Structure of Atom — Chemistry STD 11 Science — Question

2. Manganese

Explanation:

The M shell is the third energy level; the first (K) shell can contain only 2 electrons. The second (L) shell can contain up to 8 electrons, for a total of 10. The third (M) shell can contain up to 18 electrons, but when it gets to 10, the fourth or N shell starts to fill up, since its first two electrons have lower energy than the next energy level in the M shell, so we have 18 electrons in the K, L, and M shells and the next two go into the N shell (for element 20, or calcium). So calcium has 10 electrons in the M shell and two in the N shell; then the next electrons go into the M shell (the 3d orbitals) so when we get to a total of 25 electrons, we have 10 in the K & L shells, two in the N shell, and 13 in the M shell. If it is a neutral atom, the element has atomic number 25, that is, manganese.