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672 Periodic Table (Chemistry)

Element One of approximately 100 pure simple ture, we can think of today’s long form of the periodic
substances that cannot be decomposed into simpler table as the legacy of physicists as well as exceptional
ones by chemical means. Each individual element is chemists like its discoverers Mendeleev and Meyer, and
made up of atoms of identical atomic number. we can call it the physical periodic table.
Family (group) Vertical column of the periodic table Chemistry is the science which investigates the change
containing elements with similar chemical and phys- of matter. It is the science of reactions which reveals what
ical properties and similar electron configurations, is a stable and what is an unstable combination of atoms.
especially for the outer electrons. There are 18 such Each chemical reaction is thus a declaration of chemi-
columns in the modern periodic table, although earlier cal selectivity. In predicting the products, one effectively
versions sometimes grouped the members of certain selects a subset from a list of many potential outcomes,
adjacent columns into one family. i.e., one specifies the selectivity of the reaction. Hence, a
I-bond A bond represented by two arrows forming an primary goal of chemistry is to uncover the affinities of
open loop in the arrow formula. atoms for each other in order to explain why some prod-
Isotopes Atoms of the same element which differ in ucts are formed but not others. To use the philosopher’s
atomic mass, that is, in number of neutrons. terminology, the project of chemistry is selectivity.
Isoelectronic Two atoms or molecular fragments which During the last two decades of the 20th century, experi-
have the same number of valence, or outer, electrons. mental and computational data, which progressively grew
Isolobal Two atoms or molecular fragments which bind in volume, interpreted by Epiotis as being inconsistent
by using atomic orbitals of the same symmetry type. with the standard notions of periodicity. A new interpre-
Isosynaptic Isoelectronic or isolobal groups bonding by tation of the molecular quantum mechanical wave func-
the same mechanism (T, I, or E). tion provided the theoretical underpinning of a chemical
Orbital Electron cloud, generated from a wave function, periodic table, which projects the intrinsic affinities of
that represents the probability of locating an elec- atoms for one of three possible mechanisms of chemical
tron as a function of three-dimensional spatial coordi- bonding and, by extension, the affinities of atoms for each
nates. Orbitals can be atomic (AO) or molecular (MO), other. This colored periodic table, named so because it is
depending upon the region to which the electron is con- a painted version of the physical periodic table, reveals
strained. the selective ways in which elements combine to form the
Period Horizontal row of the periodic table, over which molecules and materials of the “chemical world.” Here,
the chemical and physical properties of elements usu- we present a brief historical outline of the development of
ally change gradually with increasing atomic number. the traditional periodic table [for comprehensive histories,
There are 7 periods, ranging in length from 2 to 32 see the works of Van Spronsen (1969) and Puddephatt and
elements. Monaghan (1986)], and then we focus on the revolution
Quantum number Number, in most cases an integer, promised by the colored periodic table.
specifying the fact that the associated energy, momen-
tum, or other property of a system is restricted to certain I. DEVELOPMENT OF THE
values. PERIODIC SYSTEM
T-bond A bond represented by two arrows forming a
closed loop in the arrow formula. When two electrons
A. Elementary and Atomic Concepts
of opposite spin are spanned by the arrows, we have
one T-bond. When the electrons have parallel spin, we The periodic table, the great classificatory scheme of
have one T-antibond. chemistry, is based on two of the most fundamental con-
cepts in physical science—elements and atoms. In their
rudimentary forms, both of these ideas were inventions of
THE PERIODIC TABLE as it is adopted today is an ar- the pre-Socratic Greek philosophers. The earliest of these
rangement of the elements in order of increasing atomic thinkers was Thales of Miletus (ca. 624–545 BC), who
number. This classification suggests that elements with subscribed to the idea that all matter is derived from one
similar properties appear at regular, periodic intervals. substance. While he considered water to be the elemen-
For example, elements falling in vertical columns (called tary substance, others like Anaximenes, Heraclitus, and
families or groups) have been assumed to have common Anaximander favored air, fire, and apeiron (an eternal,
characteristics because they contain the same number of unlimited element), respectively.
valence electrons assigned to atomic orbitals (AOs) of the Some time around 450 BC, Empedocles (ca. 490–430
same symmetry. This is the valence isoelectronic analogy. BC) appears to have synthesized some of these ideas in his
Since the basis of elementary periodicity is atomic struc- argument that all matter is constituted of various mixtures