The Structure Of The Nucleus

Consider the nucleus of the atom. At its simplest level, it is made up of two types of particles: neutrons and protons. The very simplest atom, Hydrogen, is made up of 1 proton. The charge of a proton is the same as that of an electron, only positive. It is +e or 1.66 x 10 - 19 C. Its mass, mp, = 1.6726 x 10 - 27 kg. Compare that to the mass of an electron (me = 9.11 x 10 -31 kg.) A neutron is a particle with a charge of zero and a mass nearly the same as the proton: mn = 1.6749 x 10-27 kg. All elements, except basic hydrogen, contain neutrons. The term nucleons refers to protons and neutrons. A nuclide is a term used to refer to the nucleus and denotes different types of nuclides. You may have seen a chart of the nuclides on the wall of your physics or chemistry lab. This
should not be confused with the periodic table of the elements which is a chart showing the basic elements. The number of protons in a nucleus determines what the element is and is referred to as the atomic number. We give
this the symbol Z. (The number of electrons in an atom determine the chemical properties of the element,) When we add the number of neutrons and protons in a nucleus we get a number called the Atomic Mass Number, or A. The number of neutrons, N, can be found by subtracting the atomic number from the atomic mass number.

We denote an element by both its atomic mass number and its atomic number as shown:

An element will always have the same number of protons (for example, Carbon has 6 protons) but it may have a variety of different numbers of neutrons. We call these variations of an element an isotope. We refer to elements by their isotopes when it is necessary to avoid confusion  (carbon 12, carbon 13, carbon 14). Carbon 12 would be written

An element will always have the same number of protons (for example,  Carbon has 6 protons) but it may have a variety of different numbers of neutrons. We call these variations of an element an isotope. We refer to elements by their isotopes when it is necessary to avoid confusion (carbon 12, carbon 13, carbon 14). Carbon 12 would be written

The percentage of an isotope that occurs naturally is called its natural abundance. The mass value of elements given in periodic tables is a weighted average based on the isotopes of each element and their natural abundance.  Trivia Note - Uranium (Z = 92) is the highest occurring natural element. Anything above that on the periodic chart is produced in a laboratory.

Since numbers such as the mass of a proton or electron are difficult to deal with, scientists have simplified their masses by using atomic mass units (u). Carbon-12 has been used as the standard. 1 atom of Carbon-12 has a value of 12.000000 u. AMUs are based on atoms with their electrons (electrically neutral.) The proton has a mass of 1.007276 u, and the neutron has a mass of 1.008665 u. Mass may also be represented using energy and the electron-volt as a unit. Einstein equated mass and energy (E = mc2) and we can use this to talk about the energies of particles. 1 u = 1.66054 x 10-27kg. Plugging this into the equation, using c = 2.9979 x 10 8 m/s and a conversion factor of 1.6022 x 10-19 J/eV, we find that 1 u = 931.5 MeV/c2.

Size of the Atom - While the overall structure of the atom prohibits finding an exact size for the nucleus, we can approximate the size of a nucleus using the formula

where r is the radius of the nucleus in a roughly spherical shape.

For more on the topic, try:

http://www.lbl.gov/abc/

http://physics.nist.gov/PhysRefData/Compositions/

A useful applet: http://lectureonline.cl.msu.edu/~mmp/kap30/Nuclear/nuc.htm 

For Practice Problems, Try: Giancoli Multiple Choice PracticeQuestions (Don't worry if you can't solve all of them just yet!)