Video Transcript
An ionic compound is made from a
metal M and a nonmetal X. The electron dot diagrams for M and
X are shown below. Which of the following molecular
formulas will the ionic compound have? (A) MX, (B) MX2, (C) MX3, (D) M2X3,
or (E) M2X.
In this question, we want to figure
out which of the formulas provided would best suit the compound in the question. We are told we have an ionic
compound. An ionic compound is a compound
formed from positively and negatively charged ions, which are held together by
strong electrostatic forces of attraction. Positively charged ions are called
cations, and negatively charged ions are called anions. The strong forces of attraction
between cations and anions are called ionic bonds. The ions are held together in a
three-dimensional ionic lattice.
In this question, however, we are
not dealing so much with the physical structure of the compound but instead are
looking for its formula. We are given electron dot diagrams
or Lewis structures for a metal M and a nonmetal X. When an ionic compound forms from a
metal and a nonmetal, the metal is transformed into a cation and the nonmetal into
an anion. The metal will donate its valence
electrons to the nonmetal.
Now, Lewis structures show valence
electrons. We can see the metal M has three
valence electrons available to donate to the nonmetal X. When the metal loses its three
valence electrons, it will therefore form M3+. The maximum number of electrons
that each nonmetal atom could gain would be two to give a full octet. So the anion that X forms would be
X2−. The net charge resulting from the
combination of these two ions is one plus, but ionic compounds are neutral
overall. So M and X cannot combine in a
ratio one as to one.
One way to think about this problem
is in terms of a puzzle. Each metal cation is missing three
electrons, and each nonmetal anion has an extra two electrons. A way to make this work so that all
of these holes are plugged or filled is to have two metal cations and three nonmetal
anions. This gives us charges of three
plus, three plus, two minus, two minus, two minus with a net or overall zero charge
for the ionic compound. So the formula must include two
metal cations and three nonmetal anions.
Which molecular formula will the
ionic compound have? The answer is (D) M2X3.
