What is a valence shell? How many electrons can be present in valence shell?

Explanation

### Step-by-Step Solution:

1. **Definition of Valence Shell**:
- The valence shell is defined as the outermost shell of an atom where the last electron is filled. It is the shell that contains the electrons that are involved in chemical bonding and reactions.

2. **Identifying the Valence Shell**:
- To identify the valence shell, we look at the electronic configuration of an element. For example, magnesium (Mg) has an atomic number of 12, which gives it an electronic configuration of 2, 8, 2. Here, the last shell (M shell) where the last electron is filled is the valence shell.

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