Study Guide to Basic Chemical Structure
Atomic Structure and the Periodic Table
*amu = atomic mass unit
**Actually, the mass of an electron is about 1/2000 that of a proton, but in terms of the chemistry we do in Biology 101, that is a
completely insignificant mass.
***Again, actually, the mass of the neutron is slightly greater than that of a proton, but we can also consider this to be an
You should be aware that, if you study chemistry in a more rigorous setting, the differences
The atom contains three subatomic particles.
Each of these has a characteristic mass and a characteristic electrical charge, and
each is found in a specific part of the atom.
In terms of the identity of an atom, the most significant thing is the number of protons in the atom.
There are 92 different
naturally occurring elements, each with its own specific number of protons.
For example, a Hydrogen atom always has one
proton, a Helium atom always has two, a Carbon atom always has 6, a calcium atom always has 20, and a Uranium atom
always has 92.
If we change the number of protons in a Carbon atom from 6 to 7, it is no longer Carbon--it becomes
(We have also performed nuclear chemistry, which is something we definitely will not be doing in Bio 101!)
An atom always has a net electrical charge of zero, which means that its positive and negative charges must balance each other
This means that an atom always has the same number of electrons as protons, since those two particles are the sources
of charge in atoms.
Each element has two important identifying numbers.
The first of these is the atomic number.
Atomic number is defined as the
number of protons in the atom.
As noted above, this number corresponds to a statement of the identity of an atom.
the atomic number of Hydrogen is 1, Helium is 2, Carbon is 6, Calcium is 20, and Uranium is 92.
though in an atom the number of protons is always equal to the number of electrons, it is not proper to define the atomic
number as the number of electrons, or as the number of protons or electrons.
This is because
under certain circumstances
many atoms gain or lose electrons, and become
If this happens to Oxygen, for instance, which has an atomic number
of 8, and whose atom has 8 protons and 8 electrons, it forms an ion which has 8 protons and 10 electrons (and a charge of -
This ion is still Oxygen, and it still has an atomic number of 8, even though it now has 10 electrons, rather than 8.
Like charges repel each other, and opposite charges attract each other.
In an atom, the electrons remain out in the electron
they are moving very rapidly around, and their forward momentum balances the force of the attraction to the
positive protons in the nucleus.
A more difficult problem to figure out is why the protons in the nucleus don’t fly apart due