The specific arrangement of electrons in atomic orbitals is called the electron configuration of the atom. It determines many physical and chemical properties of that atom. The periodic table, which is arranged in accordance with the properties of the elements, can therefore be used to predict the ground state electron configurations of atoms.
An electron configuration is written symbolically to provide three pieces of information: the principal quantum number (shell number), n; a letter that designates the subshell (s, p, d, etc.); a superscript showing the number of electrons in that particular subshell. For example, the notation 2p4 indicates 4 electrons in a p subshell (ℓ = 1) with a principal quantum number (n) of 2.
For any element, the ground state electron configuration can be built up by starting with hydrogen and following the atomic-number order through the periodic table. To go from one element to the next, add one proton (and one or more neutrons) to the nucleus and one electron to the lowest energy subshell that has an incompletely filled orbital. Repeat until you reach the desired element. This process of filling electrons into orbitals is called the aufbau principle, from the German word Aufbauen (“to build up”). Watch the video below to see how to use the aufbau principle to determine the electron configurations of oxygen and chromium.
Writing the complete electron configuration all the time can be cumbersome, so chemists often abbreviate by using the noble-gas notation. For example, the ground-state electron configuration of vanadium (V) is 1s22s22p63s23p64s23d3. The noble gas that immediately precedes V is argon (Ar); it has a ground-state electron configuration of 1s22s22p63s23p6, which can be represented as [Ar]. Thus the ground-state electron configuration of V can be shortened to [Ar]4s23d3, and it communicates the same information as the complete electron configuration. A list of ground state electron configurations for all elements in the appendix uses noble-gas notation.
The aufbau principle is based on the concept that for ground-state electron configurations, an electron occupies a lower energy atomic orbital rather than a higher energy orbital. Hence, the fact that we observe the 4s orbital fill before the 3d orbitals indicates that the 4s orbital is lower in energy. Similarly, a 6s orbital is lower in energy compared to a 4f orbital.
The energy difference between s, p, d, and f subshells causes orbitals with different n values to have similar energies. In many cases these energies are so similar that there are exceptions to the periodic-table prediction of electron configuration. These exceptions occur for d-block and f-block elements, but not for s-block and p-block elements.