The center of an atom contains protons and neutrons.
The number of protons in an atom determines the type of the atom. For example, are all carbon atoms contains six protons.
You should familiarize yourself with periodic tables so that you can go to carbon and find the number 6.
The number of neutrons can vary. Most carbon atoms have six neutrons. Some can have seven and a few can have eight.
The Atomic Mass is the sum of the number of protons and neutrons.
- A carbon with six neutrons has an atomic mass of 12.
- A carbon with seven neutrons has an atomic mass of 13.
- A carbon with eight neutrons has an atomic mass of 14.
Carbon 12 makes up 98.9% of all carbon. Carbon 13 makes up 1.1% of all carbon. Carbon 14 is present at about 1 part per trillion (1 ppt) and this is too small to affect the calculation:
(12)(0.989) + (13)(0.011) = 12.011
This is why a periodic table lists carbon as having an atomic mass of 12.011 atomic mass units.
Rob Sterling 