Question

For an element with atomic number 19, the 19th electron will occupy:
A.L-shell
B.M-shell
C.N-shell
D.K-shell

Answer 1

Every element has a certain atomic number which tells us about the number of electrons present in its ground state. From that, we can either write the electronic configuration of that element or we can directly predict using the principal quantum number which gives us the types of shells and the number of electrons they can accommodate.

Complete step-by-step answer:

The Schrodinger wave equation for a system gave us the quantized energy states which an electron can occupy and is characterized by a set of four quantum numbers. We need to know the first one so as to get the answer to this question. It is the principal quantum number which is represented by n.
It is a quantum number which is assigned to the shells or orbital in which the electron lies and it is very much similar to the obit or shells given by Bohr in his atomic model. Every shell has a fixed number of atomic orbitals and the value of n increases, there increases the number of allowed atomic orbitals accordingly.
Every shell is designated a value and the number of electrons it can accommodate which we call as principal quantum number. So, the first shell can be n = 1, second shell can be n = 2, and so on. We can further represent them in letter form as K, L, M, N, and so on. Here,
K-shell can accommodate only two electrons, the L-shell can accommodate 8 electrons, the M-shell can accommodate 18 electrons, and so on.
So, we have an element having its atomic number as 19. This means that the total number of electrons in the ground state of this element are 19. So, first two electrons will occupy K-shell, next 8 electrons will occupy L-shell and the remaining nine electrons will occupy M-shell.
From this it is clear that the 19th electron will occupy the M-shell.

Hence, the correct option is (B).

Note: We can also explain it by the Azimuthal quantum number which tells us about the sub-shells such as s, p, d, f orbitals. We use them to write the electronic configuration as per the principal quantum number and using Aufbau principle i.e. as per increasing energy levels. Since M-shell carries 18 electrons and the electronic configuration comes out to be $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$ for 19 atomic number of an element, it is clear that the 19th electron lies in M-shell.