There are two elements-calcium (atomic number 20) and argon (atomic number 18). The mass number of both the elements is 40. They are therefore known as

Isotopes-These  are  atoms  of  the elements  having  the  same  atomic number but different mass number.,  Isobars-These  are atoms of same mass number but different atomic  number.,  Isotones-These are  atoms  of  different  elements having same number of neutrons.