Biology » Chemical Foundation of Life » Atoms, Isotopes, Ions and Molecules: The Building Blocks

Electron Shells and the Bohr Model

Early Model of the Atom

It is important to stress that there is a connection between:

  • the number of protons in an element,
  • the atomic number that distinguishes one element from another, and
  • the number of electrons it has.

In all electrically neutral atoms, the number of electrons is the same as the number of protons. Thus, each element, at least when electrically neutral, has a characteristic number of electrons equal to its atomic number.

In 1913, Danish scientist Niels Bohr (1885–1962) developed an early model of the atom. The Bohr model shows the atom as a central nucleus containing protons and neutrons. It shows the atom’s electrons in circular orbits at specific distances from the nucleus. See the image below.

bohr-atom

The Bohr model was developed by Niels Bohrs in 1913. In this model, electrons exist within principal shells. An electron normally exists in the lowest energy shell available, which is the one closest to the nucleus. Energy from a photon of light can bump it up to a higher energy shell, but this situation is unstable, and the electron quickly decays back to the ground state. In the process, a photon of light is released. Image Attribution: Modified by Khan Academy from OpenStax Biology (CC BY-NC-SA 4.0)

How Electrons Fill Orbits

These orbits form electron shells or energy levels, which are a way of visualizing the number of electrons in the outermost shells. These energy levels are designated by a number and the symbol “n.” For example, 1n represents the first energy level located closest to the nucleus.

Electrons fill orbits in a consistent order: they first fill the orbits closest to the nucleus, then they continue to fill orbits of increasing energy further from the nucleus. If there are multiple orbits of equal energy, they will be filled with one electron in each energy level before a second electron is added. The electrons of the outermost energy level determine the energetic stability of the atom and its tendency to form chemical bonds with other atoms to form molecules.

Under standard conditions, atoms fill the inner shells first, often resulting in a variable number of electrons in the outermost shell. The innermost shell has a maximum of two electrons but the next two electron shells can each have a maximum of eight electrons.

Octet Rule

This is known as the octet rule, which states, with the exception of the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell. You can see some examples of some neutral atoms and their electron configurations in the image below.

bohr-diagrams

Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements (helium, neon, and argon here) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron configuration. Image Attribution: OpenStax Biology

Notice that in this image, helium has a complete outer electron shell, with two electrons filling its first and only shell. Similarly, neon has a complete outer 2n shell containing eight electrons. In contrast, chlorine and sodium have seven and one in their outer shells, respectively, but theoretically they would be more energetically stable if they followed the octet rule and had eight.

An atom may give, take, or share electrons with another atom to achieve a full valence shell, the most stable electron configuration. Elements in group 1 need to lose 1 electron in order to achieve a stable electron configuration.

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