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Understanding the Bohr Atomic Model

Posted by Hayley Milliman | Sep 24, 2019 4:00:00 PM

General Education

 

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Atoms are the basic building blocks of chemistry. They are the smallest unit which matter can be divided into without releasing electrically charged particles.

Scientists have a number of ways for describing the structure of atoms. In this article, we’ll focus on one of those models: the Bohr model, which was first proposed by Niehls Bohr in 1913.

In this article, we’ll explain what the Bohr atomic model is, give a Bohr diagram for the first element, as well as provide information for the 20 elements on the periodic table, and explain how the Bohr model is used.

 

What Is the Bohr Atomic Model?

The Bohr Model is a modification of an earlier atomic model, the Rutherford Model. The Bohr Model has an atom with a positively-charged nucleus surrounded by negatively-charged electrons that have circular, planetary-like orbits.

Today, we know that the Bohr Model has some inaccuracies, but it’s still used because of its simple approach to atomic theory. The Bohr model was also the first atomic model to incorporate quantum theory, meaning that it’s the predecessor of today’s more accurate quantum-mechanical models.

In the Bohr model, the electrons travel in defined circular orbits around the small positively-charged nucleus. The Bohr Model is known as a planetary model because these orbits look similar to that of planets orbiting the sun.

 

The Bohr Model, Explained

There are three main factors that characterize the Bohr model.

  • First, in the Bohr Model, the orbits have a set size and energy.
  • There’s a correlation between energy and size: the smaller the orbit, the less energy it has.
  • Electrons can move from one orbit to another by emitting or absorbing radiation.

According to Bohr, electron orbits could only hold a certain number of electrons. If an orbit was full, the remaining electrons would create a new orbit. That’s why, when you look at a Bohr diagram, you’ll sometimes see more than one electron orbit circling the nucleus. That happens when there are too many electrons to be housed in a single orbit.

 

Bohr Diagram: The First Element

In order to make a Bohr diagram, you need to know the number of protons, neutrons, and electrons the element has. In this section, we’ll show a sample Bohr diagram for hydrogen.

H —Hydrogen

1 proton

1 electron

0 neutrons

null

 

You can see the principles outlined in the section above at work in the Bohr model for the hydrogen atom. In the model for the hydrogen atom, a negatively-charged electron orbits a small positively-charged nucleus. In heavier atoms, there are more protons in the nucleus than in the hydrogen atom. Because of this, there are more electrons to cancel out the positive charge of the protons. That also means there will be more orbits around the nucleus.

 

Bohr Diagram: The Next 19 Elements

If you want to make a Bohr Diagram for another element, you’ll need to know the number of protons, neutrons, and electrons it needs. We outline those in this section.

Element Protons Neutrons Electrons
Helium (He) 2 2 2
Lithium (Li) 3 3 4
Beryllium (Be) 4 4 5
Boron (B) 5 5 6
Carbon (C) 6 6 6
Nitrogen (N) 7 7 7
Oxygen (O) 8 8 8
Fluorine (F) 9 9 10
Neon (Ne) 10 10 10
Sodium (Na) 11 11 12
Magnesium (Mg) 12 12 12
Aluminum (Al) 13 13 14
Silicon (Si) 14 14 14
Phosphorus (P) 15 15 16
Sulfur (S) 16 16 16
Chlorine (Cl) 17 17 18
Argon (Ar) 18 18 22
Potassium (K) 19 19 20
Calcium (Ca) 20 20 20

 

Bohr Atomic Model: What You Need to Know

As we mentioned before, the Bohr model has a few inaccuracies which were fixed in later variations of atomic models.

First, the Bohr model violates the Heisenberg Uncertainty Principle, since it states that electrons have a known radius and orbit.

The Bohr Model also provides an incorrect value for the ground state orbital angular momentum and doesn’t work as well for creating diagrams of larger atoms. The Bohr Model also does not explain the Zeeman Effect or the fine or hyperfine structure in specula lines.

The Sommerfeld model arose as a refinement to the Bohr model and fixes many of the problems with the Bohr model, including that the electrons travel in elliptical orbits rather than in circular orbits.

 

Bohr Atomic Model: Final Thoughts

The Bohr model was an advancement to an earlier atomic model, the Rutherford Model. Key features of the Bohr model include set orbits for electrons around the nucleus, as well as the ability of electrons to move from one orbit to another.

The Bohr model was later superseded by the Sommerfield model.

 

What’s Next?

Studying DNA? You'll need to know what nucleotides are. In our guide to nucleotides, we explain what they are and how they make up DNA. (coming soon)

Interested in cellular biology? Vacuoles are a key organelle in cells. Learn everything you need to know about vacuoles, as well as why they're so different in plant vs animal cells, by reading our guide to vacuoles

What are the most important science classes to take in high school? Check out our guide to learn all the high school classes you should be taking.

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Hayley Milliman
About the Author

Hayley Milliman is a former teacher turned writer who blogs about education, history, and technology. When she was a teacher, Hayley's students regularly scored in the 99th percentile thanks to her passion for making topics digestible and accessible. In addition to her work for PrepScholar, Hayley is the author of Museum Hack's Guide to History's Fiercest Females.



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