Warm-Up

1. Think of another physical analogy for a “Rutherford” atom other than a stadium.

2. Identify what represents the atom and what represents the nucleus in your analogy.

3. How good a match is your analogy for an actual atom? In what ways is it misleading?

3-4 Modern Atomic Theory

• Objectives• Describe Bohr’s model of the atom and the

evidence for energy levels.

• Explain how the electron cloud model represents the behavior and locations of electrons in atoms.

• Distinguish the ground state from excited states of an atom based on electron configurations.

Bohr’s Model of Atoms• 1913 – Neils Bohr,

Danish Scientist• Planetary model.• Positive & negative

attract.• Specific energy level• Orbit the nucleus like

planets circling the sun.

Bohr models

• p+ & n in nucleus

• e- in energy levels around nucleus

• 4 energy levels

-1st has up to 2e-

-2nd has up to 8e-

-3rd has up to 8e-

-4th has up to 18e-

e- e-e- e-e- e-

e- e-

e- e-

e- e-

e- e-

e- e-

e- e-

Electron Configuration

• Arrangement of electrons in the orbits of the atom.

• The most stable electron configuration is when the electrons are in the lowest energy state.

• When electrons are in the lowest energy level, they are said to be in the GROUND STATE.

Evidence of Bohr’s Model• When that light is

passed through a prism gives its Spectral signature.

• It gives off light unique to that element.• Kind of like wearing

team colors.

Examples of Element Spectrum

Bohr's rule for electrons

• Electrons can only be in "special" orbits.

• Fill lowest energy orbit.

Bohr Model of Lithium

34

e- e-e-

Bohr Model of Argon

1822

18

Ar

39.948

e- e-

e- e-

e- e-

e- e-

e- e-e- e-

e- e-

e- e-

e- e-

Electron Cloud Model

• Predict the location and speed of the electrons.• Could not know both.• "Cloud" - probable location of electrons• Propeller blades are a good model of electron

cloud.• Electrons move so fast, like the blades, they

are hard to see.

Electron Cloud Model