1. Bohr’s Model of the Atom
Nucleus surrounded by large volume of space
focused on electrons
constant speed
fixed orbits
Each electron has a specific amount of energy

2. Bohr’s Model of the Atom
Energy Levels
Possible energies of electrons
An electron cannot exist between energy levels.
Size of jump between energy levels determines amount of energy gained or lost.
No two elements have the same set of energy levels.

3. Bohr’s Model of the Atom
An electron can move from one energy level to another when the atom gains or loses energy.
Electron
Electrons gain or lose energy when they move between fixed energy levels
Nucleus
Bohr Model

4. Bohr’s Model of the Atom
Analogy: staircase
The landing at the bottom is the lowest level.
Each step up represents a higher energy level.
The step height represents an energy difference between levels.
You can only move in whole numbers of stairs.

5. Bohr’s Model of the Atom Evidence for Energy Levels
Scientists can measure the energy absorbed or released
Light: form of energy that can be observed.
Example- Fireworks:
Heat  Some electrons move to higher energy levels
Electrons move back to lower energy levels and release energy
visible light
Different colors: no elements have the same energy levels

6. Electron Cloud Model Electrons do not move like planets
Probability used to establish estimated electron location:
- Model
- Most likely locations for electrons
The electron cloud is a visual model of the probable locations of electrons in an atom. The probability of finding an electron is higher in the denser regions of the cloud.
The nucleus contains protons and neutrons
Electron Cloud Model

7. Analogy: airplane propellers
Electron Cloud Model
Analogy: airplane propellers
Propeller at rest: you can see the location of the blades.
Propeller is moving: you see only a blur (“electron cloud”)

8. Analogy: your path through the day
Electron Cloud Model
Analogy: your path through the day
Imagine a map of your school. Mark your exact location with a dot once every 10 minutes over one week.
The dots on your map are a model of your “orbital.”
They describe your most likely locations.

9. Electron Cloud Model
Lowest energy level: has only one orbital
Higher energy levels: have more than one orbital

10. Electron Configurations
Electron configuration: arrangement of electrons
Ground state: all electrons have lowest possible energies
Excited state: electrons move to an orbital with higher energy

11. Electron Configurations
Example: A lithium atom has three electrons.
Ground state: two electrons are in the orbital of the first energy level, third electron is in an orbital of the second energy level.
Absorbs enough energy  one of its electrons can move to an orbital with a higher energy: excited state
less stable than the ground state.
Eventually, the electron that was promoted to a higher energy level loses energy, and the atom returns to the ground state.

12. Electron Configurations
Analogy: Gymnast
The ground state of a person is on the floor.
A gymnast on a balance beam is like an atom in an excited state
When she dismounts, the gymnast will return to a lower, more stable energy level.

13. What did Bohr focus his model on?
What do we use to determine the likely locations of electrons in the electron cloud model?
If all electrons are in the lowest possible energy levels, what state is the atom in?
If electrons move from the excited state to the ground state, what type of energy might we observe being released?
Why are there different colors seen when different elements move from excited to ground states?