A has 9 protons, 9 electrons and 10 neutrons, B has 12 protons, 12 electrons and 12 neutrons. Formula of the compound between A and B is

Options

A. `BA_2`
B. `AB_2`
C. `B_2 A_3`
D. `AB_4`

Explanation

To determine the formula of the compound formed between elements A and B, we need to follow these steps:

### Step 1: Identify the Elements
- Element A has 9 protons, 9 electrons, and 10 neutrons. This means it is Fluorine (F), as the atomic number (number of protons) is 9.
- Element B has 12 protons, 12 electrons, and 12 neutrons. This means it is Magnesium (Mg), as the atomic number is 12.

### Step 2: Determine the Electronic Configuration
- For Element A (Fluorine):
- It has 9 electrons. The electronic configuration is 2 in the first shell and 7 in the second shell (2, 7).

- For Element B (Magnesium):
- It has 12 electrons. The electronic configuration is 2 in the first shell, 8 in the second shell, and 2 in the third shell (2, 8, 2).

### Step 3: Determine the Valency
- For Element A (Fluorine):
- It has 7 valence electrons and needs 1 more electron to complete its octet (8 electrons). Therefore, the valency of A is **1** (it needs to gain 1 electron).

- For Element B (Magnesium):
- It has 2 valence electrons and can lose them to achieve a stable configuration. Therefore, the valency of B is **2** (it can lose 2 electrons).

### Step 4: Write the Formula
- To write the formula of the compound formed between A and B, we use the valencies:
- A has a valency of 1 and B has a valency of 2.

- We can denote the formula as follows:
- The valency of A (1) goes down as a subscript to B, and the valency of B (2) goes down as a subscript to A.

- Therefore, the formula will be written as:
- \( \text{B}_2\text{A} \)

- Substituting A and B:
- The formula becomes \( \text{Mg}_2\text{F} \).

### Final Answer
The formula of the compound between A and B is \( \text{Mg}_2\text{F} \).

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