Question

What are the KLMN notations for K and Ca ? Why is the third digit no greater than 8 for these elements?

Answer 1

According to quantum physics, each electron configuration has a level of energy associated with it, and electrons may migrate from one configuration to another by emitting or absorbing a quantum of energy in the form of a photon under specific conditions.

Complete answer:
An electron shell is an orbit that electrons follow around the nucleus of an atom. The "1 shell" (also known as the "K shell") is the closest to the nucleus, followed by the "2 shell" (or "L shell"), then the "3 shell" (or "M shell"), and so on as you get further away from the nucleus. The shells are either named alphabetically with the letters used in X-ray nomenclature (K, L, M,...) or they correlate to the main quantum numbers (n = 1, 2, 3, 4,...).
Only a certain amount of electrons can fit into each shell: The first shell may store two electrons, the second shell eight (2 + 6) electrons, the third shell 18 (2 + 6 + 10) electrons, and so forth. The nth shell may theoretically store up to 2(n2) electrons, according to the general formula. See electron configuration for an explanation of why electrons exist in these shells. Each subshell is made up of one or more atomic orbitals, and each subshell is made up of one or more subshells.
For K it is 2, 8, 8, 1
The initial two electrons in the electron configuration for potassium will be in the 1s orbital. Because the 1s orbital can only store two electrons, Potassium's next two electrons are placed in the 2s orbital. The six electrons after that will be in the 2p orbital. Up to six electrons can be held in the p orbital. Six electrons will be placed in the 2p orbital, followed by two electrons in the 3s orbital. We'll shift to the 3p to deposit the following six electrons because the 3s is now filled. The leftover electron will now be placed in the 4s orbital.
Hence the configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{1}}$
For Ca it is 2, 8, 8, 2
In 4s, 2 electrons occupy.
Hence the configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}$
Because potassium and calcium don't need to employ 3 d orbitals, which might have held up to 10 more electrons in the M "shell," they don't. The 3d orbitals of K and Ca, in particular, are now more energetic than the 4s. When we get to Sc, though, things change.

Note:
The orbital energy of an electron is the energy associated with it. The energy of a configuration is frequently calculated as the sum of the energies of each electron, with the electron-electron interactions ignored. The ground state is the configuration that corresponds to the lowest electronic energy. Any other arrangement is a state of excitement.