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CHAPTER 1.1. fic creativity, 1910-1913FinnAaserud *AbstractPreviously unavailable correspondence between Niels Bohr and his fiancée (and later wife) Margrethe Nør lund during Niels Bohr’s stay in England from Sep tember 1911 through July 1912 indicates that Bohr’s immense creativity behind the formulation of his atom ic model in 1913 was ...
- 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]
Figure 1: Niels Bohr's first Drawing of the Atom (1912), AHQP, Harvard University. Figure 2: Niels and Margrethe Bohr riding George Gamow's motorcycle at their summer home in Tisvilde, Archive of the Niels Bohr Institute, Copenhagen, AIP Emilio Segre Visual Archives. Figure 3: Example of legible handwriting in Niels Bohr's 1912 notes.
Since the Rydberg constant was one of the most precisely measured constants at that time, this level of agreement was astonishing and meant that Bohr’s model was taken seriously, despite the many assumptions that Bohr needed to derive it. The lowest few energy levels are shown in Figure 6.14. One of the fundamental laws of physics is that ...
Margrethe Nørlund Bohr (7 March 1890 – 21 December 1984) was the Danish wife of and collaborator, editor and transcriber for physicist Niels Bohr who received the Nobel Prize. She also influenced her son, Nobel Prize winner Aage Bohr .
One of the weaknesses of Bohr's model was that he could not offer a reason why only certain energy levels or orbits were allowed. Figure 10.4.1 10.4. 1: The energy levels of the electrons can be viewed as rungs on a ladder. Note that the spacing between rungs gets smaller at higher energies (CC BY-NC; Ümit Kaya)
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Aug 26, 2023 · Figure 6.2.2 6.2. 2: The horizontal lines show the relative energy of orbits in the Bohr model of the hydrogen atom, and the vertical arrows depict the energy of photons absorbed (left) or emitted (right) as electrons move between these orbits. Since Bohr’s model involved only a single electron, it could also be applied to the single electron ...
