Video Transcript
In the molecule borane, BH3, boron
forms single covalent bonds with three hydrogen atoms. The structural formula of borane is
shown. Why does the boron atom in borane
form only three single covalent bonds? (A) There are three valence
electrons in the boron atom, so forming three covalent bonds allows it to gain a
stable octet in its outer shell. (B) There are three valence
electrons in the boron atom, so forming three covalent bonds fully utilizes its
outer shell. (C) There are five valence
electrons in the boron atom, so forming three covalent bonds allows it to gain a
stable octet in its outer shell. (D) There are six valence electrons
in the boron atom, so forming three covalent bonds allows it to gain a stable octet
in its outer shell. (E) There are six valence electrons
in the boron atom, so forming three covalent bonds fully utilizes its outer
shell.
A single covalent bond is a
chemical bond formed when two atoms share a pair of electrons. In the structural formula of
borane, each line represents a single covalent bond. So, each line represents a shared
pair of electrons. When atoms share electrons to form
a covalent bond, each atom contributes one valence electron to the pair. So, to form each of the single
covalent bonds of borane, the boron atom contributes a valence electron represented
by a dot and each hydrogen atom contributes a valence electron represented by an
X. From this we can surmise that an
atom of boron has three valence electrons. So answer choices (C), (D), and (E)
cannot be the answer to this question.
Now that we know that an atom of
boron has only three valence electrons, we need to know why it only forms three
covalent bonds in borane. Let’s go ahead and remove answer
choices (C), (D), and (E) to discuss this further.
To explain why atoms form covalent
bonds, we often use the octet rule of thumb. The octet rule of thumb states that
atoms tend to gain, lose, or share electrons to attain a full stable outer
shell. For many atoms, the outer shell can
hold a maximum of eight electrons, hence the name octet. For example, an atom of fluorine
has seven valence electrons in an outer shell that can hold a maximum of eight. When two fluorine atoms share
electrons to form a single covalent bond, each fluorine atom attains a full stable
outer shell.
Now, let’s take a look at
borane. An atom of boron has three valence
electrons in a shell that could hold eight. One of boron’s valence electrons
could form a covalent bond with an atom of hydrogen giving boron four electrons in
its outer shell. By forming a second covalent bond
with a hydrogen atom, the boron atom now has five electrons in its outer shell. After forming three single covalent
bonds with hydrogen atoms, the boron atom has six electrons in its outer shell. The boron atom hasn’t attained a
full stable outer shell. But it also doesn’t have any more
valence electrons to share. So, the three single covalent bonds
do not allow boron to gain a stable octet but do fully utilize boron’s outer
shell.
Therefore, the reason why the boron
atom in borane forms only three single covalent bonds is answer choice (B). There are three valence electrons
in the boron atom, so forming three covalent bonds fully utilizes its outer
shell.
