Final answer:
Hydrogen (H) and Oxygen (O) would be the least likely to form a compound by ionic bonding, as they are both nonmetals and tend to form covalent bonds, as seen in H₂O₂ (hydrogen peroxide).
Step-by-step explanation:
The pairs of elements least likely to form a compound by ionic bonding would be Hydrogen (H) and Oxygen (O). Hydrogen, despite being located in group 1, is a nonmetal, and Oxygen is a nonmetal found in group 16. Compounds formed between nonmetals typically exhibit covalent bonding rather than ionic bonding.
In the case of Hydrogen and Oxygen, the compound they form is H₂O₂ (hydrogen peroxide), which is a molecular compound with covalent bonds. The tendency for forming ionic bonds is higher when electronegativity difference between the bonding atoms is significant, which is common when metals from the left side of the periodic table (such as Potassium) bond with nonmetals from the right side (such as Oxygen).