August 11, 2019 53

Chemistry: Introduction to the Periodic Table

Chemistry: Introduction to the Periodic Table


The periodic table is the most powerful tool
chemists have for organizing chemical information. Without it, chemistry would be a chaotic,
confusing jumble of seemingly random observations. What makes the periodic table really invaluable
is its use as a predictive tool. You can predict a lot about the chemical behavior of an element
if you know where it is on the periodic table. Each element is represented by one square
on the periodic table, with a one or two-letter chemical symbol. Many of the chemical symbols
are derived from the English name for the element, but some come from other languages.
For example, the symbol for silver is Ag, from the Latin word argentum. The symbol for
lead is Pb, from the Latin word plumbum. Above the chemical symbol is the atomic number of the element, and below the symbol are the full name of the element and its atomic mass. Most elements are metals, and you can find
them on the left and in the middle of the periodic table. Metallic elements are typically
shiny and are good conductors of heat and electricity. Nonmetals are found on the upper
right of the periodic table (except for Hydrogen there on the left, it’s also a nonmetal).
Nonmetals generally are NOT shiny and are NOT good conductors of heat or electricity.
The dividing line between metals and nonmetals on the periodic table is drawn as a thick
staircase. The elements that are found on either side of that staircase are often called
METALLOIDS, and they have properties that fall between metals and nonmetals. Notice that the atoms are listed in order
of increasing atomic number, as you read the periodic table from left to right, top to
bottom. Each element has a unique atomic number – that’s the number of protons in the nucleus
of the atom. So why are the elements organized into rows and columns? Why don’t we just
put the elements in a long list? It turns out, if you arrange elements by atomic
number, a pattern emerges. There is a periodicity, or a repeating, of certain characteristics.
For example, every so often, an inert gas appears. Right next to it will be an element
that reacts violently with water. This periodic repetition is known as the PERIODIC LAW. This
is the basis for organizing the elements in the Periodic Table into columns. A vertical column of elements is called a
GROUP or a FAMILY. The elements in a group have similar chemical properties. We now know
that’s because they have similar valence electron configurations. There are 7 horizontal rows in the periodic
table. These rows are called PERIODS. Each row corresponds to a different energy level
occupied by electrons. The two groups on the left are the alkali
metals and the alkali earth metals. The s orbitals in the outermost shell of the atom
are being filled in these groups. On the right is a block of 6 columns. These
elements have the outermost p orbitals being filled. Notice on the far right are the Noble
gases, which all have a filled valence shell of electrons. In the middle is a block of 10 columns, the
transition metals. In these elements, the outermost d orbitals are being filled. The asterisks take you to the bottom of the
Periodic Table, where there are two rows of 14 columns, the inner transition metals – also
known as the Lanthanides and Actinides. These elements have the outermost f orbitals being
filled. The groups in the periodic table have
been numbered in a variety of ways over the years. Depending on which periodic table you
look at, it may have 1, 2, or even 3 different systems for numbering the groups.
You may see the groups labeled with Roman numerals and As and Bs – this system was popular
in North America and Europe. Unfortunately, the designations were somewhat arbitrary – in
North America, the A groups were the s and p blocks, known as the “Representative Elements,”
and the B groups were the d block, the “Transition Metals.” Meanwhile, in Europe, the A groups
were on the left, and the B groups were on the right. In both systems, there was one
triple-sized group called Roman numeral VIII. To eliminate all this confusion, the International
Union of Pure and Applied Chemistry (IUPAC) proposed a system that numbers the groups
1-18, with no As or Bs. This is an example of the sorts of refinements
that have changed the Periodic Table gradually over the years, as new discoveries were made
and chemists came to agreements about how to present the new information. You may not
even recognize the first periodic table – fewer than 70 elements had been discovered in the
mid 1800s – they didn’t know about noble gases yet. At that point, there was no agreed
upon way to list the elements that was of any help to chemists. For example, listing
them in the order of discovery didn’t tell you anything about their chemical behavior,
so that kind of list would be useless as any kind of predictive tool. This was the state
of affairs when Russian chemist Dmitri Mendeleev developed the Periodic Table. In 1869, Mendeleev came up with the idea
of listing the elements in order of increasing ATOMIC MASS. Almost simultaneously, Lothar
Meyer in Germany published a nearly identical system for classifying elements. We generally
give credit for the discovery to Mendeleev, because he devoted so much time and effort
championing this new system and he helped it become widely accepted. Mendeleev insisted that elements with similar
properties be listed together, and because of this, there were gaps in his table. Mendeleev
boldly proposed the existence of a number of elements that had not yet been found, that
would one day fill in these gaps. He named them for their positions in his table. For
example, the proposed element eka-aluminum would reside under aluminum, and eka-silica
would go under silicon. Some years later, these elements were indeed found, and their
characteristics closely matched Mendeleev’s predictions. This was a powerful example of
the utility of Mendeleev’s periodic table as a PREDICTIVE tool, something that chemists
didn’t have before. In 1913, English physicist Henry Moseley made
an important modification to the Periodic Table. Moseley, a member of Ernest Rutherford’s
research group, was probing metallic elements with X-rays and measuring the wavelength of
the X-ray emissions. He found that each element gave different results. Moseley developed
a mathematical relationship between the X ray wavelengths produced by different elements
and their atomic number, which increased by 1 for each element. Moseley suggested that
the atomic number was more significant for predicting chemical behavior than the atomic
mass as had been previously thought. Moseley reorganized the elements in the periodic
table, listing them in increasing order of atomic number instead of atomic mass. This
resolved some inconsistencies with Mendeleev’s table. For instance, Argon has a greater atomic
mass than Potassium, but a lower atomic number. Like Mendeleev, Moseley left gaps in the Periodic
Table where he proposed several yet-undiscovered elements should fit. These included atomic
numbers 43, 61, 72, and 75. Moseley’s proposed elements and many more
have since been discovered. There are 92 naturally occurring elements, and many elements not
found in nature have been synthesized. We’re running out of room to put the all the new
elements! We just might have to enlarge the Periodic Table in the near future.

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