What element has a mass number of 40?
Argon [Ar] has mass number (A) = 40.
Mass number (A) of an element is equal to sum of protons and neutrons present in its nucleus.
So, in Argon
no. of electrons = 18
no. of protons = 18
no. of neutrons = 22
A = no. of neutrons + no. of protons
A = 22 + 18 = 40