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Course: High school chemistry > Unit 1

Lesson 3: The Bohr model and atomic spectra
Science
High school chemistry
Atoms, elements, and the periodic table
The Bohr model and atomic spectra
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The Bohr model

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Learn how Bohr models are used to represent atoms.
Atoms are way too small to see with the naked eye (and even most microscopes). So, we represent atoms using models.
Models help us visualize atomic structure. They also help us explain and predict the behavior of atoms.
However, it's important to remember that no scientific model is perfect. Every model sacrifices some accuracy for simplicity, visibility, or usability. If a model was perfect, it wouldn't be a model—it would be the real thing!
Atomic models are further complicated by quantum weirdness—electrons have both wave and particle properties.
Let's take a closer look at two atomic models, each with its own strengths and limitations.

The electron cloud model

An electron cloud model of a helium-4 atom is shown below.
A model of a helium atom shows a black circle which fades to white moving from the center to the outside. At the center of the circle is a tiny nucleus, consisting of two red circles and two purple circles.
In this model, the black "cloud" represents the volume of space where electrons are likely to be found. The darker the region, the more likely electrons are to be found there.
The nucleus is shown as a tiny clump of red protons and purple neutrons in the center of the atom.

Strengths

A strength of this model is how it represents the wave behavior of electrons. The fuzzy electron cloud represents how individual electrons are actually spread out through space. Until we measure an electron's position, we don't know exactly where it is. The best we can do is describe where we're likely to find electrons around a nucleus. Quantum mechanics is weird!
Another strength of this model is how the nucleus is represented. We can see the individual protons and neutrons, represented in different colors. The nucleus is very small compared to the size of the electron cloud, which is true for real atoms. (Though the real nucleus is even smaller—it would be invisible if we drew it to scale on this model.)

Limitations

The fuzzy electron cloud does a good job representing the wave nature of electrons. However, the model doesn't show electron particles. From this model, we can't even tell how many electrons the atom has!
Since most of chemistry involves tracking what electrons are doing, it's often useful to use a another model which represents electrons in a different way.

The Bohr model

A Bohr model of a neutral helium-4 atom is shown below.
A model of a helium atom shows a green circle in the middle. Around the green circle is a gray ring. On top of the gray ring sit two black dots.
In this model, the electrons are represented as black dots that sit on a ring around the nucleus. The nucleus is shown as one green circle in the center.

Strengths

The Bohr model represents the particle nature of electrons. So, it's easy to see that the atom above contains two electrons.
As we'll discuss later in the article, atomic electrons exist at specific energy levels. The Bohr model represents these energy levels as rings. We can tell that the two electrons in the model above are at the same energy level because they are on the same ring.

Limitations

By representing electrons as particles, the Bohr model does not reflect the wave properties of electrons. The electrons appear to exist in specific locations, which is not entirely true.
Additionally, the nucleus in a Bohr model is typically shown as one circle, regardless of how many protons and neutrons are present in the nucleus.

How are electrons arranged in Bohr models?

Helium's Bohr model shows that the first two electrons are in the same energy level. But what about elements with more electrons?
It turns out that the first energy level holds a maximum of two electrons.
Beginning with lithium, a second energy level begins to fill with electrons. That second energy level can hold a maximum of eight electrons.
After the second energy level is filled with eight, a third energy level begins to fill.
Bohr models of some elements from the first three rows (periods) of the periodic table are shown below.
Bohr models of the following elements are shown, with the corresponding numbers of electrons (e), shown as dots, in each shell. H: 1 e in first shell He: 2 e in first shell Li: 2 e in first shell, 1 e in second shell Be: 2 e in first shell, 2 e in second shell O: 2 e in first shell, 6 e in second shell F: 2 e in first shell, 7 e in second shell Ne: 2 e in first shell, 8 e in second shell Na: 2 e in first shell, 8 e in second shell, 1 e in third shell Mg: 2 e in first shell, 8 e in second shell, 2 e in third shell S: 2 e in first shell, 8 e in second shell, 6 e in third shell Cl: 2 e in first shell, 8 e in second shell, 7 e in third shell Ar: 2 e in first shell, 8 e in second shell, 8 e in third shell

Energy levels

The rings in a Bohr model represent the discrete energy levels that electrons can occupy. Electrons cannot exist at energies between these levels.
The energy levels in a Bohr model are also called shells. The shells are labelled as shown for the chlorine model below.
A Bohr model of a chlorine atom shows a nucleus surrounded by three concentric rings. The ring closest to the nucleus is labelled n=1, the second ring from the nucleus is labelled n=2, and the third ring from the nucleus is labelled n=3.
The higher the shell number, the greater the energy of electrons in that shell. For example, electrons in the n=3 shell of the Cl atom are at greater energy than electrons in the n=2 shell, which are at greater energy than electrons in the n=1 shell.
To summarize for the first two shells:
Energy level (shell)Maximum number of electronsElectron energy
n=12lowest possible energy for the atom
n=28greater energy than electrons in n=1
So, keep in mind that the shells of a Bohr model represent electrons' energy levels, NOT their positions or paths. Electrons do NOT move in circular paths around the nucleus.

Valence electrons

Electrons in the outermost shell of an atom are most easily transferred or shared with other atoms. So, an atom's outer electrons are usually the most important in chemistry.
The outermost shell of an atom is called the valence shell. Any electrons in the valence shell are called valence electrons.
Any electrons in an atom which are not in the valence shell are called core electrons.
In the chlorine model below, the valence electrons are shown in red, and the core electrons are shown in black.
A Bohr model of a chlorine atom shows a nucleus surrounded by three concentric rings. The ring closest to the nucleus contains 2 black dots, the second ring from the nucleus contains 8 black dots, and the third ring from the nucleus contains 7 red dots.
So, we can tell from this model that chlorine has seven valence electrons.

Additional notes about Bohr models

Keep these things in mind when working with Bohr models:
  • The rings of a Bohr model do NOT represent circular paths followed by electrons. Electrons do NOT orbit the nucleus like planets orbit the sun. The rings are simply a convenient way to represent electron energy levels.
  • Sometimes, you may see a Bohr model with rings that get closer together as they get farther out. This represents how the difference between energy levels decreases with greater n. However, since the rings are not intended to perfectly represent electron positions or energies, the exact spacing of the rings is not important.
  • Bohr models are not meant to represent what real atoms "look" like. In fact, real atoms are too small to look like anything! The particles inside them have no color and no definite shape. Everything is fuzzy and fluctuating.
No model is perfect. Still, Bohr models are useful for explaining certain atomic behaviors, particularly atomic spectra. They are...enlightening!

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