This chapter is intended as a review of concepts covered in more depth in
general chemistry from the organic chemist's point of view. Bear in
mind that many of the topics here are emphasized with an organic bent.
Fundamentals of the Atom
An atom consists of a nucleus of protons and
neutrons surrounded by
electrons. Each of the elements in the periodic table is classified
according to its atomic number, which is the number of protons in that element's nucleus. Protons have
a charge of +1, electrons have a charge of -1, and neutrons have no charge.
Electrically, neutral atoms have the same number of electrons and protons,
but they can have a varying number of neutrons. Within a given element, atoms
with different numbers of neutrons are isotopes of that element. We will see
that isotopes typically exhibit similar chemical behavior to each other.
Electrons have such little mass that they exhibit properties of both particles
and waves. We know from Heisenberg's Uncertainty Principle that it is
impossible to know the precise location of an electron. Despite this
limitation, there are regions around the atom where the electron has a high
probability of being found. Such regions are referred to as orbitals.
Atomic Orbitals
For isolated atoms (meaning non-bonded), electrons reside in the atomic
orbitals of those atoms. Atomic orbitals are classified according to a set of
four quantum numbers which describe the energy, shape, and orientation of the
orbital.
Principle Quantum Number (n): Indicates how far the orbital is
from the nucleus. Electrons are farther away for higher values of n. By
Coulomb's Law we know that electrons which are closer to
the positively charged nucleus are more powerfully attracted and thus have lower
potential energies. Electrons of orbitals with higher values of n, being
farther away from the
nucleus, have greater potential energies. In a given atom, all the
atomic orbitals with the same n are collectively known as a shell.
n
can take on integer values of 1 or higher (ex. 1, 2, 3, etc.).
Angular Momentum Quantum Number (l): Describes the shape of the
orbital. The angular momentum number (or subshell) can be represented
either by number (any integer from 0 up n-1) or by a letter (s, p, d, f, g,
and then up the alphabet), with 0 = s, 1 = p, 2 = d, and so on. For
example:
when n = 1, l can only equal 0; meaning that shell n = 1
has only an s orbital (l = 0).
when n = 3, l can equal 0, 1, or 2; meaning that shell n =
3 has s, p, and d orbitals.
s orbitals are spherical, whereas p orbitals are dumbbell-shaped. d
orbitals and beyond are much harder to visually represent.
Figure %: s and p atomic orbital shapes
