186 The Poetry of Physics and The Physics of Poetry
The fourth entry in the periodic table is beryllium consisting of a closed
n = 1 shell and two n = 2, l = 0 electrons. Beryllium, an alkaline earth, is
quite active chemically entering into chemical bonds in which it can
surrender its two outermost electrons. The next 6 entries in the periodic
table, boron, carbon, nitrogen, oxygen, fluorine and neon correspond to
the atomic states created by adding one more additional n = 2, l = 1
electron to the preceding entry. Since the exclusion principle allows only
6 n = 2, l = 1 states by the time we arrive at neon, the n = 2 shell is closed
and naturally, we discover that neon is an inert gas. Fluorine, which is
missing only one electron to form a closed shell, is, as expected,
extremely active chemically. The same is true of the other halogens such
as chlorine, bromine and iodine, which, like fluorine, are each one
electron short of a closed shell.
The valency of an element is the number of electrons it gives up
(if the valency is positive) or gains (if the valency is negative) when
it enters into chemical combinations. Those elements with positive
valencies, which tend to readily give up electrons easily in chemical
reaction, also tend to be good electrical conductors. The valence number
corresponds exactly to the number of electrons in excess of or missing
from a closed shell as a study of the last eight entries to the periodic
table reveals. Lithium, beryllium, and boron have the valencies +1, +2
and +3, respectively, as well as 1, 2 and 3 extra electrons in addition to
their closed shell of two n = 1 electrons. Nitrogen, oxygen, and fluorine
have the valencies –3, –2 and –1 respectively, and are missing 3, 2 and 1
electrons from the n = 2 shell. Neon, with an n = 2 closed shell, has
a valency of zero. This leaves carbon, which is more complicated
since carbon can be viewed as having four electrons in excess of its
closed n = 1 shell or missing four electrons from its n = 2 shell. We
would expect on the basis of this, therefore, that carbon would enter
into complicated bonds involving valencies of either +4 or –4. This is
indeed the case, which explains the extremely complicated chemistry of
the carbon atom and why it is able to form molecules with long chains.
The next eight entries of the periodic table after neon correspond to
the filling of the n = 3 shell. First, the two l = 0 states are filled and then
the 6 l = 1 states. The eight outer electrons of the n = 3 shell of argon, an
inert gas, form a closed shell. The eight elements, sodium, magnesium,
aluminum, silicon, phosphorous, sulphur, chlorine and argon follow
exactly the same pattern of chemical properties as the previous eight
entries, with the valencies +1, +2, +3, ±4, –3, –2, –1 and 0, respectively.