4.3 Modern Atomic Theory

• Nucleus surrounded by large volume of space –focused on electrons–constant speed –fixed orbits

• Each electron has a specific amount of energy

Bohr’s Model of the Atom

4.3 Modern Atomic Theory

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.

Bohr’s Model of the Atom

4.3 Modern Atomic Theory

An electron can move from one energy level to another when the atom gains or loses energy.

Bohr’s Model of the Atom

Nucleus

ElectronElectrons gain or lose energy when they move between fixed energy levels

Bohr Model

4.3 Modern Atomic Theory

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.

Bohr’s Model of the Atom

4.3 Modern Atomic Theory

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

Bohr’s Model of the Atom

4.3 Modern Atomic Theory

Electron Cloud Model

The nucleus contains protons and neutrons

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.

Electron Cloud Model

• Electrons do not move like planets

• Probability used to establish estimated electron location:

- Model - Most likely locations for electrons

4.3 Modern Atomic Theory

Analogy: airplane propellers

• Propeller at rest: you can see the location of the blades.

• Propeller is moving: you see only a blur (“electron cloud”)

Electron Cloud Model

4.3 Modern Atomic Theory

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.

Electron Cloud Model

4.3 Modern Atomic Theory

• Lowest energy level: has only one orbital

• Higher energy levels: have more than one orbital

Electron Cloud Model

4.3 Modern Atomic Theory

• Electron configuration: arrangement of electrons

• Ground state: all electrons have lowest possible energies

• Excited state: electrons move to an orbital with higher energy

Electron Configurations

4.3 Modern Atomic Theory

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.

Electron Configurations

4.3 Modern Atomic Theory

• 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.

Electron Configurations

4.3 Modern Atomic Theory

1. What did Bohr focus his model on?

2. What do we use to determine the likely locations of

electrons in the electron cloud model?

3. If all electrons are in the lowest possible energy levels,

what state is the atom in?

4. If electrons move from the excited state to the ground

state, what type of energy might we observe being

released?

5. Why are there different colors seen when different

elements move from excited to ground states?