Question

If an element has an atomic number of 12, how many electrons are in its outermost shell?

Answer 1

For finding the electrons in the outermost orbital we need to write the electronic configuration of the element given. The electronic configuration helps in describing how the electrons are distributed in its atomic orbitals. The electronic configuration helps in determining the valency of the element and also in the prediction of the properties of groups of elements. It also helps in the interpretation of the atomic spectra.

Complete step-by-step answer:
The electron configuration of an element describes how electrons are distributed in its atomic orbitals. Electron configurations of atoms follow a standard notation in which all electron-containing atomic subshells (with the number of electrons they hold written in superscript) are placed in a sequence.

So to write the electronic configuration of the element one must follow the Aufbau principle. The Aufbau principle states that the electrons should occupy those orbitals which have lower energy. The order of the energy orbitals increases by the (n+l) rule. This is the sum of two quantum numbers and they are principal and azimuthal. So now we by the atomic number 12 we came to know it has 12 electrons in total.

So now filling the orbitals by Aufbau rule is:
$1{s^2}2{s^2}2{p^6}3{s^2}$
So we can see that the last orbital 3s has 2 electrons in its shell. So the element has 2 electrons in its outermost shell.

Note: There are some exceptions in the Aufbau principle which have been explained by the stability of the half filled and the completely filled subshells. The Hund's rule was the rule which helped in describing the order of the filling of electrons in the orbitals. It said that firstly each orbital should be singly occupied before the second electron gets occupied.