Answer:
Electrons are arranged in main energy levels and then into sublevels depending on the atom.
In summary:
The main energy levels are given a number, the sublevels are given a letter. Each sublevel contains orbitals that can hold upto 2 electrons ie 0, 1 or 2 electrons.
For elements other than hydrogen the arrangement of sublevels is as follows from the nucleus:
1s, - one s orbital ie maximum of 2 electrons. Helium has this level filled.
2s & 2p - one s and three p orbitals ie maximum of 8 electrons. Neon has this level filled.
3s & 3p - one s and three p orbitals ie maximum of 8 electrons. Argon has this level filled.
4s, 3d, 4p - one s, five d and three p orbitals ie maximum of 18 electrons. Krypton has this level filled.
(Complicated isn't it ;-)
5s, 4d, 5p - Xenon. and if we keep going the next level looks like this
6s, 5d, 6p, 4f - seven f orbitals ie upto 32 electrons. Radon has this level filled.
The sublevels of lower energy levels are 'promoted' to a higher energy level. They have enough energy to sit in the next energy level.
Hydrogen does not promote orbitals to a higher energy level so the energy levels are more straight forward.