CH6.3.1 Atomic and Ionic Radii

Which groups of elements are NOT shown in the table of ionic radii?

• Alkali metals

• Alkaline earth metals

• Transition and inner transition metals

• Halogens

• Noble gases

Answer explanation

The noble gases and transition metals are not shown.

General rule of thumb: the noble gases and the transition metals will pretty much always break the rules we learn.

Remember: the lanthanides and actinides are the inner transition metals.

In what unit is atomic and ionic radius measured? Express this unit in scientific notation.

Answer explanation

pico is a prefix that is attached to the dimension of measurement. In this case, atom radius is a length, so pm = picometer. 1 picometer = 1E-12 m

Which elements have the smallest and largest atomic radii shown?

• Smallest: Li, Largest: Cs

• Smallest: Be, Largest: Ba

• Smallest: O, Largest: Sr

• Smallest: He, Largest: Cs

What happens to atomic radii across a period?

• Atomic radii decrease

• Atomic radii increase

• Atomic radii remain constant

• Atomic radii fluctuate randomly

What accounts for the trend in atomic radii across a period?

• Increase in the number of protons

• Increase in the number of neutrons

• Increase in the number of electrons

• Increase in the number of valence electrons

Answer explanation

As the number of protons increases across a period, the number of shells does not increase. Thus, the effective nuclear charge increases while the shielding effect does not change.

What happens to atomic radii within a group as the atomic number increases?

• Atomic radii decrease

• Atomic radii increase

• Atomic radii remain constant

• Atomic radii fluctuate randomly

Answer explanation

The atomic radius increases down a group because the number of shells added to the atom also increases. Thus, the effective nuclear charge decreases.

In the table of ionic radii, how is the charge of the ions of elements in groups 1 and 2 related to the group number of the elements?

• The charge of the ions is equal to the group number

• The charge of the ions is one less than the group number

• The charge of the ions is one more than the group number

• The charge of the ions is two more than the group number

Answer explanation

Ion charge in groups 1 and 2 is related to the group number because these elements tend to lose electrons to achieve a stable electron configuration. Group 1 elements have one valence electron, so they lose that electron to form 1+ ions. Group 2 elements have two valence electrons, and they lose both electrons to form 2+ ions.

The ion charge corresponds to the number of electrons lost in a chemical reaction to have outer shell of eight electrons (or 2), satisfying the octet rule.

What is the charge of the ion with the smallest ionic radius in the table?

• 1+

• 2+

• 3+

• 4+

Answer explanation

Carbon has an ionic radius of 15 pm.

Cite any exceptions to the generalization you stated in your answer to question 10.

• There are no exceptions

• The generalization is always true

• The generalization is not applicable to transition metals

• The generalization is not applicable to noble gases

What accounts for the trend in ionic radii within a group?

• Increase in the number of protons

• Increase in the number of neutrons

• Increase in the number of electrons

• Increase in the number of valence electrons

Answer explanation

When electrons are added to an anion, the number of shells does not change. There is an increase in electron-electron repulsion within the highest energy level or outermost shell.

When electrons are removed from a cation, the number of shells does not change. The removal of electrons reduces the electron-electron repulsion, leading to a decrease in the repulsion within the highest energy shell.