The atom is the smallest indivisible unit of an element. Scientists have discovered more than 100 particles even smaller than the atom, but for the purposes of biology we need go no further than the atomic level to begin our discussion.

Neon Atom

Each atom is made up of positively charged protons, neutral neutrons, and negatively charged electrons. The protons and the neutrons can be found in the center, crunched into a dense ball called the nucleus. The electrons surround this nucleus, attracted to the protons because of their electrostatic charges. This attraction between the positive protons and the negative electrons is just like the attraction between two oppositely charged magnets. Because the proton and the electron have equal but opposite charges, the number of protons and the number of electrons are always equal in an individual stable atom.

Two common properties of an atom are the atomic number and atomic mass. One element is distinguished from another by its atomic number. This number is simply the amount of protons in an element. Hydrogen, for example, has only one proton in its nucleus and so has an atomic number of one. Another property of an atom is the atomic mass, or atomic weight. Because almost all of the mass of an atom is in its nucleus, the atomic mass of one atom can be determined by simply adding the number of protons and neutrons that the atom has. Since this is the definition of atomic weight, this number should always be a whole integer. If one looks at the periodic table of elements, though, the atomic masses given always have a fraction. Why aren't they all nice, round numbers? Because though each element has to contain a certain number of protons, they can have a varying number of neutrons. For example, the carbon atom must have six protons to be considered carbon, but sometimes it has six neutrons, sometimes seven. These different forms are called isotopes of an element. When the atomic mass for an element is calculated, then, the mass of each isotope must be evaluated while taking into consideration how common each isotope is in nature. From this data, then, scientists can finally find a weighted average for the atomic mass of each element, and this is what we see on the periodic table.