Activity 5: SKILL: Bohr model of atom –
Connections: We now should have a firm grasp of the structure of the atom at least internally (protons and neutrons). We will study the energy changes that keep protons in a very small volume of the entire atom in nuclear chemistry at the end of the course next year. Those forces in the nucleus are the strongest in universe which lead to fission and fusion nuclear reactions. Right now and the rest of the your time with me we will discuss the energy changes in atoms and subsequently in chemical reactions by the changes in energies of electrons. We have discussed the discovery of electrons by William Thompson (It’s a corpuscle !) and how they are lost or gained to become ions but now we need to learn that their arrangement in the atom and the change of this arrangement is what creates the energy changes in a chemical reaction. We have learned that chemical reactions can be exothermic (overall energy released) or endothermic (overall energy absorbed) and we have studied that some compounds are more stable than others.
The products in the following reaction, CO2 and NO are more stable (lower in energy) than reactants, CO and NO2 thus the reaction is EXOTHERMIC as the energy is released. The extra energy of the reactants (CO and NO2 ) is released when they form the products (CO2 and NO). We have learned that the ΔH = negative value here because of the energy released or is lowered. WHY? Because of how the electrons are arranged! Chemistry in terms of energy changes deal with electrons arranged in either a stable low energy state or higher excited state. Remember it was the electrons in C-H bonds in sugar that cell respiration uses for energy!
Khan Academy image
The first model of the atom that dealt with energy changes due to electron arrangements was Niels Bohr. His model of the atom that used principle developed from Albert Einstein and Max Plank was the start of the quantum revolution. It changed everything Forever!
1. Please view the lecture below and follow along with me using the worksheets below as I teach you the basics of the Bohr model.
2. Please complete both worksheets that you started with the video above and review with the keys below:
3. Please read my notes on each of the following demonstrations and then view the demonstration video.
4. Please complete a Form after the Demonstrations below on the Bohr model
Bright line Spectrum: of hydrogen Demo – (gas discharge tube of hydrogen and a diffracting lens)
This is a gas tube filled with hydrogen that electricity is being shot through. I am using a diffracting lens to separate the individual photons (spectral lines) given off by the gas when electricity is shot through it.
Every element has a unique arrangement of electrons with a unique number of protons thus each element has a unique spectral “fingerprint” that can be used to identify elements. Hydrogen has the 4 classic spectral lines!
In this demo I had the gas discharge tube off to the left and then I placed the diffracting lens off to the right of my camera so that you can see both the raw light and the individual photons that create that light. Those photons are tied to a single electron leap back to it original position!
It is important to realize that the Bohr model was the first quantum model meaning that these electron energy levels were quantized meaning the can only absorb or emit exact amounts of energy thus each element can only give off a specific spectral lines. The energy of a photon emitted by an electron returning from its excited state to its ground state is measured by the color of the light, which depends on the wavelength or frequency of the light based on the following formula:
1: Energy = h x v or E = hv
h = Planks Constant = 6.63 x 10-34 Js (Joule seconds)
v = frequency = how many waves per time period (light is an electromagnetic wave!)
Thus, higher the frequency, higher the energy = greater energy leap!!
Also the higher the frequency the shorter the wavelength which is given by this inverse relationship (like Pressure and Volume):
2: c = wavelength (λ) x frequency (v) = c = λ x v
c = speed of light
wavelength = distance of each wave
So if the speed of light is constant if the frequency increases then the wavelength must decrease.
We can see this inverse relationship below:
image from https://www.cyberphysics.co.uk/
Blue Light has higher energy Red Light has lower energy
So Red photons have longer wavelengths with SMALLER frequencies while Blue photons have shorter wavelengths and higher frequencies.
Ruben’s Tube Demo – (propane, a tube with holes connected to a speaker).
In this demo I use sound as a way to visualize the relationship between wavelengths and frequencies. Sound is not an electromagnetic wave but is a transverse or mechanical wave so it behaves using the second equation, except C is not the speed of light its the speed of sound. c = λ x v . Notice the size of the wave decreases as the frequency increases (or the pitch gets higher). An inverse relationship.
Sunset Demo Demo – (HCl is added to a solution of sodium thiosulfate as light is projected threw it).
This demonstrates how the different colors of light have unique energy levels. The Red/yellows have the lowest energy and the blue Violet has the highest energy. The higher energy light is getting trapped (or is scattered more) as the beaker thickens ( as the chemical reaction continues). The light shining through the beaker is made of lower energy light that can get through the thickening beaker because it scatters less. (Blue/Violet light has more energy to scatter because it has a shorter wavelength and higher frequency. Higher energy light scatters more because it interacts with a medium (thickening solution) more as smaller waves hit the medium more!
Phosphorescence Demo – (light sticks of different colors affect the glow in the dark material)
This demonstrates that only a certain light colors have enough energy to make the glow in the dark substance glow. Only the blue light stick had high enough energy to “excite” the paper. The green and red light stick did not have enough energy. This also demonstrates that the paper can only accept certain energies to glow. That means that atoms are quantized!!!! Thank you Bohr!
Remember the 4th of July a few weeks ago? Why are there different colors in fireworks?
Chemistry is all around you! You just have to look!
3. Please use the following worksheet below to complete the form below on the Bohr model.
Readiness Assessment Test 25 – Bohr model lecture 2.12R.pdf