Question

What is the electronic configuration of Lithium [Atomic number=3, Mass number =7]
A. $2$
B. $2,3$
C. $2,2$
D. $2,1$

Answer 1

Hint: Lithium is the first alkali metal in the periodic table. It is represented by symbol Li. The metal with lowest density is Lithium. It is soft, white and lustrous.
Lithium belongs to group 1 and period 2 in the periodic table.
Since Lithium belongs to group 1 , the outermost orbital is the s-orbital
Lithium is an s-block element and readily loses its electrons.

Complete step by step answer:
s, p, d, f etc. are the representation given to the orbitals which hold the electrons in atoms. There are three rules for giving an electronic configuration to any element in the periodic table.

Aufbau Principle: According to the Aufbau principle,orbitals are filled in order of increasing energy .This means the orbital with lowest energy will be filled first.

According to the diagram given , the order of increasing energy is as follow
1s<2s<2p<3s<3p<4s<3d<4p<5s
 
Hund’s Rule : According to Hund’s rule, the electrons should occupy all the degenerate orbitals first ,before doubling in the orbitals

 
Pauli Exclusion principle: If two electrons are filled in the same orbital they must have opposite spins
i.e.
Now, Lithium has atomic number 3, so the total electrons to be filled are 3 and it belongs to group 1  ,so therefore the outermost orbital is s
Applying all the principles, we get

Hence the correct answer is option D,that is, 2,1.

Note: Lithium is the only alkali metal that reacts with nitrogen to form black nitrure. It Is the lowest density metal and has two isotopic form i.e. Li6 and Li7 . Lithium is very reactive, Since Lithium is very reactive with water and air, it is wrapped in paraffin wax.