* Atomic Number (Number of Protons): X has an atomic number of 18, while Y has an atomic number of 20. This means that X has 18 protons and Y has 20 protons.

* Mass Number (Protons + Neutrons): Both elements have a mass number of 40. This means that both X and Y have a total of 40 protons and neutrons combined.

The Relationship:

Elements X and Y are isotopes. Isotopes of an element have the same number of protons (atomic number) but differ in the number of neutrons.

Here's how it works:

* Element X: With an atomic number of 18 (18 protons), it needs 22 neutrons to achieve a mass number of 40 (18 protons + 22 neutrons = 40).

* Element Y: With an atomic number of 20 (20 protons), it needs 20 neutrons to achieve a mass number of 40 (20 protons + 20 neutrons = 40).

Conclusion:

Elements X and Y are isotopes. X has 22 neutrons, while Y has 20 neutrons. They are different forms of the same element, but their differing neutron counts give them slightly different properties.

Note: Element X is Argon (Ar) and element Y is Calcium (Ca).