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Dmitri Mendeleev’s New Way to Classify Elements
By the middle of the 19th century, scientists had discovered 63 elements. There was no system to classify
them, so they arranged them alphabetically. Grouping them alphabetically, however, resulted in
awkward groups (aluminum, a metal, would be grouped with argon, a gas).
Scientists needed a way of classifying elements so that elements with similar properties would be
placed together.
Mendeleev noticed a repetition of properties or periodicity. He used the pattern in the elements to
create a periodic table.
His periodic law states: if elements are arranged according to their atomic mass, a pattern can be seen
in which similar characteristic properties occur regularly.
Below is Mendeleev’s Periodic
Each of the vertical groups contain elements with similar characteristic properties. Note that the
elements increase in atomic mass across each row.
Mendeleev noticed a pattern, but also found that there were gaps in the pattern (much like how there
were gaps in the cookie pattern). His genius was evident by the fact that he left gaps in these places and
concluded that these elements just hadn’t been discovered yet.
Not only was he right, he was able to predict the characteristic properties of the missing elements by
looking at the pattern in the row (like the brownness of the cookies) and the pattern in the column (like
the type of topping on the cookies).
There was a problem, however. When he organized the elements in atomic mass order, a few elements
did not fit the pattern and needed to be reversed. For example, he switched iodine and tellurium to fit
the pattern, rather than by putting them in order of their atomic mass.
Henry Moseley helped develop a solution to Mendeleev’s atomic mass problem. Moseley’s work led to
a better understanding of the nucleus of the atom and from his work the concept of atomic number
arose. Moseley realized the importance of the number of protons. Elements in the present-day periodic
table are arranged according to atomic number (the number of protons in an element). This fixed
Mendeleev’s problem with some elements not fitting the pattern when arranged by atomic mass.
 Mendeleev transformed an earlier understanding about the classification of elements into a
grand plan called the periodic table.
 Mendeleev discovered (with a few exceptions) that arranging elements in order of increasing
atomic mass across rows and similar characteric properties down the columns would result in
elements obeying the periodic law (they repeated their characteristic properties regularly).
 Mendeleev believed that missing elements had yet to be discovered. Mendeleev left these
spots open, predicted their characteristic properties accurately, and expected that these
elements were yet to be discovered.
 The exceptions to the increasing atomic mass rule were iodine and tellurium. Mendeleev
handled the problem by placing the element in the periodic table according to characteristic
properties and ignored the atomic mass.
 Moseley solved Mendeleev’s problem by arranging elements in order of increasing atomic
number. This is because though arranging atoms according to their mass is makes an almost
perfect pattern, arranging atoms according to their number of protons makes a completely
perfect pattern.