Calcium reacts vigorously even with cold water.

Answer

### Step-by-Step Solution:

1. **Identify the Element**: Calcium (Ca) is the element in question. It belongs to the S-Block of the periodic table and has an atomic number of 20.

2. **Determine the Electronic Configuration**: The electronic configuration of calcium can be written as:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
- This shows that calcium has two electrons in its outermost shell (4s²).

3. **Understand the Position of Electrons**: The outermost electrons (4s²) are located in a shell that is further away from the nucleus compared to the inner shells. This distance makes these electrons less tightly bound to the nucleus.

4. **Explain the Reactivity**: Because the outermost electrons are farther from the nucleus, they can be lost more easily. This means calcium has a high tendency to lose these two electrons.

5. **Reaction with Water**: When calcium is placed in water, it reacts vigorously. The reaction can be represented as:
- Ca + 2H₂O → Ca(OH)₂ + H₂↑
- Here, calcium reacts with water to form calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂).

6. **Conclusion**: The vigorous reaction of calcium with cold water is due to the ease with which it loses its two outermost electrons, leading to the formation of calcium hydroxide and the release of hydrogen gas.