General Chemistry Principles, Patterns, and Applications, 2011

valence electrons, boron forms unique aggregates that contain multicenter bonds, including metal
borides, in which boron is bonded to other boron atoms to form three-dimensional networks
or clusters with regular geometric structures. All neutral compounds of the group 13 elements are electron
deficient and behave like Lewis acids. The trivalent halides of the heavier elements form halogen-bridged
dimers that contain electron-pair bonds, rather than the delocalized electron-deficient bonds
characteristic of diborane. Their oxides dissolve in dilute acid, although the oxides of aluminum and
gallium are amphoteric. None of the group 13 elements reacts directly with hydrogen, and the stability of
the hydrides prepared by other routes decreases as we go down the group. In contrast to boron, the
heavier group 13 elements form a large number of complexes in the +3 oxidation state.
K E Y T A K E A W A Y S
Compounds of the group 13 elements with oxygen are thermodynamically stable.
Many of the anomalous properties of the group 13 elements can be explained by the
increase in Z eff moving down the group.
C O N C E PTUAL P R OBLEMS
1. None of the group 13 elements was isolated until the early 19th century, even though one of these elements
is the most abundant metal on Earth. Explain why the discovery of these elements came so late and describe
how they were finally isolated.
2. Boron and aluminum exhibit very different chemistry. Which element forms complexes with the most ionic
character? Which element is a metal? a semimetal? What single property best explains the difference in their
reactivity?
3. The usual oxidation state of boron and aluminum is +3, whereas the heavier elements in group 13 have an
increasing tendency to form compounds in the +1 oxidation state. Given that all group 13 elements have
an ns 2 np 1 electron configuration, how do you explain this difference between the lighter and heavier group
13 elements?
4. Do you expect the group 13 elements to be highly reactive in air? Why or why not?
5. Which of the group 13 elements has the least metallic character? Explain why.
6. Boron forms multicenter bonds rather than metallic lattices with delocalized valence electrons. Why does it
prefer this type of bonding? Does this explain why boron behaves like a semiconductor rather than a metal?
Explain your answer.
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