The elements that are most likely to combine with each other to form an ionic compound are those from Group 1 (alkali metals) and Group 17 (halogens). This is because the alkali metals have one valence electron that they can easily give up, while the halogens have seven valence electrons and need only one more to fill their outer shells. When an alkali metal and a halogen react, the alkali metal atom loses its valence electron to the halogen atom, forming a positive ion (cation) and a negative ion (anion), respectively. The electrostatic attraction between the positive and negative ions holds the compound together, forming an ionic bond.

For example, when sodium (Na) and chlorine (Cl) react, the sodium atom loses its valence electron to the chlorine atom, forming Na+ and Cl- ions. These ions then combine to form the ionic compound sodium chloride (NaCl).

Other examples of ionic compounds formed between Group 1 elements and Group 17 elements include potassium fluoride (KF), calcium iodide (CaI2), and lithium bromide (LiBr).