The size of an atom is difficult to describe because atoms have no definite outer
boundary. Unlike a volleyball, an atom does not have a definite circumference.
To overcome this problem, the size of an atom is estimated by describing its
radius. In metals, this is done by measuring the distance between two nuclei in the
solid state and dividing this distance by 2. Such measurements can be made with
X-ray diffraction. For a nonmetallic element that exists in pure form as a
molecule, such as chlorine, measurements can be made of the distance between
nuclei for two atoms covalently bonded together. Half of this distance is referred
to as the covalent radius. The method for finding the covalent radius of the
chlorine atom is illustrated in the following diagram.
Figure 10 shows the relative atomic and ionic radii for some elements. As
you review this chart, you should note two trends:
- Atomic radii decrease from left to right across a period in the Periodic
Table (until the noble gases). - Atomic radii increase from top to bottom in a group or family.
The reason for these trends will become clear in the following discussions.
TIP
Know these trends in atomic radii.Atomic Radii in Periods
Since the number of electrons in the outer principal energy level increases as you
go from left to right in each period, the corresponding increase in the nuclear
charge because of the additional protons pulls the electrons more tightly around
the nucleus. This attraction more than balances the repulsion between the added