An atom's electron configuration describes the distribution of its electrons in the atomic orbitals ordered by the orbitals' energy levels. The simplest configuration is for Hydrogen: 1s1. This shows that there is one electron in the 1s orbital.
In the periodic table beyond Helium (He), each element's electron configuration is shown in an abbreviated form that starts with the symbol of the noble gas that precedes it.
|Principal energy level
(Quantum number: n)
(Quantum number: l)
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
For example, for Potassium (K) (atomic #19), the preceding noble gas is Argon (Ar) (atomic #18).
Thus, the configuration shown for Potassium is [Ar]4s1.
Potassium (K) atom has 19 electrons. The full electron configuration of Potassium (K) is 1s22s22p63s23p64s1.
The abbreviated form - [Ar]4s1 - means the electron configuration of Argon (Ar), plus one electron in the 4s orbital. Argon has 18 electrons. The one additional electron configuration completes the picture for 19 electrons of Potassium.
Note that in the electron configuration of both K and Ca, the 4s orbital is filled before the 3d orbital. The reason for this is that the energy level of orbital 4s is slightly lower than that of orbital 3d. Therefore, orbital 4s is filled with electrons prior to orbital 3d.
Starting with Scandium (Sc, atomic #21), the 3d orbital has a lower energy than the 4s. Thus, the electron configuration of Sc is [Ar] 3d1 4s2. For a detailed explanation of these aspects, see the reference by Schwarz listed below.
The table below shows the abbreviated and full forms of the electron configurations of noble gases.
|Name||Symbol||Group||Electron configuration||Electron configuration - Full|
|10||20.180||Neon||Ne||18||[He] 2s2 2p6||1s22s22p6|
|18||39.948||Argon||Ar||18||[Ne] 3s2 3p6||1s22s22p63s2 3p6|
|36||83.800||Krypton||Kr||18||[Ar] 3d10 4s2 4p6||1s22s22p63s2 3p63d10 4s2 4p6|
|54||131.293||Xenon||Xe||18||[Kr] 4d10 5s2 5p6||1s22s22p63s2 3p63d10 4s2 4p64d10 5s2 5p6|
|86||222.000||Radon||Rn||18||[Xe] 4f14 5d10 6s2 6p6||1s22s22p63s2 3p63d10 4s2 4p64d10 5s2 5p64f14 5d10 6s2 6p6|
Abbreviations and Definitions:
No. - Atomic number: The number of protons in an atom. Each element is uniquely defined by its atomic number.
Atomic mass: The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. Atomic mass is measured in Atomic Mass Units (amu) which are scaled relative to carbon, 12C, that is taken as a standard element with an atomic mass of 12. This isotope of carbon has 6 protons and 6 neutrons. Thus, each proton and neutron has a mass of about 1 amu.
Schwarz E.W. The full story of the electron configurations of the transition elements. Journal of Chemical Education (2010) 87(4) 444-448. https://doi.org/10.1021/ed8001286