Question

The element with atomic number 56 is likely to have the same outer shell configuration as the element with atomic number __________.
A. 12
B. 18
C. 14
D. 30

Answer 1

Hint: First of all, we will see the definition of electronic configuration. And then find the electronic configuration for the atomic number 56 to get its valence electron. Finally we check from the option for the same number of valence electrons.

Complete answer:
The distribution of electrons into orbitals of an atom is called its electronic configuration.
The filling of electrons into the orbitals of different atoms takes place according to the aufbau principle which is based on Pauli's exclusion principle, the Hund’s rule of maximum multiplicity and the relative energies of the orbitals.
Aufbau Principle:
The word ‘aufbau’ in German means ‘building up’. The building up of orbitals means the filling up of orbitals with electrons. The principle states : In the ground state of the atoms, the orbitals are filled in order of their increasing energies. In other words, electrons first occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled.
Pauli Exclusion Principle:
The number of electrons to be filled in various orbitals is restricted by the exclusion principle,
According to this principle: No two electrons in an atom can have the same set of four quantum numbers. The Pauli exclusion principle can also be stated as : “Only two electrons may exist in the same orbital and these electrons must have opposite spin.”
Hund’s Rule of Maximum Multiplicity:
This rule deals with the filling of electrons into the orbitals belonging to the same subshell (that is, orbitals of equal energy, called degenerate orbitals). It states : pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has one electron each i.e., it is singly occupied.
The electrons in the completely filled shells are known as core electrons and the electrons that are added to the electronic shell with the highest principal quantum number are called valence electrons. Let us see the electronic configuration of elements with atomic number 56.
56= 2, 8, 8, 18, 18, 2.
So, it has 2 valence electrons.
Now from the option, option A which is 12 can be written as:
12= 2, 8, 2.
So, it also has 2 valence electrons.
Therefore, the element with atomic number 56 is likely to have the same outer shell configuration as the element with atomic number 12.

So, option A is correct.

Note:
The lower the value of (n + l) for an orbital, the lower is its energy. If two orbitals have the same value of (n + l), the orbital with lower value of n will have the lower energy. The maximum number of electrons in the shell with principal quantum number n is equal to \[2{n^2}\].