Bohr's Model of the Atom

Bohr's Model of the Atom

and the ongoing search for a more perfect model.

Thompson's Model

The best model based on what we know about charges (like charges repel and opposite charges attract)

Multiple Choice

Which observation did Thompson's model NOT explain.

• like charges repel, opposite charges attract

• negative charges can leave the atom

• different elements have different amounts of mass

• different elements give off different colors of light

Rutherford's Experiment

Showed Thompson's model can't be true. All of the positive charges is in the center.

Multiple Choice

What did Rutherford expect to happen to the alpha particles bombarding the gold foil?

• they would be absorbed by the gold.

• they would be deflected by the gold.

• they would go completely through the gold foil.

• the would be changed into beta radiation by the gold foil.

Multiple Choice

What did Rutherford discover in his gold foil experiment?

• atoms are indivisible and have a dense positive nucleus.

• atoms are divisible and have multiple layers of electrons scattered uniformly about the atom.

• atoms are like plum pudding with positive and negative charges uniformly scattered throughout the atom.

• atoms are mostly empty space with a dense positive nucleus in the center.

Two Problems

• How can all the positive charge be in the center if positive charges repel each other?

• Where are the negative charges and what are they doing?

Problem 1: Positive Charges in the Center

• The force that repels two positive charges is called the electrostatic force.

• If two positive charges are sticking together that must mean that there is a force stronger than the electrostatic force.

Open Ended

What would you call a force that is stronger than the electrostatic force?

Solution

The Strong Force

At close distances, this force overcomes the electrostatic force and holds protons (positive charges) together.

Problem 2: Negative Charges (Electrons)

• Where are they?

• What are they doing?

• Oh, and by the way, we never explained where those different colors of light were coming from.

Solution: Bohr's Model of the Atom

• Positive (and Neutral) charges in the center

• Electrons are in "orbits" with different amounts of energy

Multiple Choice

How are electrons arranged in an atom?

• In groups of five

• In energy levels

• By color

• By shape

Multiple Choice

What the acronym that can help you remember the order of the visible spectrum?

• ROY G. BIV

• MVEMJSUN

• GBADF

• NESW

Multiple Choice

Which color has the longest wavlength?

• Yellow

• Red

• Green

• Purple

Multiple Choice

Which color has the highest energy?

• Yellow

• Red

• Green

• Purple

What about the colors?

• When an atom is "excited" (like the gas in our light tubes we looked at) the electrons jump up to a higher energy orbit.

• Then, the electrons fall back down to their original energy level.

• The farther they fall, the more energy they give off and a different color of light is produced.

Did we finally find the perfect model?

Almost. One big problem with the Bohr model is that the electrons don't really move in orbits like planets do around the Sun.

The colored areas show the area that an electron is likely to be for each energy level.

Poll

Which picture of the atom makes the most sense to you?

Orbits are not really Orbits but . . .

• They are useful for picturing and counting how many electrons are in each energy level.