Which of the following is equal to number of electrons in the valence shell ?

Options

A. Atomic mass
B. Group number
C. Period number
D. Atomic volume

Explanation

To determine which of the following is equal to the number of electrons in the valence shell, we need to analyze the options provided in the question. The valence shell is the outermost shell of an atom and contains the electrons that are involved in chemical bonding.

### Step-by-Step Solution:

1. **Understanding Valence Electrons**:
- Valence electrons are the electrons present in the outermost shell of an atom. They are crucial for determining how an element will react chemically.

2. **Identifying Relevant Terms**:
- The question mentions several terms: atomic mass, group number, atomic number, and atomic volume. We need to evaluate each of these in relation to valence electrons.

3. **Evaluating Atomic Mass**:
- Atomic mass is the weighted average mass of an atom's isotopes and does not directly relate to the number of valence electrons. Thus, atomic mass is not the correct answer.

4. **Evaluating Atomic Number**:
- The atomic number is the number of protons in an atom, which also equals the number of electrons in a neutral atom. However, it does not specifically indicate the number of valence electrons. Therefore, atomic number is not the correct answer.

5. **Evaluating Atomic Volume**:
- Atomic volume is a measure of the space occupied by an atom and is not related to the number of valence electrons. Hence, atomic volume is also not the correct answer.

6. **Evaluating Group Number**:
- The group number in the periodic table indicates the number of valence electrons for the main group elements (s and p block elements). For example, elements in Group 1 have 1 valence electron, and elements in Group 2 have 2 valence electrons. This makes the group number a direct indicator of the number of valence electrons.

7. **Conclusion**:
- Based on the analysis, the correct answer is the **group number**, as it corresponds to the number of electrons in the valence shell for main group elements.

### Final Answer:
The number of electrons in the valence shell is equal to the **group number**.