1. The Bohr Model The Electron Cloud
Today we will be covering the Bohr model and its importance to the electron cloud, and how the Bohr model can show chemical behavior.
The Bohr Model

2. Learning Objective
Express the arrangement of electrons in atoms through electron configurations.
Today you will be able to express the arrangement of electrons through electron configurations, specifically the Bohr model.

3. Vocabulary
Planetary model: an atomic model patterned after the motions of the planets around the sun. The electrons are arranged in concentric circular paths, or orbits, around the nucleus.
The important vocabulary for today is the idea of the planetary model. This shows the electrons arranged in concentric circular paths, or orbits, around the nucleus, like the motions of the planets around the sun.

4. The Bohr Model In 1913, Niels Bohr came up with a new atomic model.
He proposed that electrons are arranged in concentric circular paths, or orbits, around the nucleus.
Bohr’s model is often referred to as the planetary model.
According to the Bohr model of the atom, electrons exist on fixed energy levels and cannot exist between energy levels. To more from one energy level to another, an electron must gain or lose just the right amount of energy.
Here’s a little background on the Bohr model. As you know, the previous model was done by Rutherford in 1910, based on the results of the gold foil experiment. However, the Rutherford model didn’t explain the behavior of electrons and why different elements have different colors in emission spectroscopy. Niels Bohr used new theories of how light works in order to come up with a new atomic model in In this model, electrons are arranged in “orbits” around the nucleus, like planets rotating around the sun. According to the Bohr model, electrons cannot exist between energy levels. To move around, the electrons must gain and lose just the right amount of energy.

5. The Bohr Model Flaws
Bohr’s model of the atom was consistent with the emission spectrum produced by the hydrogen atom, but the model does not work for more complicated atoms.
Although the Bohr model is not the current model of the atom, his model is still commonly used to help explain chemical behavior and periodic trends.
When other scientists tested Bohr’s model, they found that it was not entirely accurate. The model worked for hydrogen, but not the other 90 elements. However, the model can help explain chemical behavior, and is easy to draw, so it is commonly used for these purposes.

6. Drawing Bohr Models for Atoms
The electrons are drawn on energy levels.
The first energy level can hold 2 electrons.
The second energy level can hold 8 electrons.
The third energy level can hold 18 electrons.
The fourth energy level can hold 32 electrons.
Although the third and fourth energy level can hold more than 8 electrons, an outer energy level must never have more than eight electrons on it. This will affect the elements with atomic numbers greater than 18.
Here are a few things to know about drawing Bohr models for atoms. The electrons are drawn on those fixed energy levels. Only certain amounts of electrons can fit on each level. The first energy level can hold 2 electrons. The second energy level can hold 8 electrons. The third energy level can hold 18 electrons, and the fourth energy level can hold 32 electrons.
Although the third and fourth energy levels can hold more than 8 electrons, the electrons will only fill the outer level up to 8 (we will learn more about this later). This will be explained further, and affects elements with atomic numbers greater than 8.

7. Example 1
Let’s look at an example of the Bohr model for the oxygen-16 atom.
How many protons and electrons does an oxygen atom have?
How many neutrons does an oxygen atom have?
Two electrons will go on the first energy level and six electrons will go on the second energy level.
An easier way of writing this is 2.6
Now let’s look at an example of the Bohr model. Let’s use oxygen-16. How many protons and electrons does an oxygen atom have? According to the periodic table, Oxygen has 8 protons, and therefore 8 electrons. How many neutrons does an oxygen atom have? In the case of oxygen-16, the mass number is 16, and the atomic number is =8, so oxygen has 8 neutrons.
Now let’s focus on those 8 electrons. Remember that 2 electrons will go in to the first energy level. How many electrons are next? We have six left, so we don’t have to worry about that 8 rule I talked about earlier. These six electrons can all fit in that outer energy level. Let’s also write in the 8 protons and 8 neutrons that the atom has. Now, a shorthand way of writing this information, if you can’t draw it, is to simply write the energy levels and differentiate them with a period. For this atom, it would be written as 2.6 –the two for the first energy level and six for the second energy level.

8. Example 2
Let’s look at an example of the Bohr model for aluminum-27 atom. How many protons and electrons does an aluminum atom have? How many neutrons? Two electrons will go on the first energy level. Eight electrons will go on the second energy level and three electrons will go on the third energy level.
Let’s look at a second example, the Bohr model for aluminum-27. How many protons and electrons does an aluminum atom have? All aluminum atoms have 13 protons, and 13 electrons. How many neutrons will this aluminum atom have? Our mass number is 27, and our atomic number is 13, so this atom has 14 neutrons. Let’s go ahead and write in the protons and neutrons in the middle. Now let’s look at the electrons. We have 13 to work with. 2 will go into the first energy level. We have 11 left. 8 can go into the second energy level, the maximum it can hold. We are left with three, which will fit nicely into that third energy level. Our shorthand notation will be And we’re done!
An easier way of writing this is 2.8.3

9. Example 3
Now let’s look at an example of a Bohr model for calcium-40. Calcium has more than 18 e-. How many protons and electrons does a calcium atom have? How many neutrons? Two electrons will go on the first energy level. Eight electrons will go on the second energy level. Although the third energy level can hold up to 18 electrons, only 8 electrons will be placed in it. Remember: only eight electrons can be on an outer energy level. The remaining electrons go on the fourth energy level.
Let’s do one last example to make sure we have it down. Let’s look at an example of a Bohr model for calcium-40. Since calcium has more than 18 electrons, we can see what we have to do with the 8 electron rule. How many protons does a calcium atom have? It has 20, and therefore 20 electrons. How many electrons does this calcium atom have? Since it has a mass of 40 amu, the calcium atom has 20 neutrons. Let’s write the protons and neutrons into the center of our atom. Now let’s get started with the electrons. Once again, 2 electrons will go in the first energy level. Eight electrons will go into that second level. Although the third energy level can hold up to 18 electrons, we can’t put all 10 of the electrons left into that level. So we cap the electrons at 8, and take the two left and put them in a fourth energy level. Finally, let’s write that short hand notation:
An easier way of writing this is

10. Bohr Model Note
If you needed to add more electrons for a higher number element, you would put them on the third level until you reached 18.
For example, the element zinc has 30 electrons. There would be 2 electrons on the first energy level, 8 electrons on the second energy level, 18 electrons on the third energy level, and then 2 electrons the fourth energy level. The shorthand way of writing this would be
A note on high number elements: if you don’t have to worry about the 8 rule, then you fill those higher energy levels to full. For example, zinc has 30 electrons. There would be 2 electrons in the first energy level, 8 in the second energy level, 18 electrons in the third energy level, and 2 electrons in the fourth energy level. The rules have been maintained. The shorthand method is