Valence Electron and Core Notation

Core Notation:
As we know that, writing a complete electron configuration takes a bit time and it's easy to make mistakes even we memorize it. So Core Notation can save us sometimes.
Basically it uses another element from the group "Nobel gas" before itself to replace the first part of electron configuration, and doesn't affect the electrons number of the original atom.

For example:
Electron Configuration : Al = 1s2 2s2 2p6 3s2 3p1
Core Notation: Al = [Ne] 3s2 3p1

Electron Configuration: O = 1s2 2s2 2p4
Core Notation: O = [He] 2s2 2p4

Electron Configuration : K = 1s2 2s2 2p6 3s2 3p6 4s1
Core Notation : K = [Ar] 4s1

little notes: the number of the noble gas is actually repersenting the first part of the electron configuration, so once they add up, the electrons number won't be affected as a result.

In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Basically, as we know from the Bohr diagram, to be able to achieve a reaction, both reactant needed to have either Open shell or Closed shell to allow or provide electrons. And to predict the valence electrons, we used to draw a diagram, but now we have one more method from electron configuration.

Frist we have to know the role, valence electrons are not counted in the d- and f- subshells. which only refer to s- and p-.

For example:
Electron Configuration : Al = 1s2 2s2 2p6 3s2 3p1
Core Notation: Al = [Ne] 3s2 3p1

From here  [Ne] 3s2 3p1,   2 + 1 = 3
so the valence electrons are 3 from Al

Electron Configuration: O = 1s2 2s2 2p4
Core Notation: O = [He] 2s2 2p4
Valence electrons = 2 + 4 = 6

Electron Configuration : K = 1s2 2s2 2p6 3s2 3p6 4s1
Core Notation : K = [Ar] 4s1
Valence electrons = 1