Before we discuss the more complicated aspects of organic chemistry, it's wise to start with the basics; namely, the electronic structure of atoms. This knowledge will come in handy later and will review anything you may have covered in general chemistry.
An atom contains a small, dense nucleus composed of neutrons and positively charged protons. Most of the mass of an atom is contained in its nucleus, which is surrounded by a space containing negatively charged electrons.
The probability of finding an electron in various regions of space relative to the nucleus is defined by shells. The energy of the electrons inside these shells are quantized, which means that only specific values of energy are possible, as opposed to a continuum of values. These shells only occur at quantized energies in which three important effects balance each other.
The first is the electrostatic attraction that the electrons have to the nucleus and that draws them toward the nucleus, the second is the electrostatic repulsion between the electrons, and the third is the wavelike nature of an electron that prefers to be delocalized. Delocalization refers to the spreading of electron density over a larger volume of space.
Electron shells are identified by the principal quantum numbers 1, 2, 3, and so on. Each shell can contain up to 2(n^2) where n is the number of the shell. Therefore, the first shell can contain 2 electrons, the second can contain 8, the third 18, and so on. Electrons in the first shell are nearest to the positively charged nucleus and are held most strongly. Likewise, electrons in the further shells are held less strongly, due to decreased proximity to the positively charged nucleus.
Shells are divided into subshells designated by the letters s, p, d, and f. Within these subshells, electrons are grouped in orbitals. An orbital is a region of space that can hold two electrons and has a specific quantized energy. The first shell contains a single orbital called a 1s orbital. The second shell contains one s orbital and three p orbitals.
The three 2p orbitals reflect orthogonal angular momentum states in three-dimensional space. Orthogonal in this context results in 90 degree angles between the orbitals, but in all cases orthogonals also means that the orbitals have no net overlap. As a point of reference, to discuss the 2p orthogonal orbits, we consider them to be directed along the x-, y-, and z- axes and give them designations, 2px, 2py, and 2pz.
The third shell contains one 3s orbital, three 3p orbitals, and five 3d orbitals. The shapes of s and p orbitals will be discussed later when we cover quantum or wave mechanics.