Having too few valence electrons to ever obtain an octet.

• Hydrogen, Beryllium and Boron have two few valence electrons to ever obtain a full octet. See this.
• Hydrogen can have at most 2 valence electrons after it shares its electron with another atom.
• Beryllium will have 4 valence electrons after it has finished bonding.
• Boron will have 6 valence electrons after it shares its valence electrons with other atoms.

Expanding the octet to have 10, 12 or 14 valence electrons instead of 8.

• elements in periods 3, 4, 5, 6 and 7 can expand their octet to have 10, 12, or 14 valence electrons. See how this can happen.