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Page 225
Scientists study the tracks
left behind by particles
after they collide in the
cyclotron. To an expert,
these markings show the
masses of the particles.
Seaborg went on to predict the properties of elements with even
higher atomic numbers. It wasn’t long before all these elements had
been produced. Some were first made in labs at Berkeley. Seaborg’s
predictions turned out to be true. Much as Mendeleev had done
almost a century before, Seaborg had made predictions about
unknown elements based on their location in the periodic table.
The periodic table shows trends in
atomic structure.
Remember, a group is one column of the periodic table. Atoms of the
elements in a group have the same number of electrons in their outer
energy shells. This gives the elements similar chemical properties. It
also gives rise to the trends in the periodic table. Some trends are very
evident. Others are not.
The Sizes of Atoms
We measure the size of an atom by the size
of its electron cloud. The size of the cloud
depends a lot on how many electrons it
contains. It also depends on how strongly
the protons in the nucleus pull on the
electrons. If the nucleus exerts a strong pull
on the electrons, the atom is smaller. If the
nucleus pulls only weakly on the electrons,
the atom is larger.
The excerpt from the periodic table shows
how the sizes of atoms vary within groups
and across periods. Notice that within each
group, atomic size increases as you read
down the periodic table. The electron cloud around the nucleus grows
as more energy shells are filled. Notice that as you read from left to
right across each period, atomic size decreases. Even though there are
more electrons, the nucleus pulls more strongly on these electrons, so
the atoms are smaller.
Groups 3 to 12 are not
shown here.
Chapter 7: Atomic Structure and the Periodic Table 225