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.