- Alkali metals and halogens (e.g., NaCl)

- Alkaline earth metals and halogens (e.g., CaF2)

Less ionic:

- Alkali metals and oxygen (e.g., Na2O)

- Alkaline earth metals and oxygen (e.g., CaO)

Least ionic:

- Covalent bonds (e.g., H2, Cl2)

The more electronegative the difference between two atoms, the more ionic the bond will be.