Chapter 1: Structure Determines Properties

Let's review some key aspects of atomic structure:
 

atoms have a nucleus at their core and are surrounded by electrons, e- the nucleus is comprised of protons, p+  and neutrons, n0 protons are positively charged, +1 unit neutrons have no charge electrons are much smaller and negatively charged, -1 unit

the number of protons in the nucleus of an atom is given by a unique atomic number, Z  which defines the type of atom the total number of protons and neutrons in the nucleus is given by the mass number, A a neutral atom must have the same number of electrons surrounding the nucleus as there are protons in the nucleus, so Z also defines the number of electrons in a neutral atom. in a collection of neutral atoms, some may have nuclei with different numbers of neutrons. These are known as isotopes of that element.

Z Symbol A   2 He 4.00

The above characteristics can easily be figured out by looking at a periodic table (which is arranged based on increasing atomic number).   It is important to note that when obtaining the mass number from a periodic table, A, is the isotopic mass number or the weighted average of the isotopic masses based on a normal sample.
In introductory organic chemistry the vast majority of the atoms involved in the organic compounds we will be interested in will be based on only a small group of elements, H, C, N, O, and the halogens Cl, Br and I.