$(a)$ Calculate the number of electrons,protons,and neutrons in an atom of an element with atomic number $20$ and mass number $40$. Write the electronic configuration and draw the structure of the atom.
$(b)$ An atom has complete $K$ and $L$ shells. Is this an atom of a metal,non-metal,or noble gas? Justify.

(N/A) For an element with atomic number $Z = 20$ and mass number $A = 40$:
Number of protons $= Z = 20$
Number of electrons $= Z = 20$
Number of neutrons $= A - Z = 40 - 20 = 20$
Electronic configuration: $K=2, L=8, M=8, N=2$.
The structure of the atom consists of a nucleus containing $20$ protons and $20$ neutrons,surrounded by four shells $(K, L, M, N)$ containing $2, 8, 8,$ and $2$ electrons respectively.
$(b)$ An atom with complete $K$ and $L$ shells has an electronic configuration of $2, 8$. The total number of electrons is $10$,which corresponds to the element Neon $(Ne)$. Since its outermost shell ($L$-shell) is completely filled (octet is complete),it is chemically inert and is classified as a noble gas.