An element `M` has an atomic mass 19 and atomic number 9. Its ion is represented by

Explanation

To solve the question regarding the element `M` with an atomic mass of 19 and an atomic number of 9, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Element**:
- The atomic number of an element is unique and indicates the number of protons in its nucleus. In this case, the atomic number is 9.
- By referring to the periodic table, we find that the element with atomic number 9 is **Fluorine (F)**.

2. **Determine the Atomic Mass**:
- The atomic mass given is 19. The most common isotope of fluorine has an atomic mass of approximately 19 (specifically, it is about 18.998). Thus, this aligns well with our identification of the element as fluorine.

3. **Write the Electronic Configuration**:
- The electronic configuration of fluorine can be determined based on its atomic number. Fluorine has 9 electrons, which can be arranged as follows:
- 1s² 2s² 2p⁵
- This shows that fluorine has 2 electrons in the first shell (1s) and 7 electrons in the second shell (2s and 2p).

4. **Determine the Valence Electrons**:
- The valence shell for fluorine is the second shell (n=2), which contains 7 electrons (2 in 2s and 5 in 2p).
- To achieve a stable electronic configuration similar to the nearest noble gas (Neon), fluorine needs to gain 1 more electron.

5. **Formation of the Ion**:
- When fluorine gains one electron, it becomes negatively charged because it now has more electrons than protons.
- The charge of the ion formed is -1, and thus the ion can be represented as **F⁻**.

6. **Representation of the Ion**:
- The ion of element `M` can be represented as **M⁻**, where `M` corresponds to fluorine. Therefore, the final representation of the ion is **F⁻**.

### Final Answer:
The ion represented by element `M` is **F⁻** (fluoride ion).

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