In this tutorial, you will learn about electron shells, the different subshells, and the orbitals where electrons can be found.
Topics Covered in Other Articles
- How to Write Electron Configurations
- Electron Orbitals and Orbital Shapes
- Sigma and Pi Bonds
- Valence Bond Theory
- Bohr Model of the Atom
The Bohr Model
Niels Bohr developed an early model of the atom in 1913. According to the Bohr model electrons occupy fixed circular orbits around the nucleus of an atom. Every electron shell has a different energy level. The shells closest to the nucleus have the lowest energy and the shells farther away have higher energies. The distance between each shell and the nucleus determines its number. Each quantum number is given the letter n after its energy level. The quantum numbers begin at 1 and increase by 1 for each energy level. For example, the energy level closest to the nucleus is 1n.
The Bohr model is not an accurate representation of where electrons are at a specific moment in time. This approach is still taught due to its simplicity.
*Note the nucleus or center of the atom, contains the protons and neutrons.
In very simple terms an electron shell is the outside part of an atom that surrounds the atomic nucleus. The shells are orbital paths that are followed by electrons around the nucleus. Like everything in chemistry, electrons like to follow the path of least resistance. This means that electrons will usually fill up the shells of an atom from the inside out. Starting at the lowest energy level and working its way out. Each shell has a maximum amount of electrons it can hold. For example, shell 1n can hold 2 electrons, shell 2n can hold 8 electrons, and shell 3n can hold 18 electrons. The rule to calculate the number of electrons that each shell can hold is 2n2. E.g. the first shell is 2(1)2 which gives you 2 electrons.
In the diagram above the energy levels are depicted as the rings around the nucleus of the atom.
The outermost layer of electrons is known as its valence shell. The valence shell electrons determine an element’s affinity to bond. When valence shells are full this usually is a good indicator that the electron is stable. The magic number when it comes to valence shells is the number eight. Most elements follow the octet rule, which means that in order for an element to be stable it needs eight electrons in its outermost energy level.
*Note the octet rule is not true for every element on the periodic table.
Energy levels are split into subshells. Each energy level contains a certain number of subshells. Subshells are smaller parts within each electron shell. The most common, and the first four subshells, are s, p, d, and f.
Since we know that each electron shell has its own energy level when the principal quantum number increases the shell increases in energy levels. This is the same for each subshell within a shell. Subshell f has the highest energy level than d, p, and s. Each subshell in one of the electron shells will have an energy level that is lower than the shell above it. So, the subshells in energy level 3 will have lower energy than those in energy level 4.
One of the flaws in the Bohr model is the depiction of electrons moving around the nucleus of an atom in perfectly circular orbits. Electrons can be anywhere inside the electron cloud at any moment in time. Orbitals come in different shapes: s, p, d, f. each of these orbitals is shaped differently, and depict the way an electron might float around the nucleus.
*Note the definition of an orbital is the defined regions of space where electrons can be found 95 percent of the time.
So as seen above, the S orbital may have one spherical orbital, but the p subshell has three dumbbell-shaped orbitals. There are different ways of depicting orbital diagrams, which can be found in this article.
You can use what is known as an electron configuration to depict where the electrons are in a molecule or an atom. The electron configuration will tell you what energy level, subshell, and how many electrons occupy that subshell.
The electron configuration in the picture above tells us that the electrons are housed in energy level one in the s subshell with only two electrons.