2.45. Mendeleev’s Periodic Table http://www.ck12.org
FIGURE 2.89
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 theFigure2.90. 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
columnsgroups. They are sometimes called families, because elements within a group are similar but not identical
to one another, like people in a family.
FIGURE 2.90
Mendeleev’s table of the elements is called aperiodic tablebecause 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 termperiodrefers to the interval between repetitions. For example,