Lithium, beryllium and boron often behave differently from the heavier members of their groups. Which of the statements below are correct?

• Statement I: The maximum covalency of a second-period element is 4, because it has only four valence orbitals.
• Statement II: Boron forms the ion [BF₆]³⁻ just as aluminium forms [AlF₆]³⁻.
• Statement III: Lithium shows several similarities to magnesium, an example of a diagonal relationship.
• Statement IV: Sodium oxide, Na₂O, is amphoteric.