What is the number of valence electrons in the atom of an element A having atomic number 20? Name the valence shell of this atom.

Explanation

To determine the number of valence electrons in the atom of an element A with atomic number 20 and to identify its valence shell, follow these steps:

### Step 1: Write the Electronic Configuration
The atomic number of element A is 20, which means it has 20 protons and, in a neutral atom, 20 electrons. The electronic configuration is determined by filling the shells in the order of their energy levels.

- The first shell (K) can hold a maximum of 2 electrons.
- The second shell (L) can hold a maximum of 8 electrons.
- The third shell (M) can also hold a maximum of 8 electrons.
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