Electrons are mapped out in terms of energy levels, sublevels, and orbitals. The electron cloud is divided into sections, called energy levels, each containing a different number of sublevels and electrons. The energy levels are represented by whole numbers, starting with energy level 1 closest to the nucleus.
Each orbital holds a maximum of two electrons. Orbitals with the same potential energy are grouped together to form sublevels, which are labeled with letters such as s, p, d, and f.
An atom’s orbitals are arranged by their potential energy values. Each energy level of an atom is organized by its energy: the more potential energy, the farther the energy level is from the nucleus.
Building out from the nucleus, each energy level contains an additional sublevel, meaning that each energy level can hold more electrons than the level below it.
Energy level 1 contains an s-sublevel, meaning it holds a total of 2 electrons.
Energy level 2 contains an s-sublevel and a p-sublevel, meaning it holds a total of 8 electrons.
Energy level 3 contains an s-sublevel, p-sublevel, and d-sublevel. This means the third energy level can hold a total of 18 electrons.
Energy level 4 contains an s-sublevel, p-sublevel, d-sublevel, and f-sublevel. This means the fourth energy level can hold a total of 32 electrons.
Each energy level above the fourth level contains more sublevels and orbitals, meaning they can hold even more electrons. The additional sublevels in these higher energy levels are labeled in alphabetical order (g, h, i, etc.). The quantum model of the atom mathematically maps out an atom’s orbitals well beyond what is occupied by ground state electron configurations.
Just as you saw that the rules for dorm occupancy affected the arrangement of residents in the dormitory building, there are some rules that will help you write electron configurations correctly.
The word aufbau means ‘building up’ in German. The Aufbau Principle tells us that an electron occupies the lowest energy orbital (closest to the nucleus) available in an atom’s electron cloud. This means that when you map out the arrangement of an atom’s electrons, you always start at the lowest energy level first.
The order that orbitals are filled: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…etc.
The order in which atomic orbitals are filled is based on the potential energy of each position. The 1s fills first because it has the lowest energy, followed by the 2s, 2p, 3s, and 3d sublevels. Notice that the 4s sublevel is filled after the 3p sublevel, and then followed by the 3d sublevel. This is because the potential energy of the 4s sublevel is lower than the 3d sublevels. It is important to fill orbitals and sublevels based on their potential energy and not by the overall energy level that they occupy.
Remember that objects with the same electrical charge repel each other. Because negatively charged electrons repel each other, they spread out as much as possible within the orbitals of a given sublevel. This is described by Hund’s rule, which says that orbitals of equal energy are each occupied by one electron before any orbital will receive a second electron.
In the example below, an atom has seven electrons. Electrons can be represented by arrows and orbitals represented by lines.
Following the Aufbau Principle, we start filling the first energy level. The first energy level has an s-sublevel, and the second energy level has an s-sublevel and p-sublevel. The s-sublevel in the energy level is filled first, followed by the p-sublevel.
1s is filled with two electrons (represented by two arrow each point in opposite directions). 2s is filled with two electrons (represented by two arrow each point in opposite directions). 2p is filled with three electrons. One electron is in orbital (represented by three arrows one in each orbital all pointing in the same direction).
In the 2p sublevel, where there are three orbitals with equal potential energy, the electrons spread out as much as possible. This is following Hund’s rule, placing one electron in each orbital of a sublevel before any electron in the sublevel receives a second electron.
Hund’s rule also says that electrons in the same orbital will spin in opposite directions (shown by the arrows in the s orbitals pointing in opposite directions) and that electrons in singly occupied orbitals will all spin in the same direction (shown by the single arrows in the p sublevel all pointing in the same direction).