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

The Nucleus

The Electrons

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

The number of protons determines the element.

• Just 1 proton in the nucleus, orbited by only 1 electron and no neutron.

Simplest atom: Hydrogen

Atoms really are mostly Atoms really are mostly empty space.empty space.

Quantum Mechanics Electrons can only orbit the nucleus in certain orbits.

n =1 First orbital: Ground State)

The Electron prefers the Ground StateThe Electron prefers the Ground State

Ground State - Lowest energy stateGround State - Lowest energy state

For an electron to For an electron to move to a higher move to a higher orbit, it must orbit, it must gaingain energy E3 - E2energy E3 - E2

So the atom So the atom must must absorbabsorb a a pphotonhoton of just of just the correct the correct energy and jump energy and jump to a higher orbit.to a higher orbit.

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

E3 – E2E3 – E2

So the atom So the atom must must emitemit a a photonphoton of of energy E3 – E2energy 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.

Up absorption

Down emission

Hydrogen Spectrum

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.

Hydrogen emission spectrum(“Balmer lines”)

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 hottest stars because, 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.

Molecules

• Sometimes Atoms combine to form molecules

• A molecule is the smallest amount of a compound

Molecules can complicate Spectra , Molecules can complicate Spectra , because molecules can:because molecules can:

Rotate: produces Radio linesRotate: produces Radio lines

Vibrate: produces Infrared linesVibrate: 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 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.

The Uncertainty Principle

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

Nick Stobel

Richard Pogge

John Pratt

NASA, JPL, OSHO

W.H.Freeman & Company

Spectrum of a Fluorescent Light

Mercury