Some atoms of K have 20 neutrons, others have 21, and others have 22. 40 K is radioactive and decays to 40 Ar and 40 Ca. Structure of Atoms Electrons orbit around the nucleus in different shells, labeled from the innermost shell as K, L, M, N, etc. Each shell can have a certain number of electrons. Thus, atoms often loose electrons or gain electrons to obtain stable configuration.
Noble gases have completely filled outer shells, so they are stable.
Examples He, Ne, Ar, Kr, Xe, Rn. Others like Na, K loose an electron. This causes the charge balance to become unequal. The drive to attain a stable electronic configuration in the outermost shell along with the fact that this sometimes produces oppositely charged ions, results in the binding of atoms together. When atoms become attached to one another, we say that they are bonded together.