Electron configuration of atoms
Atoms consist of a nucleus surrounded by electrons, which occupy regions of space called electron shells. These shells are organized around the nucleus in increasing levels of energy, and electrons within them are arranged according to specific rules based on quantum mechanics. The distribution of electrons across shells and subshells determines the chemical properties of an element.
Orbital is the the name of the space in an atom occupied by an electron. A subdivision of the available space within an atom for an electron to orbit the nucleus. Each orbital has a fixed size and shape and can hold up to two electrons.
| Principal energy level Quantum number: n | Sublevels available Quantum number: l |
|---|---|
| 1 | 1s |
| 2 | 2s 2p |
| 3 | 3s 3p 3d |
| 4 | 4s 4p 4d 4f |
| 5 | 5s 5p 5d 5f 5g |
| 6 | 6s 6p 6d 6f 6g 6h |
| Subshell | Number of orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
| g | 9 | 18 |
| h | 11 | 22 |
Electron configuration is the arrangement of electrons in an atom’s shells and subshells. This configuration follows a specific set of rules:
- Aufbau Principle: Electrons fill orbitals starting from the lowest energy level to the highest.
- Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons with opposite spins.
- Hund’s Rule: Electrons will fill degenerate orbitals (orbitals of the same energy, like those in the same subshell) singly before pairing up.
The simplest configuration is for Hydrogen: 1s1. The superscript shows that there is one electron in the 1s orbital.
In the periodic table beyond Neon (Ne), each element's electron configuration is shown in an abbreviated form that starts with the symbol of the noble gas that precedes it. For example, the abbreviated configuration for Sodium is [Ne] 3s. The configuration for Neon [Ne] is 1s22s22p6. Thus, substituting the config of Ne gives the full config for Sodium: 1s22s22p63s
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 (see Table below).
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 an explanation of these aspects, see the reference by Schwarz listed below.
The table below shows the full forms of the electron configurations of noble gases.
| No. | A. weight | Name | Symbol | Group | Electron configuration |
|---|---|---|---|---|---|
| 2 | 4.002 | Helium | He | 18 | 1s2 |
| 10 | 20.180 | Neon | Ne | 18 | 1s22s22p6 |
| 18 | 39.950 | Argon | Ar | 18 | 1s22s22p63s2 3p6 |
| 36 | 83.798 | Krypton | Kr | 18 | 1s22s22p63s2 3p63d10 4s2 4p6 |
| 54 | 131.293 | Xenon | Xe | 18 | 1s22s22p63s2 3p63d10 4s2 4p64d10 5s2 5p6 |
| 86 | 222.000 | Radon | Rn | 18 | 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.
References
- 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
- Kramida, A., Ralchenko, Yu., Reader, J., and NIST ASD Team (2023). NIST Atomic Spectra Database (ver. 5.11),
Also see: The Orbitron: A gallery of atomic orbitals and a few molecular orbitals