³⁷K and ⁴²K
Both atoms are of the same element. which means they have the same atomic number. Atomic number is the number of protons. Atomic number is characteristic for the element. Both having the same atomic number means they both have the same number of protons
the superscripts are for their masses. Atomic mass is the sum of the protons and neutrons. the 2 K atoms have different masses but same number of protons
which means the number of neutrons vary. These are called isotopes. Isotopes are atoms of the same element with different numbers of neutrons.
therefore An atom of potassium-37 and an atom of potassium-42 differ in their total number of neutrons
