Question

An atom has mass number 40 and atomic number 20.
(i) How many electrons are revolving around the nucleus?
(ii) How many electron shells are there in the atom?

Answer 1

Hey there !!

Atomic number = Z = 20

We know that 
Number of protons = Atomic number 
Hence , Number of protons = 20

Mass number = 40

1) How many electrons are revolving around the nucleus?
ans :-
The number of electrons in an atom is equal to the no: of protons.
Hence , no: of electrons = 20

2)How many electron shells are there in the atom?
Ans:-
No: of electrons = 20
Electronic configuration = 2 , 8 , 8 , 2

Hence , there are 4 electron shell :-
K shell [ 2 electrons]
L shell [ 8 electrons ]
M shell [  8 electrons ]
N shell [ 2 electrons ]

Answer 2

the element is calcium.
1.total 20electrons are revolving around nucleus.
2.k shell(2)L shell(8),M shell(18).so 4shells.