For the energy levels in an atom , which of the following statement is //are correct ?

Options

A. There are serven principle electron energy levels
B. The second principal energy level cab be have four sub-shell energy level and contains a maximum of eight electrons
C. The M energy level can have a maximum energy than the `3d` sub-energy level
D. The `4s` sub-energy level is at a lower energy than the `3d` sub-energy level

Explanation

Statement (a) and (d) are true statement (b) is wrong because for `n = 2,l = 0,1` (two sub-energy levels) c is wrong because M shell means `n = 3` Maximum electrons d can have `= 2n^(2) = 2 xx 3^(2) = 18`
Hence a and d are correct