Valency expresses

Options

A. total electrons in an atom
B. atomicity of an element
C. oxidation number of an element
D. combining capacity of an element.

Explanation

### Step-by-Step Solution

1. **Definition of Valency**: Valency is defined as the combining capacity of an element to form compounds. It indicates how many electrons an atom can gain, lose, or share when it reacts with other atoms.

2. **Determining Valency**: To find the valency of an element, we look at the number of electrons it needs to gain, lose, or share to achieve a stable electron configuration, typically an octet (8 electrons in the outer shell).

3. **Understanding Options**:
- **Option A**: Total electrons in an atom - This is incorrect as valency is not simply the total number of electrons.
- **Option B**: Atomicity of an element - Atomicity refers to the number of atoms in a molecule of the element (e.g., O2 has an atomicity of 2) and is not related to valency.
- **Option C**: Oxidation number - This represents the hypothetical charge of an atom in a compound and can vary, while valency is a fixed number based on electron gain/loss/sharing.
- **Option D**: Combining capacity of an element - This is the correct definition of valency.

4. **Example of Valency**: For instance, nitrogen has an electronic configuration of 2-5. To complete its octet, nitrogen needs to gain 3 electrons. Therefore, its valency is 3. However, its oxidation state can vary from -3 to +5 depending on the compounds it forms.

5. **Conclusion**: Based on the definitions and explanations provided, the correct answer to the question "Valency expresses" is that it expresses the combining capacity of an element to form compounds.