electrons...

Every electron is the same. Electrons are able to move from one energy level to another by emission or absorption of a quantum of energy, in the form of a photon. Because of the Pauli exclusion principle, no more than two electrons may exist in a given atomic orbital; therefore an electron may only leap to another orbital if there is a vacancy there.

Shells and subshells (also called energy levels and sublevels) are defined by the quantum numbers, not by the distance of its electrons from the nucleus, or even their overall energy. In large atoms, shells above the second shell overlap.

type s orbital 　　max 2 electrons
type p orbitals　　max 6 electrons
type d orbitals　　max 10 electrons
type f orbitals　　max 14 electrons
type g orbitals　　max 18 electrons


Shell 1 1s2
Shell 2 2s2 2p6
Shell 3 3s2 3p6 3d10
Shell 4 4s2 4p6 4d10 4f14
Shell 5 5s2 5p6 5d10
Shell 6 6s2 6p6
Shell 7 7s2

1s2 < 2s2 < 2p6 < 3s2 < 3p6 < 4s2 < 3d10 < 4p6 < 5s2 < 4d10 < 5p6 < 6s2 < 4f14 < 5d10 < 6p6 < 7s2 ...

element 19(K - Potassium) is 2,8,8,1
element 20(Ca - Calcium) is 2,8,8,2
because the energy for 4s2 is a bit lower than 3d10
this is not the only case, as 5s2 is also a bit lower than 4d10 , same as 6s2 is also a bit lower than 4f14 and 5d10. 7s2 is also a bit lower than 5f, 5g, 6d, 6f, and 6g levels.

element 37(Rb - Rubidium) is 2,8,18,8,1
element 38(Sr - Strontium) is 2,8,18,8,2
element 55(Cs - Caesium) is 2,8,18,18,8,1
element 56(Ba - Barium) is 2,8,18,18,8,2
element 87(Fr - Francium) is 2,8,18,32,18,8,1
element 88(Ra - Radium) is 2,8,18,32,18,8,2
etc.....

The orbital labels s, p, d, and f originate from a now-discredited system of categorizing spectral lines as sharp, principal, diffuse, and fundamental, based on their observed fine structure. When the first four types of orbitals were described, they were associated with these spectral line types, but there were no other names. The designation g was derived by following alphabetical order. Shells with more than five subshells are theoretically permissible, but this covers all discovered elements. For mnemonic reasons, some call the s and p orbitals spherical and peripheral.

In the ground state of an atom (the condition in which it is ordinarily found), the electron configuration generally follows the Aufbau principle. According to this principle, electrons enter into states in order of the states' increasing energy; i.e., the first electron goes into the lowest-energy state, the second into the next lowest, and so on.

The order of increasing energy of the subshells can be constructed by going through downward-leftward diagonals of the table above (also see the diagram at the top of the page), going from the topmost diagonals to the bottom. The first (topmost) diagonal goes through 1s; the second diagonal goes through 2s; the third goes through 2p and 3s; the fourth goes through 3p and 4s; the fifth goes through 3d, 4p, and 5s; and so on. In general, a subshell that is not "s"; is always followed by a "lower"; subshell of the next shell; e.g. 2p is followed by 3s; 3d is followed by 4p, which is followed by 5s, 4f is followed by 5d, which is followed by 6p, and then 7s.

I hope this can help explain it to you.