Question

quick lab
purpose to use a graph to iden-
tify period and group trends
materials

• graph paper
• pencil

periodic trends in ionic radii
procedure
use the data presented in figure 6.23 to
plot ionic radius versus atomic number.
analyze and conclude

1. compare how does the size change when an atom forms a cat

and when an atom forms an anion?

1. describe how do the ionic radii vary within a group of metals

how do they vary within a group of nonmetals?

1. describe what is the shape of a portion of the graph that corre-

sponds to one period?

1. compare and contrast is the trend across a period similar or

different for periods 2, 3, 4, and 5?

1. explain propose explanations for the trends you have described

ionic radii within groups and across periods.
figure 6.23 atomic and ionic radii
atomic and ionic radii are an
indication of the relative size of atoms
and ions. the data listed are reported
in picometers (pm).
156—atomic radius
li
60
ionic radius
metal atom
cation
metalloid atom
anion
nonmetal atom
180 chapter 6 • lesson 3

Explanation:

Since the problem is about analyzing periodic trends in ionic radii (part of atomic structure and periodicity in chemistry), let's take question 1: "How does the size change when an atom forms a cation and when an atom forms an anion?"

• Cation Formation: When an atom loses electrons to form a cation (e.g., $\text{Na} \to \text{Na}^+$), the number of protons in the nucleus remains the same, but there are fewer electrons. The remaining electrons experience a stronger effective nuclear charge (since there's less electron - electron repulsion), so they are pulled closer to the nucleus. Thus, the ionic radius of a cation is smaller than the atomic radius of the parent atom.
• Anion Formation: When an atom gains electrons to form an anion (e.g., $\text{Cl} \to \text{Cl}^-$), the number of electrons increases. This leads to greater electron - electron repulsion among the valence electrons. The electrons spread out more, and the effective nuclear charge per electron is reduced. As a result, the ionic radius of an anion is larger than the atomic radius of the parent atom.

Answer:

When an atom forms a cation, its size (ionic radius) decreases compared to its atomic radius. When an atom forms an anion, its size (ionic radius) increases compared to its atomic radius.