1. “Spanish Steps”, Francesco de Sanctis, 1726, Rome, Italy
Niels Bohr’s Atom
Vocabulary:
bright line spectrum
continuous spectrum
Niels Bohr
Bohr model
energy level
ground state
excited state
photon
quantized
“Spanish Steps”, Francesco de Sanctis, 1726, Rome, Italy

2. Bright Line Spectra
When electricity passes through a gaseous element, the element gives off a distinctive color of light.
hydrogen helium neon mercury krypton

3. Bright Line Spectra Low energy Red Orange Yellow Green Blue Indigo
If we take light and pass it through a prism, the light is broken up into all the colors that make up that light. For white light, that would be every color of the rainbow, called a continuous spectrum (shown below.)
Low energy
High energy
Red
Orange
Yellow
Green
Blue
Indigo
Violet

4. Bright Line Spectra
However, you probably noticed that none of the elements gave off white light. Each element gave off a distinctive color of light, each of which is made of specific colors of light called a bright line spectrum. For instance, when you pass hydrogen’s light through a prism you get the bright line spectrum shown below.
Hydrogen’s light only contains these four colors, not a continuous spectrum.

5. Bright Line Spectra
The bright line spectra for each element shown in the first slide is shown below.
Low energy High energy
hydrogen
helium
neon
mercury
krypton

6. Bright Line Spectra
In the early 1900’s, Danish physicist Niels Bohr was trying to explain why each gas’ bright line spectrum only contained certain colors of light rather than a continuous spectrum.

7. The Bohr Model
To do this, he postulated that electrons could only exist on certain energy levels and travelled around the nucleus of the atom much like planets around the sun. Electrons could not be at any position in the atom other than these energy levels.

8. The Bohr Model
When energy was added to the atom (such as with an electric current) the electrons could move from a low energy level (the ground state) to a higher energy level (an excited state.)
Ground state
Excited state
Add energy

9. The Bohr Model
Excited states are unstable, however. The electron will then quickly return to the ground state, releasing energy in the form of a particle of light called a photon. The energy (color) of this light is directly related to the energy difference between the two energy levels.
Ground state
Excited state
Photon Released
Release energy

10. The Bohr Model
The rings (energy levels) of the Bohr model can more easily be represented using straight lines.
We can then show energy changes using arrows.
n = 5
n = 4
n = 3
n = 2
n = 1

11. The Bohr Model Let’s say an electron is on n=1 and is excited to n=4.
The electron could then return to n=3, n=2, or n=1. Each of these would release a photon of different (but specific) energy/color.
n = 5
n = 4 or or
n = 3
n = 2
n = 1

12. hydrogen’s bright line spectrum
The Bohr Model
Let’s see how this applies to the electron movement in hydrogen and its bright line spectrum.
n = 6
n = 5
n = 4
n = 3
photon released
photon released
photon released
photon released
n = 2
hydrogen’s bright line spectrum
energy absorbed
energy absorbed
energy absorbed
energy absorbed
n = 1

13. The Bohr Model
Because the energy released by an atom is only available in specific amounts, we say that the energy is quantized.
This is the foundation of the Bohr Model of the atom. Electrons are in specific energy levels travelling in orbits around the nucleus like planets around the sun.