Ocr chemistry a level

Could you be a bit more specific? I don't know exactly what you're referring to

Okay so what you're referring to is the octet rule, where to maximise stability, atoms have a tendency to prefer having 8 electrons in its valence shell. There are 2 exceptions to this rule: Incompletion of the octet rule and Expansion of the octet rule

Incompletion:
Where the atom is still stable with less than 8 electrons in its valence shell.
Groups 1,2,3, have atoms with less than 4 valence electrons, so they do not normally form covalent bonds. Boron is an exception, and so can act as an example of the incompletion of the octet rule. The electronic configuration of Boron is - 1s^2 2s^2 2p^1
Here, Boron has 3 valence electrons in its valence shell (2s and 2p are different subshells of the SAME SHELL, remember not to get confused between orbitals, subshells and shells).
Boron is able to form the compound BF3, where Boron forms 3 covalent bonds, resulting in 6 valence electrons surrounding the Boron atom. This compound is still stable with less than 8 electrons surrounding an atom, and thus is an exception.

I'm not aware of any other examples of incompletion of the octet rule.

Expansion:
Where the atom is stable with MORE than 8 electrons in the valence shell. This occurs from period 3 onwards, because the valence shell becomes the 3rd shell, and the 3rd shell can hold up to 18 electrons (because of its electron configuration - 3s^2 3p^6 3d^10 = 2+6+10 = 18 electrons) And so, the d subshell has become accessible for accepting electrons ---> period 3 elements do NOT store their valence electrons in the d subshell, BUT the d subshell has now become accessible to accept electrons, thus allowing MORE than 8 electrons in its valence shell.

Examples include:
PF5 - Phosphorous in elemental form has 5 electrons in its valence shell - 1s^2 2s^2 2p^6 3s^2 3p^3 - and so can form up to 5 covalent bonds. In this compound there are 10 valence electrons surrounding the phosphorous atom and it is still stable

SF4 - Sulphur in elemental form has 6 valence electrons - 1s^2 2s^2 2p^6 3s^2 3p^4 - and so can form up to 6 covalent bonds. In this compound there are 10 valence electrons surrounding the sulfur atom (6+4 =10)
SF6 - In this compound there are 12 valence electrons surrounding the sulfur atom.

ClF3 - Chlorine in elemental form has 7 valence electrons - 1s^2 2s^2 2p^6 3s^2 3p^5 - and so can form up to 7 covalent bonds. In this compound there are 10 valence electrons (7+3) surrounding the chlorine atom.
ClF5 - In this compound there are 12 valence electrons (7+5) surrounding the chlorine atom.
ClF7 - In this compound there are 14 valence electrons (7+7) surrounding the chlorine atom.

IF7 - Iodine in elemental form has 7 valence electrons - 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 4d^10 5s^2 5p^5 - and so can form up to 7 covalent bonds. In this compound there are 14 valence electrons (7+7) surrounding the iodine atom.