The atomic numbers of elements X, Y, Z are 19, 21 and 25 respectively. The number of electrons present in the 'M' shells of these elements follow the order:

Explanation

To determine the number of electrons present in the 'M' shells of elements X, Y, and Z, we need to consider the electron configuration of each element. The 'M' shell corresponds to the third principal energy level (n=3). Element X (atomic number 19) is potassium (K). Its electron configuration is 1s2,2s2,2p6,3s2,3p6,4s1 .This electronic configuration shows that M shell has 8 electrons. Element Y (atomic number 21) is scandium (Sc). Its electron configuration is 1s2,2s2,2p6,3s2,3p6,4s2,3d1 .This electronic configuration shows that M shell has 9 electrons. Element Z (atomic number 25) is manganese (Mn). Its electron configuration is 1s2,2s2,2p6,3s2,3p6,4s2,3d5 .This electronic configuration shows that M shell has 13 electrons. Therefore, the order of the number of electrons present in the 'M' shells of elements X, Y, and Z is: 8 (element X) < 9(element Y) < 13(element Z).