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The Elements. Spectra Part II. The Nucleus. The Nucleus contains positively charged Protons and neutral Neutrons; almost all of the Atom’s mass. The number of protons determines the element. The Electrons.

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The Elements

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The Nucleus

The Nucleus contains positively chargedProtons and neutral Neutrons; almost all of the Atom’s mass.

The number of protons determines the element.

The Electrons

  • Negatively charged electrons, which are much lighter, orbit the Nucleus moving quite fast.

Simplest atom: Hydrogen

  • Just 1 proton in the nucleus, orbited by only 1 electron and no neutron.
  • Atoms really are mostly empty space.

Quantum Mechanics

Electrons can only orbit the nucleus in certain orbits.

n =1First orbital: Ground State)

The Electron prefers the Ground State

Ground State - Lowest energy state


For an electron to move to a higher orbit, it must gain energy E3 - E2

  • So the atom must absorb a photon of just the correct energy and jump to a higher orbit.

For the electron to move to a lower orbit, it must lose energy of the correct amount

  • E3 – E2
  • So the atom must emit a photon of energy E3 – E2

For convenience, the energy levels (orbits) of an atom are represented by horizontal lines. Energy increases as you move upward in the diagram.


Hydrogen Spectrum

Up absorption

Down emission

All Balmer lines star or end with the second level (n=2)


Hydrogen Emission Lines: Balmer Lines

When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.


Hydrogen Absorption Lines: Balmer Lines

An electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital. No more, no less.


All stars are made of roughly 75% Hydrogen, 20% Helium, and 5% everything else.

Different absorption line spectra are not due to different chemical composition, but rather the effect of different surface temperatures.


Hydrogen lines are absent in the hotteststarsbecause,photons ionize electrons.

They are also absent in the coolest stars because, photons don’t have enough energy to move the electrons from n=2 to higher energy levels.

  • Sometimes Atoms combine to form molecules
  • A molecule is the smallest amount of a compound

Molecules can complicate Spectra , because molecules can:

  • Rotate: produces Radio lines
  • Vibrate: produces Infrared lines

If there are magnetic fields present, the atomic energy levels are split into a larger number of levels and the spectral lines are also split. This splitting is called the Zeeman Effect.

The Zeeman Effect


The Uncertainty Principle

  • The uncertainty principle of QM states that we cannot know both where something is and how fast it is moving.
    • Thus we cannot predict exactly what will happen in a given experiment.
    • We can only give the probability of an outcome.

Thanks to the following for allowing me to use information from their web site :

Nick Stobel

Richard Pogge

John Pratt


W.H.Freeman & Company