How many electron in Ca (Z =20) have n = 2, l = 0 ?

Options

A. 1
B. 0
C. 2
D. 3

Answer

To determine how many electrons in calcium (Ca) with atomic number Z = 20 have the principal quantum number n = 2 and azimuthal quantum number l = 0, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Electron Configuration of Calcium (Ca)**:
- The atomic number of calcium is 20, which means it has 20 electrons.
- The electron configuration can be written as:
\[ \text{Ca: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \]

2. **Understand the Quantum Numbers**:
- The principal quantum number (n) indicates the energy level of the electron.
- The azimuthal quantum number (l) indicates the shape of the orbital. For l = 0, the orbital is an s orbital.

3. **Identify the Relevant Orbitals**:
- For n = 2, the possible orbitals are:
- 2s (where l = 0)
- 2p (where l = 1)
- Since we are interested in l = 0, we focus on the 2s orbital.

4. **Count the Electrons in the 2s Orbital**:
- From the electron configuration, we see that the 2s orbital has:
\[ 2s^2 \]
- This indicates that there are 2 electrons in the 2s orbital.

5. **Conclusion**:
- Therefore, the number of electrons in calcium (Ca) with n = 2 and l = 0 is **2**.

### Final Answer:
The number of electrons in Ca (Z = 20) with n = 2 and l = 0 is **2**.