Scientists
spend a lot of time organizing information into useful patterns. Before they
can organize information, however, they must possess it, and it must be
correct. Botanists had enough information about plants to organize their field
in the eighteenth century. Because of uncertainties in atomic masses and
because many elements remained undiscovered, chemists were not able to organize
the elements until a century later.
We
can distinguish one element from all others by its particular set of observable
physical properties. For example, sodium has low density of 0.971 g/cm3
and a low melting of 97.81 degree Celsius. No other element has this same combination
of density and melting point. Potassium, though, also has a low density (0.862
g/cm3) and low melting point (63.65 degree Celsius), much like
sodium. Sodium and potassium further resemble each other in that both are good
conductors of heat and electricity, and both react vigorously with water to
liberate hydrogen gas. Gold, conversely, has a density (19.32 g/cm3)
and melting point (1064 degree Celsius) that are very much higher than those of
sodium or potassium, and gold does not react with water or even ordinary acids.
It does resemble sodium potassium and in its ability to conduct heat and electricity,
however. Chlorine is very different still from sodium, potassium, and gold. It is
a gas under ordinary conditions, which means that melting point of solid
chlorine (-101 degree Celsius) is far below room temperature. Also chlorine is
a non-conductor of heat and electricity.
Even
from these very limited data, we get an inkling of a useful classification scheme
of the elements. If the scheme is to group together elements with similar
properties, then sodium and potassium should appear in the same group. And if
the classification scheme is in some way to distinguish between elements that
are good conductors of heat and electricity and those that are not, chlorine
should be set apart from sodium, potassium, and gold.
The
classification system we need is the one shown in Figure below (and inside the
front cover), known as the periodic
table of the elements. In upcoming blog posts, I will describe how
the periodic table was formulated, and you will also learn its theoretical
basis. For the present, we will consider only a few features of the table.
Features of the Periodic Table
In
the periodic table, elements are listed according to increasing atomic number
starting at the upper left and arranged in a series of horizontal rows. This
arrangement places similar elements in vertical groups, or families. For
example, sodium and potassium are found together in a group labeled 1 (called
the alkali metals). We should expect other members of the group, such as cesium
and rubidium, to have properties similar to sodium and potassium. Chlorine is
found at the other end of the table in a group labeled 17.
Some
of the groups are given distinctive names, mostly related to an important property
of the elements in the group. For example, the group 17 elements are called the
halogens, a term derived from Greek, meaning "salt former".
Each
element is listed in the periodic table by placing its symbol in the middle of
a box in the table. The atomic number (Z) of the element is shown above the
symbol, and the weighted-average atomic mass of the element is shown below its
symbol. Some periodic tables provide other information, such as density and
melting point, but the atomic number and atomic mass are generally sufficient
for our needs. Elements with atomic masses in parentheses, such as plutonium,
Pu (244), are produced synthetically, and the number shown is the mass number
of the most stable isotope.
It
is customary also to divide the elements into two broad categories metals and
nonmetals. In Figure above, colored backgrounds are used to distinguish the metals
(tan) from the nonmetals (blue and pink). Except for mercury, a liquid, metals
are solids at room temperature. They are generally malleable (capable of being
flattened into thin sheets), ductile (capable of being drawn into fine wires), and
good conductors of heat and electricity, and have a lustrous or shiny
appearance.
The
properties of nonmetals are generally opposite those of metals; for example,
nonmetals are poor conductors of heat and electricity. Several of the nonmetals,
such as nitrogen, oxygen, and chlorine, are gases at room temperature. Some,
such as silicon and sulfur, are brittle solids. One bromine is a liquid.
Two
other highlighted categories in Figure are a special group of nonmetals known
as the noble gases (pink), and a small group of elements, often called
metalloids (green), that have some metallic and some nonmetallic properties.
The
horizontal rows of the table are called periods. (The periods are numbered at
the extreme left in the periodic table inside the front cover.) The first period
of the table consists of just two elements, hydrogen and helium. This is followed
by two periods of eight elements each, lithium through neon and sodium through
argon. The fourth and fifth periods contain 18 elements each, ranging from
potassium through krypton and from rubidium through xenon.
The
sixth period is a long one of 32 members. To fit this period in a table that is
held to a maximum width of 18 members, 15 members of the period are placed at
the bottom of the periodic table. This series of 15 elements start with lanthanum
and these elements are called the lanthanides. The seventh and final period is
incomplete (some members are yet to be discovered), but it is known to be a
long one. A 15-member series is also extracted from the seventh period and
placed at the bottom of the table. Because the elements in this series start
with actinium they are called the actinides.
The
labeling of the groups of the periodic table has been a matter of some debate
among chemists. The 1-18 numbering system used in Figure 2-15 is the one most
recently adopted. Group labels previously used in the United States consisted
of a letter and a number, closely following the method adopted by
Mendeleev,
the developer of the periodic table. As seen in Figure 2-15, the A groups 1 and
2 are separated from the remaining A groups (3 to 8) by B groups 1 through 8.
The International Union of Pure and Applied Chemistry (IUPAC) recommended the
simple 1 to 18 numbering scheme in order to avoid confusion between the
American number and letter system and that used in Europe, where some of the A
and B designations were switched! Currently, the IUPAC system is officially
recommended by the American Chemical Society (ACS) and chemical societies in
other nations. Because both numbering systems are in use, we show both in
Figure 2-15 and in the periodic table inside the front cover. However, except
for an occasional reminder of the earlier system, we will use the IUPAC
numbering system in this text.
Useful Relationships from the Periodic Table
The
periodic table helps chemists describe and predict the properties of chemical compounds
and the outcomes of chemical reactions. Throughout this text, we will use it as
an aid to understanding chemical concepts. One application of the table worth
mentioning here is how it can be used to predict likely charges on simple
monatomic ions.
Main-group
elements are those in groups 1, 2, and 13 to 18. When main-group metal atoms in
groups 1 and 2 form ions, they lose the same number of electrons as the IUPAC
group number. Thus, Na atoms (group 1) lose one electron to become and Ca atoms
(group 2) lose two electrons to become Ca 2+. Aluminum in group 13
loses three electrons from Al 3+ (here the charge is “group number
minus 10”). The few other metals in groups 13 and higher from more than one
possible ion, a matter that we deal with in next blog posts.
When
nonmetal atoms form ions, they gain electrons. The number of electrons gained
is normally 18 minus the IUPAC group number. Thus, an O atom gains 18 – 16 = 2
electrons to become O2-, and a Cl atoms gains 18 – 17 = 1 electrons
to become Cl-. The “18 minus group number” rule suggests that an atom
of Ne in group 18 gains no electrons: 18 – 18 = 0. The very limited tendency of
the noble gas atoms to form ions is one of several characteristics of this
family of elements.
The
elements in groups 3 to 12 are the transition elements, and because all of them
are metals, they are also called the transition metals. Like the main group metals,
the transition metals form positive ions, but the number of electrons lost is
not related in any simple way to the group number, mostly because transition
metals can form two or more ions of differing charge.
No comments:
Post a Comment