Bohr Model And Aes

How does this work? (Neon Signs)

What do we know about neon signs?

• Electricity is passed through tube full of neon gas

• Neon atoms in tube absorb this energy– What happens when something absorbs energy?

• Neon atoms in tube become excited– Stable of Unstable?– What happens when something is unstable?– What do we see released energy as?

• Electromagnetic radiation…visible light!!!

EM spectrum • What happens when we pass sunlight through

a prism?– Continuous spectrum of colors– ROYGBIV

What happens when we pass light from neon gas or hydrogen gas

through prism?

• Separation of colors

• Discontinuous spectrum

• This is called…

ATOMIC EMISSION SPECTRUM(AES)

• AES of an element is the set of frequencies of the electromagnetic radiation emitted by the atoms of that element

• Individual lines of color• Only certain lines of color appear for certain

elements…– What does this mean…????

• Every element has a unique AES• Why is this important?

Hydrogen Atom

• Why did scientists want to use hydrogen?– How many protons?– How many electrons?– Do you think it is easy to use?– Check out the AES of hydrogen gas…

Neils Bohr (1913)

• Danish Physicist• Worked with

Rutherford• Quantum Model of

Hydrogen atom– Predicted lines of

Hydrogen AES

• Hydrogen has only one electron but why do we get different colored lines on AES???– We get hydrogen atoms excited…– Electrons move to excited levels

• H has certain allowable energy states….– The lowest energy state is called the

GROUND STATE

Bohr’s Hydrogen Orbits…

• He related H’s energy states to the motion of an electron in an atom

• Single electron in moves around nucleus in circular orbits

• Smaller orbit, smaller radius, closer to nucleus means…?– Lower energy level

• Larger orbit, larger radius, farther from the nucleus means…?– Higher energy level

Bohr’s Quantum Model

• Assigned quantum numbers, n, to each orbit

• Calculated orbits radius– Chart on page 127

• 1st orbitn=1 (first energy level)

• 2nd orbitn=2 (second energy level)

• 3rd orbitn=3 (third energy level)

When we add energy, what happens to electron?

• Electron excited• Moves to next energy level• Excited=?

– unstable

• What happens when something is unstable?– Wants to get back to being stable– Releases energy– Goes back down to lower energy level

• Photon is emitted corresponding to the 2 different energy levels associated with the 2 orbits

NEW EQUATION

• /_\ E= E higher e- orbit - E lower e- orbit =E photon=hv

• Only certain energies are possible so only certain frequencies, v, of EM radiation are emitted

• Lets look at the AES of Hydrogen…

•How many lines are there?

•So how many different types of radiations are we seeing?

•There are 4 electron transitions account for lines in the hydrogen spectrum

•Going from 3rd orbital to 2nd orbital…

•Going from 4th orbital to 2nd orbital…

•Going from 5th orbital to 2nd orbital…

•Going from 6th orbital to 2nd orbital…

Names for these lines…

• Balmer Series– The 4 visible color lines– Electrons that drop into n=2

• Other electrons transitions not visible– Lyman series

• Ultraviolet light• Electrons drop into n=1

– Paschen series• Infrared • Electrons drop into n=3

Problems with Bohr’s Model

• Predicted AES lines of H but not any other elements

• Did not account for all chemical behavior

• Big problem…– Electrons don’t move in circular orbits– Time for a new model…