(3) Atomic number and atomic weight. Two important figures commonly
used when discussing an atom are its atomic number and its atomic weight.
(a) Atomic number. The atomic number of an atom is equal to the
number of protons in the nucleus of the atom. For example, a carbon atom has six
protons in its nucleus; therefore, the atomic number of carbon is six.
(b) Atomic weight. The atomic weight of an atom is equal to the
number of protons in the nucleus of the atom (one amu each) plus the number of
neutrons in the nucleus of the atom (one amu each). Therefore, a carbon atom with six
protons and six neutrons has an atomic weight of 12.
c. The Outer Structure. The particles that orbit the nucleus (as the planets
orbit the sun) are called electrons. These particles have an electrical charge of negative
one (-1), but their mass is so small that it is considered to be zero. Actually, the mass of
the electron is 1/1837 of the mass of a proton, but the mass, which contributes to the
atom is so small that it is not important. The symbol for the electrons is e or .
(1) Electron configuration. Since we may have many electrons going
around the nucleus, It might appear that there could be a collision of electrons.
Collisions do not occur because the electrons are located in orbits, which are different
distances from the nucleus and because of the repulsion between like charges. The
number of electrons and their locations are called the electron configuration. This
electron configuration is different for each element.
(2) Electron shell. The term electron shell (or energy level) describes where
electrons are located (i.e., a specific region around the nucleus). Since electrons can
be forced to leave their atoms, the term energy level indicated the amount of energy
required to remove the electrons from the various levels or shells. A nucleus can have
seven shells, but more chemicals of medicinal importance contain electrons in the first
four, which are labeled the K, L, N, and N shells. The K shell is the closest to the
nucleus and the N shell is the farthest from the nucleus (figure 1-1). These shells
contain different numbers of electrons. The maximum number each shell can hold is
equal to 2N2, where N is the number of the shell (K=1, L=2, M=3, and so forth.). Thus
the maximum number of electrons that each of the first four shells can hold Is:
Since, for example, the M shell can contain as many as 18 electrons, the possibility for
collision might still appear to exist. The reason collisions do not occur is that a shell is
subdivided into smaller energy levels, called subshells and orbitals, which we will not
need to consider.