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Jun 28, 2024 · Bohr model, description of the structure of atoms proposed in 1913 by the Danish physicist Niels Bohr. The Bohr model of the atom, a radical departure from earlier, classical descriptions, was the first that incorporated quantum theory and was the predecessor of wholly quantum-mechanical models.
- Niels Bohr
Niels Bohr (born October 7, 1885, Copenhagen, Denmark—died...
- Atomic Model
In the years after Dalton described his atomic model,...
- Niels Bohr
People also ask
What is the Bohr model of atom?
What does a Bohr model represent?
Why are the shells of a Bohr model important?
Why is Bohr model called planetary model?
- Overview
- The electron cloud model
- Strengths
- Limitations
- The Bohr model
- How are electrons arranged in Bohr models?
- Energy levels
- Valence electrons
- Additional notes about Bohr models
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.
An electron cloud model of a helium-4 atom is shown below.
[What do the scales mean on this model?]
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.
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.)
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.
A Bohr model of a neutral helium-4 atom is shown below.
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.
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.
[Why are some electrons paired?]
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.
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:
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.
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 .
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 !
[Image credit]
In atomic physics, the Bohr model or Rutherford–Bohr model is an obsolete model of the atom, presented by Niels Bohr and Ernest Rutherford in 1913. It consists of a small, dense nucleus surrounded by orbiting electrons.
Jul 16, 2020 · The Bohr model or Rutherford-Bohr model of the atom is a cake or planetary model that describes the structure of atoms mainly in terms of quantum theory. It’s called a planetary or cake model because electrons orbit the atomic nucleus like planets orbit the Sun, while the circular electron orbits form shells, like the layers of a cake.
Jan 27, 2020 · The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the sun (except that the orbits are not planar).
- Anne Marie Helmenstine, Ph.D.
The Bohr model represents the structure of an atom developed by Danish physicist Niels Bohr in 1913. According to this model, the atomic structure is similar to that of the solar system. The nucleus represents the sun, and the electrons represent the planets orbiting around the nucleus.
Key points. Bohr's model of hydrogen is based on the nonclassical assumption that electrons travel in specific shells, or orbits, around the nucleus. Bohr's model calculated the following energies for an electron in the shell, n. : E ( n) = − 1 n 2 ⋅ 13.6 eV.
