Video Transcript
Boron trifluoride can react with a
fluoride anion to produce boron tetrafluoride, BF4 1−. Which of the following is the
correct Lewis structure of BF4 1−? (A), (B), (C), (D), or (E).
In this question, we want to
identify the Lewis structure of boron tetrafluoride. As a first step to solving this
problem, let’s review the types of bonds we see in the answer choices.
Conventional covalent bonds contain
valence electrons from both of the bonding atoms or ions. A standard single bond has one
electron from each bonding species. A conventional double bond has two
electrons from each bonding species. Some compounds contain
unconventional coordinate covalent bonds. The coordinate covalent bonds are
unusual because the bonding electrons come from one rather than both bonding
species. A single straight line between
chemical symbols represents a conventional single bond.
We can see that all of the Lewis
structures have at least one of these single lines between boron and fluorine. A pair of straight lines between
two chemical symbols represents a double bond. Answer choice (B) contains a double
bond between boron and fluorine. A single-sided arrow describes a
coordinate covalent bond. The arrowhead identifies the
electron pair acceptor. Several answer choices show
coordinate covalent bonds.
Let’s draw a dot-and-cross diagram
to understand the composition of all covalent bonds between the boron and fluorine
species in boron tetrafluoride. The question tells us that boron
trifluoride, shown in the dot-and-cross diagram, will react with a fluoride
anion. Fluoride has eight electrons rather
than seven that fluorine has as a neutral atom because it has a one-minus charge
state. Boron tetrafluoride results from a
coordinate covalent bond with both of the electrons coming from the fluoride
ion. The boron tetrafluoride compound
has an overall one-minus charge, as it forms from a neutrally charged BF3 compound
and a single F 1− ion.
Answer choice (D) is the only Lewis
structure that shows three single covalent bonds and one coordinate covalent bond
with boron as the electron pair acceptor and an overall negative charge. Therefore, the correct Lewis
structure for BF4 1− is answer choice (D).
