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PROBLEMS WITH THE RUTHERFORD MODEL

According to this model, an orbiting electron should continuously emit electromagnetic radiation and lose energy. This would cause it to collapse towards the nucleus since it is attracted to the (+) charge.

This does not happen…

WE NEED A NEW MODEL!

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Spectroscopy is a technique used for analyzing spectra (plural of spectrum) to determine properties of their source. Light passes through a sample, then is dispersed by a prism. The spectrophotometer scans the spectrum and calculates the amount of light absorbed or transmitted at each wavelength.

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ATOMIC SPECTRUM OF THE HYDROGEN ATOM

Hydrogen gas exposed to a spark.

H atoms absorb that energy and electrons get excited

Some H-H bonds are broken and excited H atoms release energy

light of various wavelengths is emitted

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EMISSION SPECTRUM - the spectrum of E

MR emitted by an atom. There are two types…

1) CONTINUOUS SPECTRUM - an emission spectrum that contains all the wavelengths in a specific region of the electromagnetic spectrum

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2) LINE SPECTRUM - an emission spectrum that contains only those wavelengths characteristic of the element being studied.

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Each element has its own unique line spectrum, like a fingerprint.

New spectra = discovery of new element!

Application in astronomy.

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NOW BACK TO ATOMIC STRUCTURE…

Learning about hydrogen’s line spectrum shows us that the electron of the hydrogen atom can only exist at discrete energy levels.

This is consistent with Planck’s quantum theory!

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THE BOHR MODEL OF THE ATOM

(consistent with line spectrum observations)

electrons can only move in specific orbits around the nucleus
each orbit corresponds to a state of constant (quantized) energy
farther from the nucleus = higher energy level
if an electron got excited (gained more energy) it could move farther from the nucleus

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THE BOHR MODEL OF THE ATOM

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BALL - STAIRCASE ANALOGY

The ball (electron) can only ever be found on a stair (orbit), so can only ever be a specific distance from the ground (nucleus).

The higher the ball, the more potential energy it has.

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The movement of an electron from one energy level to another is called a TRANSITION.

Transition to higher energy level…

electron absorbs energy (ex. is struck by a photon)

Transition to lower energy level…

electron emits a photon (or quantity of energy)

GROUND STATE?

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