The periodic table of elements provides specific information about each element. The atomic number is the number of protons found inside the nucleus of an atom. Protons have a positive charge. Once the number of protons is known, there is an equal number of negative electrons orbiting around the nucleus in specific energy levels. 

The atomic mass of an atom is the sum of the protons and neutrons it contains. Neutrons are neutral particles found in the nucleus. The atomic mass is calculated using the protons and neutrons but not the electrons, as their mass is negligible.

In an example, the element oxygen has an atomic number of 8 and an atomic mass of 16. Therefore, 8 protons are in the nucleus, and if we subtract the atomic number from the atomic mass, (16-8) we will find there are 8 neutrons also located in the nucleus.

To place the 8 electrons in their orbitals, the first energy level holds two electrons. That leaves 6 electrons to be placed in the next energy level which can hold up to 8. 

I have included an interactive periodic table link along with a link to a diagram of an oxygen atom.