My textbook states,
Period 2 elements typically obey the octet rule. Period 3 and later elements can expand their valence shells. Elements in Periods 5 and 6 of the p-block show variable valence (the inert-pair effect).
I was surprised that it said period 2 elements typically obey the octet rule. What are some circumstances in which period 2 elements break the octet rule? I though they couldn't because they don't have available d orbitals.
Typically the octet rule becomes less likely to work, regardless of the period, when we move to a group where there are too few valence electrons to go around. Prominent examples in Period 2 include compounds of beryllium in Group 2 (such as the fluoride, especially when gaseous or liquid) and boron in Group 13 (as in alkyl boranes like this one). Carbon can also form species that are most stable with fewer electrons than the octet rule would predict, a "violation" most prominently seen in the cyclopropenyl cation, where another "rule" that is much more reliable when properly used comes into play.
