Light and Electrons

Draw an atom of Lithium.

Did you draw this?

Why did you draw the electrons in rings around the nucleus?

How did scientists figure out electron “rings”?

Could they see them?

Scientists got clues from light.

Atoms absorb light and emit light in distinct ways.

What does a prism do?The prism breaks light into all of its separate colors. If the light is sunlight, all the colors are present.

This is called a continuous spectrum, because every color is present.All the colors flow together continuously. Every possible energy level of visible light is present.

A rainbow is another example of the continuous spectrum of sunlight..

You get a continuous spectrum from sunlight. But when scientists look at the light coming off just one element, hydrogen for instance, they don't see the whole rainbow.Instead they just get bright lines of certain colors.

This is called a line spectrum because only certain lines of colors are present.

This is the line spectrum of hydrogen.

Here is the line spectrum of neon.

In fact, every element has a unique line spectrum.

That’s how scientists figured outwhat stars are made of.

They analyzed the light coming from the stars, and compared it to the spectra

of known elements.

But why don’t pure elements emit a continuous spectrum like sunlight?

Remember, the color of a light corresponds to its energy.

For example, red light has lower energy than yellow light. Why don’t hydrogen atoms emit all the colors?

Atoms are only emitting light of certain energies.

Why don’t they emit light of the other energies?

To explain this puzzle, Neils Bohr came up with a radical model of the atom.

Interesting side note: Bohr was so excited by his newidea that he postponed his honeymoon to write hislandmark paper about it.

In order to explain the line spectra, Bohr came up with an extraordinary rule the electrons seemed to follow: Electrons can only orbit the nucleus in certain energy levels. All other energy levels just

are not possible.

It’s like climbing a ladder: You can stand on one step or another step, but

not really between steps.

Think of each step on this ladder as an energy level for an electron.Electrons can be in one energy level or jump to another energy level, but they’renot allowed in between the allowed orbits.

• It takes energy to move an electron away from the nucleus. Why?

• Electrons can’t be in between levels.

First

Second

Third

Fourth

Fifth

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Here’s how it’s not like a ladder:

An electron moving between orbits disappears from one and reappears instantaneously in another without visiting the space between.

This jump from one orbit to another is called the “quantum leap.”

Sound strange to you?

Then congratulations!

Bohr himself once said that a person who wasn’t outraged on first hearing about quantum theory didn’t understand what had been said.

Just like going up a ladder, it takes energy for an electron to move away from the positively charged nucleus.

Remember, opposites attract.The electron “wants” to be near the

nucleus.It takes energy to move a negative

charge away from a positive charge.

Summary:

Atoms make light in a three-step process:

•They start off in their stable "ground state" with electrons in their normal places.

•When they absorb energy, one or more electrons are kicked out farther from the nucleus into higher energy levels. We say the atom is now "excited."

•However, an excited atom is unstable and quickly tries to get back to its stable, ground state. So it gives off the excess energy it originally gained as a photon of energy (wiggly line): a packet of light.

Let’s see how this works at

http://www.colorado.edu/physics/2000/quantumzone/bohr.html

http://www.cs.sbcc.edu/physics/solar/sciencesegment/bohratom.swf

The atoms you’ve been learning to draw are called Bohr models.

When you draw the electron rings, you’re drawing the energy levels that

are allowed for the electrons.

Summary:

Did you learn …

The fact that atoms give off characteristic colors of light (line spectra) provides clues about how electrons are arranged in atoms.

The line spectra gave rise to two important ideas:

1. Electrons exist only in certain energy levels around the nucleus.2. Energy is absorbed when an electron moves to a higher level; energy is released when an electron moves to a lower level.

Go back to your hydrogen line spectrum and label the electron jumps that these lines represent.

The pink light is made by electrons dropping from the third energy level to the second energy level.The blue line represents energy emitted by electrons dropping from the 4th level to the 2nd level.The purple line is made by electrons dropping from the 5th level to the second level.

Summary diagram: