An element is represented as ${}_{20}{}^{40}A$.

(a) What is its atomic number?

(b) What is its mass number?

(c) What is the number of protons in its nucleus?

(d) What is the number of electrons in its atom?

(e) What is the number of neutrons in its nucleus?

(g) What is its valency?

(a) Atomic number is defined as the total number of protons present inside the nucleus of the atom

Atomic number = 20

(b) Mass number is defined as the sum of the number of protons and number of neutrons present inside the nucleus of an atom.

Mass number = number of protons+ number of neutrons

Mass number = 40

(c) The number of protons in its nucleus = Atomic number = 20

(d) The number of electrons = Atomic number= number of protons = 20

(e) The number of neutrons = Mass number - Atomic number = 20

(g) Valency is the the number of valence electrons lost or gained by an atom in order to attain its stability.

electronic configuration of A = 2,8,8,2

Therefore valency = 2