When dealing with electronic structure, the concept of a wave is the most important thing. Electrons travel in waves at certain frequencies, which causes the different shades of light, x-rays, and radio waves. The basic equation that sums this concept up is shown below:
speed of light = wavelength * frequency
Planck's equation, which relates energy and the previous equation is:
energy = Planck's constant (h) * frequency
In 1905, Einstein came up with the concept of the photoelectric effect.
Basically, this meant that in order for an electron to be released from
an object, a certain amount of energy (quantum) was required. Einstein
also stated that the amount of energy (intensity) was the only variable
that affected the release of electrons, not the quantity of electrons.
Atoms are complex particles, even when dealing with only the electrons. In order to keep track of electrons in an atom, quantum numbers were invented. Four such numbers are used. These are n, l, ml, and ms.
"n" is the main quantum number. This is the number that keeps track of the energy level of the electron. The valid range is from 1 to infinity.
"l" is the quantum number that gives the sublevel of the electron. The range of this number is from 0 to n-1. 0 corresponds to s, 1 corresponds to p, 2 corresponds to d, and 3 corresponds to f.
"ml" is the magnetic quantum number. This keeps track of the orientation of the subshell. The valid range of this number is -"l"...0...+"1" where the "l" is the sublevel number.
"ms" is the final quantum number. This is tells you
the spin of the electron. The two valid numbers are +1/2 and -1/2.