Bohr-ing!

Toward a Modern Atomic Theory

LG: I can explain Bohr’s model of the atom and the experimental evidence

to support this

Recall: Rutherford/ Chadwick Model

• Wasn’t able to answer new questions:– If nucleus contains +ve charges, how does it stay

together?– Why don’t the electrons crash into the nucleus?

Bohr’s Answer:

Bohr suggested that:

Electrons do not travel randomly around the nucleus, but are confined to discrete energy levels

Only a specified number of electrons can fit in each energy level

“Planetary Model”

Energy level Maximum Number of Electrons

1st (K) 2

2nd (L) 8

3rd (M) 8

4th (N) 18

How do they come up with this stuff?Bohr observed that when elements were excited by heat or electricity, a pattern of distinct lines of light could be observed.

He noticed that the pattern was unique for each element

Energy levels and Line SpectraBohr suggested that electrons could “jump” to unfilled orbits if it gained the required amount of energy; called a transition.

Electrons will always return to their original orbit “ground state” but first must release their excess energy.

Electromagnetic SpectrumThe energy released when an electron return to ground state corresponds to a frequency on the electromagnetic spectrum; this energy can be release as ultraviolet, infrared, or visible light

HomeworkOld Book: Pg. 42 # 1-3 (below)

Questions:1. When a gas is heated, the gas will emit light. Explain why

this occurs using the Bohr model of the atom2. What do the different colours in a line spectrum represent?3. Why do different elements show different spectra?

Read Lab: Pg. 1.4.1 and 1.4.2 – Summarize purpose of lab, procedure, and materials (or handout) and make a table to record observations throughout the lab. You will need one table for each part of the lab.

RecallElectrons exist in orbits that correspond to specific amounts of energy

When excited, electrons can use energy they have gained to jump to higher, unfilled orbits = transition

Electrons always return to their original orbit = ground state, by giving off the same amount of energy they absorbed

The energy released corresponds to a frequency on the EM spectrum

RecallDifferent elements produce different colours when their electrons return to ground state because:

1) Elements in different periods have different valence shells2) Difference in energies between consecutive orbits is

different for every element