Note-A-Rific: Nuclear

In the early part of this century, physicists were uncertain about the structure of the nucleus of an atom. By the 1930s a model had been developed that answered many questions. This was based on information that had been discovered by people like Millikan and Thomson.

The nucleus is made up of two types of particles (referred to as “nucleons”)

1.      Proton

·        positive charge = +e (remember “e” stands for “elementary charge”, and does not necessarily have anything to do with electrons)

·        mass à mp = 1.6726 x 10-27kg

·        number of protons in a nucleus à atomic number (symbol is “Z”)

·        It is the number of protons in the nucleus that determine which element you are dealing with.

·        a single proton is the entire nucleus of the simplest atom, hydrogen

2.      Neutron

·        no charge

·        mass à mn = 1.6749 x 10-27kg

 

The total number of nucleons (protons + neutrons) in a nucleus is called the atomic mass number (symbol “A”).

·        An atom with 7 protons and 8 neutrons would be written as Z = 7 and A = 15.

·        To write this out quickly and also identify the element…

which is nitrogen (see periodic table, near the top right).

·        Quite often we skip writing the value for Z (you can always look it up on the periodic table) so…

·        We read this as “nitrogen fifteen”

·        Notice that to find the number of neutrons in the nucleus, do the calculation A – Z.

 

In a neutral atom (which is what we will usually be dealing with) the number of electrons equals the number of protons.

·        How the atom reacts chemically with other atoms depends on the number of electrons orbiting the nucleus.

·        The physical characteristics of the atom depend on the number of protons in the nucleus

·        It is possible to find atoms of the same element that have different numbers of neutrons (the number of protons and electrons stays the same).

·        These are called ISOTOPES.

                                                                          

Notice that the Z value (number of protons) stays the same, so we’re always dealing with carbon here. At the same time the total number of nucleons, the A value, is increasing. The only way for this to happen is if we are adding neutrons.

·        Although these are all isotopes of carbon, some of them are more common than others.

·        About 98.9% of carbon found in nature is , about 1.1%  is

·        These percentages are called the “natural abundances”.

·        The masses on your periodic table (e.g. Carbon is 12.01) are weighted averages of the natural abundances.

 

·        Masses of atoms are sometimes given as unified atomic mass units (“u” or “amu”).

 

·        Although this is getting a bit old, and is not part of the metric system, it is still popular when discussing nuclear reactions.

·        The system is based on a neutral atom of , which is given a mass of exactly 12.000000u.

·        Based on this…

mp = 1.007276u

mn = 1.008665u

m of   = 1.007825u

* The mass of an electron is so small compared to these that it is usually ignored.