Ni are all transition metals and are in the same period (4th period).
- They have similar atomic sizes because they are d-block elements and experience similar effective nuclear charge.
- **Conclusion**: Approximately equal in size.

- **Option 4: Mn+, Fe2+, Cr**
- Mn+ has 25 protons and 24 electrons.
- Fe2+ has 26 protons and 24 electrons.
- Cr has 24 protons and 24 electrons.
- These are not isoelectronic and have different sizes due to different nuclear charges.
- **Conclusion**: Not approximately equal in size.

3. **Final Answer**:
- The triad that has approximately equal sizes is **Option 3: Fe, Co, Ni**.