Chemistry Reference
In-Depth Information
Understanding the size relationships is equally straightforward if the
number of electrons is constant in several species and the number of protons
changes. The greater the positive charge in the nucleus, the greater is the
attraction for the set of electrons and, therefore, the smaller is the atom
or ion.
EXAMPLE 13.2
Cl
,
Ca
2
.
Compare the sizes of
Ar, and
Solution
Each of these atoms or ions has 18 electrons. The greater positive charge in the
nucleus (20 protons) has a greater pull on the electrons than do the smaller
charges in Ar (18 protons) and
Ca
2
Cl
Ca
2
(17 protons). Thus,
is the
smallest
and
Cl
is the
largest
of these three.
O
2
,
Ne, and Na
.
Practice Problem 13.2
Compare the sizes of
ENRICHMENT
When we investigate the effects of changes
on some property, we like to make only one change at
a time so that we know that any effect is caused by that
change. If we were to make more than one change, we
would not know which of the changes caused the final
effect. This procedure is known as doing a
controlled
experiment.
When we change the number of protons
in a series of atoms and ions, we first hold the number
of electrons constant. We then know that the change in
size is due to the effect of the protons. After we know
that effect (and the effect of changing the number of
electrons only), we can change both and figure out what
is happening.
Predicting size variations of neutral atoms of neighboring elements in the
periodic table, in which both the number of protons and the number of elec-
trons change, is more difficult. It turns out that increasing the number of pro-
tons, which tends to make the atom or ion smaller, is more important than
increasing the number of electrons, which tends to make the atom or ion larger,
except when the last electron starts a new shell of electrons.
That is, as we com-
pare neighboring elements in a given period of the periodic table, the atoms get
somewhat smaller as we go to the right. Starting a new shell of electrons (at
each alkali metal) causes a
very large increase
in size, so that in going down a
group, the atoms get larger. Figure 13.1 indicates the periodic variation in size
of the neutral atoms, and Figure 13.2 shows a representation of the actual sizes
of atoms and ions.
Smaller
Smaller
Figure 13.1
General Periodic
Trends in Atomic Size
