Question

An element has an atomic number 19. How many electrons are there in K, L, M, N shells of its atom?

• A. 2, 8, 8, 1
• B. 2, 7, 8, 2
• C. 3, 7, 7, 2
• D. 3, 7, 8, 1

Answer 1

The correct option is A 2, 8, 8, 1

The atomic number of the given element is 19.
So, the electronic configuration will be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$.
where 1, 2 ,3 ,4 represents the principal quantum number K, L, M, N shells.
From electronic configuration, the total number of electrons present in each shell can be calculated as follows:
$$\begin{array}{ccc}Shell& Subshell& No.ofelectrons\\ K(n=1)& 1s^2& 2\\ L(n=2)& 2s^2,2p^6& (2+6)=8\\ M(n=3)& 3s^2,3p^6& (2+6)=8\\ N(n=4)& 4s^1& 1\end{array}$$
So the number of electrons in the K, L, M, N shells are 2, 8, 8, 1 respectively.