Mendeleev's Periodic Table
Look at the left-hand photo above. What a messy closet! Do
you have a messy closet too? If you do, then you know how
hard it can be to find a specific item of clothing. If you don’t
have a messy closet, just imagine trying to find a particular
shirt or pair of jeans in the closet above. It could take a long
time, and it would probably make you late for school! Now look
at the closet on the right. It’s very neat and well organized.
With a closet like this, it would be easy to find whatever item
you wanted.
Q: What do these two closets have to do with science?
A: They show why it’s important to keep things organized,
including the elements, which are the pure substances that
make up all kinds of matter.
Organizing Elements
For many years, scientists looked for a good way to organize
the elements. This became increasingly important as more and
more elements were discovered. An ingenious method of
organizing elements was developed in 1869 by a Russian
scientist named Dmitri Mendeleev, who is pictured below.
Mendeleev’s method of organizing elements was later revised,
but it served as a basis for the method that is still used
today. You can learn more about Mendeleev and his work at
this URL: http://videos.howstuffworks.com/sciencechannel/27862-100-greatest-discoveries-the-periodic-tablevideo.htm.
Mendeleev was a teacher as well as a chemist. He was writing
a chemistry textbook and wanted to find a way to organize
the 63 known elements so it would be easier for students to
learn about them. He made a set of cards of the elements,
similar to a deck of playing cards. On each card, he wrote the
name of a different element, its atomic mass, and other known
properties. Mendeleev arranged and rearranged the cards in
many different ways, looking for a pattern. He finally found it
when he placed the elements in order by increasing atomic
mass.
Q: What is atomic mass? Why might it be a good basis for
organizing elements?
A: Atomic mass is the mass of one atom of an element. It is
about equal to the mass of the protons plus the neutrons in an
atom. It is a good basis for organizing elements because each
element has a unique number of protons and atomic mass is an
indirect way of organizing elements by number of protons.
Groups and Periods
You can see how Mendeleev organized the elements in
the Figure below. From left to right across each row,
elements are arranged by increasing atomic mass. Mendeleev
discovered that if he placed eight elements in each row and
then continued on to the next row, the columns of the table
would contain elements with similar properties. He called the
columns groups. They are sometimes called families, because
elements within a group are similar but not identical to one
another, like people in a family.
Mendeleev’s table of the elements is called a periodic
table because of its repeating pattern. Anything that keeps
repeating is referred to as periodic. Other examples of things
that are periodic include the monthly phases of the moon and
the daily cycle of night and day. The term period refers to
the interval between repetitions. For example, the moon’s
phases repeat every four weeks. In a periodic table of the
elements, the periods are the rows of the table. In
Mendeleev’s table, each period contains eight elements, and
then the pattern repeats in the next row.
Filling in the Blanks
Did you notice the blanks in Mendeleev’s table? They are
spaces that Mendeleev left blank for elements that had not
yet been discovered when he created his table. He predicted
that these missing elements would eventually be discovered.
Based on their position in the table, he even predicted their
properties. For example, he predicted a missing element in row
5 of group III. He also predicted that the missing element
would have an atomic mass of 68 and be a relatively soft metal
like other elements in this group. Scientists searched for the
missing element, and they found it just a few years later.
They named the new element gallium. Scientists searched for
the other missing elements in Mendeleev’s table and eventually
found all of them.
An important measure of a good model is its ability to make
accurate predictions. This makes it a useful model. Clearly,
Mendeleev’s periodic table was a useful model. It helped
scientists discover new elements and made sense of those
that were already known.
Summary

In 1869, Dmitri Mendeleev developed a method for
organizing elements based on their atomic mass. His method
was later revised, but it served as a basis for the method
used today.

Mendeleev created a periodic table of all the elements that
were known at the time. The rows of the table, called
periods, each contained eight elements that increased in
atomic mass from left to right. The columns of the table,
called groups, contained elements with similar properties.

Mendeleev’s periodic table was a good model because it
could be used to predict unknown elements and their
properties. All of these missing elements were eventually
discovered.
Explore More
Do the activity at the URL below to simulate how Mendeleev
developed his periodic table. After you organize your element
cards, answer the questions provided. Don’t look at the
answers until you are finished.
http://www.education.com/reference/article/mendeleevperiodic-table/?page=2
Review
1. How did Mendeleev develop his periodic table of the
elements?
2. What are the groups in Mendeleev’s table?
3. Describe the periods in Mendeleev’s table.
4. Why was Mendeleev’s periodic table a good model?