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Course: High school chemistry > Unit 1
Lesson 3: The Bohr model and atomic spectraThe Bohr model
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.
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.
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.
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.
The higher the shell number, the greater the energy of electrons in that shell. For example, electrons in the shell of the atom are at greater energy than electrons in the shell, which are at greater energy than electrons in the shell.
To summarize for the first two shells:
Energy level (shell) | Maximum number of electrons | Electron energy |
---|---|---|
lowest possible energy for the atom | ||
greater energy than electrons in |
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 , and the core electrons are shown in .
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
. 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... !
Want to join the conversation?
- Under the subtitle Energy levels, it is stated that "Electrons cannot exist at energies between these [energy] levels." Why is that? Thank you in advance.(24 votes)
- This is a fundamental property of quantum mechanics. How I think about it is that there is no continuous range of energy between these levels, so the electrons have to exist at one of these discrete quantized levels of energy. Unlike when we consider an infinite range of imaginary values between 0-1 in mathematics, this isn't the case at a quantum level, where there is no in between. Quantum energy is not continuous, it is discrete, and this is where the word "quantum" gets it's meaning of "a discrete quantity of energy"(37 votes)
- why are there only 3 shells(6 votes)
- Because the lower you get on the periodic table the more shells are added to store the extra electrons, Chlorine is in the 3rd row, so it has 3 shells.
Don't ask me about the lanthanides and actinides, I don't know.(13 votes)
- I know this is partly irrelevant but I am really stuck. Since the Octet Rule has a pattern of 2, 8, 8 for the maximum of electrons that can be in each shell. Why did I read somewhere that some atoms are different and they dont follow this Octet Rule and their third shell can hold up to 18 electrons? I am really confused, someone please help.(5 votes)
- You are right that the 3rd shell can hold above 15 electron. That's why the article did not include the 3rd shell it's too complex we will learn about it later(9 votes)
- When it says "Electrons do NOT orbit the nucleus like planets orbit the sun. The rings are simply a convenient way to represent electron energy levels". Then whats the purpose of it then?(0 votes)
- It serves as a convenient way to visualise them.(13 votes)
- why is there only 3 shells?(1 vote)
- Well, if you look at the atoms above chlorine, we see that all the atoms in a row have two shells. And the ones above that row have only one shell. This trend tells us the deeper into the rows you go the more shells you get. So if you had Bromine, for example, it would have 4 shells. So the number of shells is dependent on the row number it is in.
Hope this helps!
FreeRadical(7 votes)
- Also, why does electrons further away from the nucleus have more energy?(4 votes)
- Electrons further from the nucleus have more energy due to their increased distance, shielding effect, weaker attraction, quantum mechanics principles, and orbital arrangement.(1 vote)
- What is the maximum number of electrons the third layer can hold?(3 votes)
- The third shell (or layer) of an atom can hold a maximum of 18 electrons. This is calculated based on the formula for the maximum number of electrons in a shell, which is
2×n^2
, wheren
is the shell number. For the third shell(n=3)
, the calculation is2×3^2 = 18
.(2 votes)
- The word "quantum is used twice in this section - "quantum weirdness" and "Quantum Mechanics is weird!" What does quantum mean and what is Quantum Mechanics? I didn't see them defined in this presentation, just mentioned. Thank you.(1 vote)
- Quantum is a latin word meaning a measurement, specifically numbers. In physics, it means a measurement of small energy. So when someone says quantum mechanics, or quantum physics, it means the physics and mechanics of small amounts of energy, such as atoms and their protons, neutrons and electrons.(4 votes)
- Can you explain why there is a maximum to each electron shell?(2 votes)
- What is the maximum amount of electrons per shell? Thank you in advance.(2 votes)
- It is 2n*n where n is the number of the shell.
Example: you have to find the maximum no. of electrons in 4th shell so, no. of electrons=2*4*4=32[n=4]
Therefore no. of electrons in the 4th shell of an atom=32 electrons.(1 vote)