What is the mass number of an element, the atom of which contains two protons, two neutrons and two electrons?

The number of electrons and neutrons in an element is 18 and 20 respectively. Its mass number is

If an atom X has 10 electrons and its mass number is 22, calculate number of protons for X

What is the maximum number of electrons that can occupy the first shell of an atom ?

Atomic mass of an element is equal to the sum of number of

The atoms, in which the number of protons is same but the number of neutrons is different, are known as –