(3) Number of electrons. What determines the number of electrons an atom
will contain? For an atom to exist freely in nature, it must be electrically neutral (without
a charge). There are two particles in an atom that have charges--the proton, which is
positive, and the electron, which is negative. For electrical neutrality, the sum of the
charges must be zero. In other words, the number of electrons (negative charges) must
equal the number of protons (positive charges).
Figure 1-1. First four electron shells.
d. Atomic Structure of Elements. As previously stated, each element consists
of a single type of atom. Since all atoms consist of the three basic particles we have
just discussed (except hydrogen, which usually has no neutrons), the only ways in
which elements can differ are atomic number (the number of protons) and atomic
weight, (the number of protons and neutrons). There are over 106 different elements
which scientists know to have a different atomic number and atomic weight. These
elements have a large assortment of properties. Two elements are liquids at room
temperature, eleven are gases, and all others are solids.