An element M has an atomic number 9 and atomic mass 19. Its ion will be represented by
(1) M (2) `M^(2+)` (3) `M^(-)` (4) `M^(2-)`

Explanation

To solve the question about the element M with atomic number 9 and atomic mass 19, we need to determine the correct representation of its ion. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Element
- The atomic number of the element M is 9. This means it has 9 protons and, in a neutral state, also 9 electrons.
- The element with atomic number 9 is Fluorine (F).

**Hint:** The atomic number tells you the number of protons and electrons in a neutral atom.

### Step 2: Determine the Electron Configuration
- The electron configuration for Fluorine (atomic number 9) is:
- 1s² 2s² 2p⁵
- This means that Fluorine has 2 electrons in the first shell (1s) and 7 electrons in the second shell (2s and 2p).

**Hint:** Use the atomic number to write the electron configuration, filling the orbitals according to the Aufbau principle.

### Step 3: Analyze the Tendency to Gain or Lose Electrons
- Fluorine has 7 electrons in its outer shell (2s² 2p⁵) and needs 1 more electron to achieve a stable octet configuration (2s² 2p⁶).
- Therefore, Fluorine has a strong tendency to gain 1 electron.

**Hint:** Elements in Group 17 (Halogens) typically gain electrons to achieve a full outer shell.

### Step 4: Determine the Ion Formed
- When Fluorine gains 1 electron, it becomes negatively charged. The ion formed is represented as F⁻.
- In the context of the question, we represent this as M⁻ since M is the symbol for our element.

**Hint:** Gaining electrons results in a negative charge, while losing electrons results in a positive charge.

### Step 5: Choose the Correct Representation
- The options given are:
1. M
2. M²⁺
3. M⁻
4. M²⁻
- Since Fluorine (M) gains 1 electron to become F⁻, the correct representation of the ion is M⁻.

**Hint:** Compare the charge of the ion with the options provided to select the correct one.

### Final Answer
The correct representation of the ion for element M (atomic number 9) is **(3) M⁻**.