Final answer:
Hypervalency is when some anion-cation bonds take more than half of the anion charge, relating to molecules having more than eight electrons in their valence shell, contrary to the octet rule.
Step-by-step explanation:
The term you are looking for when some anion-cation bonds take more than half of the anion charge is hypervalency. This concept is especially relevant in the context of molecular chemistry, where a hypervalent molecule contains at least one main group element that has more than eight electrons in its valence shell. Ionic bonds occur when the electronegativity difference between bonded atoms is large, leading to a complete transfer of electrons and the formation of cations and anions.
An example of hypervalency can be seen in molecules such as sulfur hexafluoride (SF6), where sulfur has twelve electrons in its valence shell despite having only six valence electrons in its neutral ground state. This arrangement is attributed to sulfur's ability to expand its octet under certain conditions, thus permitting the formation of additional bonds beyond the standard octet rule.
Contrastingly, in a double covalent bond like that in oxygen gas (O=O), two atoms of oxygen share two pairs of electrons to achieve stability. The sharing of electrons in covalent bonds can lead to either nonpolar covalent bonds, where electrons are shared equally, or polar covalent bonds, where electrons are shared unequally, leading to a dipole moment.