Everything you should know about atomic physics:

• a photon is a particle of light with energy = h X f

• an atom is composed of a nucleus and orbiting electrons

• the nucleus is composed of protons and neutrons

• the number of protons determines the element (2 protons = Helium)

• a neutral atom has the same number of electrons as protons

• an ion is an atom with one or more electrons knocked off

Quantum Mechanics and electron orbits:

• electrons orbit the nucleus with only certain allowed values of energy

• higher energy orbits are farther from the nucleus

• a cold atom has very little energy, so electrons are found in lowest level

• a warm atom is moving fast, collisions bump electrons into higher orbits

• an electron can absorb a photon if the photon energy lifts the electron into a higher, allowed level

• an electron can emit a photon if it has a lower energy level into which it can drop