Key Concepts:
isotope  nuclear reaction  atomic number   nuclear decay    mass number   radiation  atomic mass
The atomic number of any atom is determined by the number of protons in its nucleus. All atoms of the same element have the same number of protons. Atoms of different elements have different numbers of protons. For example, all Oxygen atoms have 8 protons, all Nitrogen atoms have 7 protons. It is the number of protons, and therefore the atomic number, that identifies the element. If you know that an atom has 18 protons then it must be an atom of the element Argon, because ALL Argon atoms have 18 protons.
The mass number of any given atom depends solely on the number of protons and neutrons in its nucleus. The mass number of any atom can be determined by adding the number of protons and neutrons. (Mathematically this is stated as Mass Number = Protons + Neutrons). For instance, a Carbon atom with 6 protons and 6 neutrons will have a mass number of 12AMU. However, a Carbon atom with 6 protons and 8 Neutrons will have a mass number of 14AMU. They are both Carbon atoms, however they each have a different mass number. Atoms of the same element that have different numbers of neutrons, and therefore, different mass numbers, are called isotopes. Isotopic symbols are used to indicate isotopes of the same elements. In the following isotopic symbols the lower number is the atomic number…it is the number of protons. The upper number is the mass number, it represents to sum of the protons and neutrons in the atoms nucleus.
                 
Carbon 12    Oxygen 16     Uranium 235
Here are 3 Isotopes of Hydrogen

Hydrogen 1    Hydrogen 2       Hydrogen 3
Hydrogen       Deuterium         Tritium    ß These are the commonly used names of the three Isotopes of Hydrogen. Make no mistake about it, they are all Hydrogen atoms because they each have an atomic number of 1. They are Isotopes only because they each have a different number of neutrons, and therefore, different mass numbers.
The net charge of the nucleus is positive. Since like electrical charges repel each other it would stand to reason that the positively charged protons should be repelled from each other, causing the nucleus to fly apart. The Strong Nuclear Force prevents this from happening. The Strong Nuclear Force acts like a glue; stronger than the repulsive electrical forces between the Protons, it acts across very short distances to hold the nucleus together…IF there are not to many Neutrons. Too many Neutrons weaken the Strong Force allowing the nucleus to kick out the extra mass, some of which may exit the nucleus as particles of mass, some of which may exit as radiant energy. This loss of extra mass from the nucleus of a Isotope is called Nuclear Decay. The matter and energy released from this unstable nucleus are commonly referred to as Nuclear Radiation. Nuclear Radiation takes three forms, Alpha, Beta, and Gamma radiation.   Click here to view a video of Alpha particle tracks in a cloud chamber.
Most elements have naturally occurring isotopes. The Atomic Mass of an element is the weighted average of the naturally occurring isotopes of that element in nature. This is why most of the atomic masses on the Periodic Table are not whole numbers. If the atomic mass is the weighted average of the masses of all the naturally occurring isotopes of an element, it is easy to calculate the weighted averages if you know the percentages of each isotope present. For example:
If the Statue of Liberty is 69% copper-63 and 31% copper-65, just multiply the mass of each isotope by its percentage and add them up to find the atomic mass of copper in the statue.
(63 x .69) = 43.47 AMU   
(65 x .31) = 20.15 AMU       so...
(43.47) + (20.15) = 63.62 AMU which is the atomic mass or weighted average mass of copper in the statue.
Click here to see a video that explains how average atomic mass is calculated.
TRY THIS: Calculate the atomic mass of boron, which occurs naturally at 20% boron-10 and 80% boron-11.