Bohr model

In atomic physics, the Bohr model is an old way of thinking about atoms that used some early ideas from quantum science. It was made between 1911 and 1918 by Niels Bohr. This model improved on earlier ideas about atoms, like the plum pudding model created by J. J. Thomson.

The Bohr model shows a tiny, heavy center called the atomic nucleus with electrons moving around it, like planets around the Sun. But instead of gravity holding the planets, it is an electrostatic force that keeps the electrons in place. The model also says that the electrons can only have certain fixed amounts of energy.

One big success of the Bohr model was that it helped explain why hydrogen, the simplest atom, gives off light in certain colors. Before this model, scientists knew these colors existed but did not know why. The Bohr model gave a clear reason for this pattern of colors. Even though we now use more advanced ideas to understand atoms, the Bohr model is still useful because it is simple and helps students learn the basics before moving on to more complicated theories.

Background

Main article: History of atomic theory

Before the 1900s, ideas about what atoms look like were mostly guesses. Even the idea that atoms exist was something some scientists did not fully accept.

Planetary models

In the late 1800s, some scientists thought atoms might look like tiny planets, with tiny particles called electrons moving around them. But there was a big problem with this idea. If electrons moved in circles, they would shake and send out energy, which would make them spiral into the center. This would mean atoms could not stay stable.

Thomson's atom model

Main article: Plum pudding model

When Niels Bohr started working on his ideas about atoms in 1912, the best model was made by J. J. Thomson. Thomson thought atoms looked like plum pudding, with small electrons stuck inside a big positive sphere. But this model had problems. It could not explain why atoms looked the way they did in experiments, or why they gave off certain colors of light when heated.

Rutherford nuclear model

Main articles: Rutherford atom and Rutherford scattering experiments

In 1911, scientists Ernest Rutherford and his team found that atoms have a tiny, dense center called a nucleus. This nucleus holds most of the atom's mass and is surrounded by electrons. This idea replaced Thomson's model and helped Bohr develop his own model of the atom.

Atomic spectra

Scientists knew that when atoms are heated, they glow with certain colors. These colors are like fingerprints for each element. By the early 1900s, they had formulas to describe these colors, but they did not understand why these colors happened. Bohr would later explain that these colors come from electrons moving between different energy levels.

Haas atomic model

In 1910, Arthur Haas suggested another model for the hydrogen atom. He thought the electron moved around the edge of a sphere. Haas used a special number called the Planck constant to connect the electron's energy to its movement. This idea would later help Bohr in his own work.

Influence of the Solvay Conference

In 1911, some of the world's best scientists met at the Solvay Conference. They talked about how atoms work and why old ideas did not fully explain things. These talks encouraged Bohr to think differently about atoms and to use new ideas that went beyond old physics.

Nicholson atom theory

In 1911, John Nicholson made a model of the atom based on stars and their light. He used the Planck constant to explain how electrons moved. This work helped Bohr see how important certain numbers were in understanding atoms.

Bohr's previous work

Before Bohr made his famous atom model, he finished his PhD in 1911. His work looked at how electrons behave in metals. He found that old ideas did not fully explain things like heat or magnetism in metals. Later, he worked in labs and learned more about atoms, especially after Rutherford discovered the nucleus. This knowledge helped Bohr create his new model of the atom.

Development

Niels Bohr developed his model of the atom between 1911 and 1918. He learned that certain patterns in light from hydrogen, called the Balmer series, could be described by a simple formula. This helped Bohr understand how electrons move around the nucleus.

Bohr suggested five key ideas to explain how electrons behave in an atom. He said that electrons can only move in certain fixed paths, or orbits, around the nucleus. These orbits are stable, meaning the electron won’t spiral into the nucleus. When an electron jumps between these orbits, it either gives off or absorbs a particle of light, called a photon, with a specific energy. Bohr’s ideas helped explain why atoms only give off certain colors of light and not others. Later scientists built on Bohr’s work to create more complete models of the atom.

Refinements

Main article: Bohr–Sommerfeld model

Some changes were made to the Bohr model, especially the Sommerfeld or Bohr–Sommerfeld models. These models said that electrons move in oval paths around the nucleus instead of circular ones. This helped explain some things better, but it also had problems.

Eventually, scientists developed a new way to understand atoms using quantum mechanics. This new method, created by Wolfgang Pauli in 1925 and Erwin Schrödinger in 1926, replaced the older models. Even so, the older Bohr–Sommerfeld model could still explain some effects correctly, like changes in light from atoms under certain conditions.

Replacement

In 1925, scientists created a new way to understand how tiny parts of atoms work, called quantum mechanics. This new idea improved on an older model made by Niels Bohr. Werner Heisenberg wrote a paper that helped start this new theory, and later, Max Born and Pascual Jordan added more ideas using something called matrix mechanics. Around the same time, Erwin Schrödinger found another way to describe these ideas using waves. He looked at how electrons move around the center of a simple atom, trapped by its positive charge.

Electron energy levels

The Bohr model works best for systems with two charged particles, like a single electron orbiting a nucleus. This includes simple atoms such as hydrogen atom, singly ionized helium, and doubly ionized lithium. It also applies to positronium and Rydberg states where one electron is far from the nucleus.

To understand the model, we need two main ideas:

• Classical mechanics: The electron moves in a circular orbit because of the attraction to the nucleus. The force pulling the electron in equals the force that keeps it moving in a circle.

• A quantum rule: The electron’s angular momentum must be a whole number multiple of a very small constant called ħ (h-bar).

This model helped scientists understand how electrons behave in simple atoms and led to important discoveries about atomic structure.

Rydberg formula

Main article: Rydberg formula

In the late 1860s, scientists like Johann Balmer, Johannes Rydberg, and Walther Ritz created formulas that matched the colors of light given off by atoms. These formulas helped explain the patterns seen in experiments.

Later, Niels Bohr used his ideas about atoms to show why these formulas worked. He explained how the energy of light matched the differences in energy levels of electrons moving around the atom. This helped confirm his model and showed how the Rydberg constant relates to basic parts of nature, like the charge and mass of an electron and a special number called the Planck constant.

Shell model (heavier atoms)

Main article: Electron shell

Niels Bohr’s early ideas about atoms mainly explained simpler elements. He thought of electrons arranged in circles or “rings” around the nucleus. Later, scientists realized these rings could be called “shells.” Each shell could hold a certain number of electrons before filling up and starting a new shell.

Bohr’s ideas helped explain why some elements behave differently. For example, small atoms get a bit smaller as you move across a row in the periodic table, and then suddenly get larger again. This happens because shells fill up one by one. When a shell is full, the next atom adds an electron to a new, outer shell, making the atom larger. This also helps explain why some elements, like helium and neon, don’t easily react with others—they have full outer shells.

Moseley's law and calculation (K-alpha X-ray emission lines)

In 1913, Henry Moseley discovered a relationship between the strongest X-ray line that atoms emit when bombarded by electrons, called the K-alpha line, and their atomic number, which is a unique number for each element.

Moseley found that this X-ray line comes from an electron moving between two energy levels in the atom. His work helped confirm the idea that atoms have a small, dense center, or nucleus, and that elements are arranged in order of their atomic number.

Shortcomings

The Bohr model had several problems. It gave the wrong value for the angular momentum of electrons in their lowest energy state. In modern science, we know electrons are more like a fuzzy cloud around the nucleus, not like planets orbiting.

The Bohr model also could not explain many things we see in experiments, like the detailed patterns of light that atoms give off, or how atoms behave in magnetic fields. It worked only for very simple atoms and missed important details about how electrons actually behave. Later scientific ideas improved on the Bohr model to explain these missing pieces.

Main article: spherical cloud of probability

Further information: lithium, Rydberg formula, Stark effect, hyperfine structure, Zeeman effect, magnetic fields, Umdeutung paper

Model of the chemical bond

Niels Bohr created a way to explain how atoms stick together to form molecules. In his idea, the electrons in two atoms that join together make a shape like a ring. This ring stands straight up compared to the line connecting the two atoms' centers. The atoms stay balanced because the pull between the centers and the ring of electrons matches up with the push away from each other that the centers feel. Bohr’s idea also considered how electrons push each other away when they are far apart in this ring.

Symbolism of planetary atomic models

Even though scientists now use more advanced ideas about atoms, many people still picture atoms like tiny planetary systems. This idea of an atom with electrons moving around a center has become a popular symbol. It is used in many places, such as:

• The logo of the United States Atomic Energy Commission, which helped develop nuclear fission technology.
• The flag of the International Atomic Energy Agency features a spinning atom design surrounded by olive branches.
• The US minor league baseball Albuquerque Isotopes logo shows baseballs like electrons orbiting a large letter "A".
• The symbol called the atomic whirl is used to represent atheism.
• The Unicode Miscellaneous Symbols includes a symbol (⚛) that looks like a planetary atom model.
• The TV show The Big Bang Theory uses a similar image in its logo.
• The JavaScript library React also uses a planetary-like image for its logo.
• On maps, this symbol marks a nuclear power site.