Question

An element is represented as ${}_{20}{}^{40}\mathrm{X}$. Name the shell which has valence electrons and the number of valence electrons in it.

Answer 1

Step-1: Find out the number of electrons present in the element

Atomic number = 20 (Given from the representation ${}_{\mathrm{Atomic}\mathrm{number}}{}^{\mathrm{Mass}\mathrm{number}}\mathrm{X}$)

As atomic number = Number of protons

In an atom, Number of protons = Number of electrons

Therefore, number of electrons = 20

Step-2: Write the electronic configuration

Electronic configuration = 2, 8, 8, 2

Electrons begin to fill in the orbits from inside out i.e.; from the innermost to the outermost orbit according to the Bohr-Bury scheme.

Step-3: Find the valence shell and valence electrons

Valence shell: The outermost shell of all the shells that surround a nucleus.

Valence electrons: The electrons present in the valence shell.

• The shells are named from the alphabet K onwards with K being the first or the innermost shell.
• K, L, M, N shells occupy 2, 8, 8, 2 electrons respectively.
• As electrons begin to fill from K shell or first shell and last electrons get filled in N shell or the outermost or valence shell, therefore valence shell is N-shell.
• Number of electrons in N-shell are 2. Therefore, the number of valence electrons are 2.