For the energy levels in an atom which one of the following statements is correct?

Options

A. There are seven principal electron energy levels
B. The second principal energy level can have four sub energy levels and contains a maximum of eight electrons
C. The Menergy level can have a maximum of 32 electrons
D. The 4s subenergy level is at a higher energy than the 3d subenergy level.

Answer

To solve the question regarding the correct statement about the energy levels in an atom, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Understanding Principal Energy Levels:**
- The principal energy levels in an atom are denoted by the principal quantum number \( n \). The values of \( n \) can be 1, 2, 3, 4, 5, 6, and 7.
- Therefore, the statement "there are seven principal electron energy levels" is **correct**.

2. **Analyzing the Second Statement:**
- The second principal energy level (\( n = 2 \)) can have a maximum of \( n \) sublevels, which means it can have 2 sublevels (s and p).
- The maximum number of electrons in a principal energy level is given by the formula \( 2n^2 \). For \( n = 2 \), this is \( 2(2^2) = 8 \) electrons.
- However, the statement claims that the second principal energy level can have four sub energy levels, which is **incorrect**. It can only have two sublevels.

3. **Analyzing the Third Statement:**
- The M energy level corresponds to \( n = 3 \).
- The maximum number of electrons in the M energy level is calculated as \( 2n^2 = 2(3^2) = 18 \) electrons.
- The statement claims that the M energy level can have a maximum of 32 electrons, which is **incorrect**.

4. **Analyzing the Fourth Statement:**
- The 4s sublevel corresponds to \( n = 4 \) and \( l = 0 \) (s subshell).
- The 3d sublevel corresponds to \( n = 3 \) and \( l = 2 \) (d subshell).
- To compare their energies, we use the \( n + l \) rule:
- For 4s: \( n + l = 4 + 0 = 4 \)
- For 3d: \( n + l = 3 + 2 = 5 \)
- Since 5 > 4, the 3d sublevel has higher energy than the 4s sublevel. Thus, the statement that "4s sub energy level is at higher energy than 3d" is **incorrect**.

5. **Conclusion:**
- The only correct statement among the options provided is: "There are seven principal electron energy levels."

### Final Answer:
**The correct statement is: A. There are seven principal electron energy levels.**

---