Video Transcript
Why is the formula for sodium
chloride NaCl, but the formula for magnesium chloride is MgCl2? (A) Magnesium forms two-plus ions,
so it requires one chloride ion to form an electrically neutral compound. (B) Chlorine forms two-minus ions,
so it requires two magnesium ions to form an electrically neutral compound. (C) Magnesium forms a one-plus ion,
so it requires two chloride ions to form an electrically neutral compound. (D) Chlorine forms a one-minus ion,
so it requires two magnesium ions to form an electrically neutral compound. (E) Magnesium forms two-plus ions,
so it requires two chloride ions to form an electrically neutral compound.
We are asked why sodium chloride
and magnesium chloride have different formulas. Specifically, the question implies
that we need to explain why the number of chloride ions differs. Chlorine has a subscript of one in
sodium chloride and a subscript of two in magnesium chloride. Sodium chloride and magnesium
chloride are both ionic compounds.
Ionic compounds are those compounds
composed of positively and negatively charged ions. Although these compounds contain
charged species, they are electrically neutral overall. Positively charged ions are called
cations, and negatively charged ions are called anions. The cations and anions are held
together by strong electrostatic forces of attraction. Specifically, the electrostatic
forces of attraction between cations and anions in an ionic compound are called
ionic bonds.
Many ions together are held in a
giant 3D ionic lattice. These ions and the strong forces of
attraction between them give ionic compounds specific properties. These include extremely high
melting points. Ionic compounds are generally
soluble in polar solvents, such as water. And ionic compounds have the
ability to conduct electricity in solution or in the molten phase.
But if ionic compounds tend to have
similar properties, why is there a difference in the subscript of the chloride ions
in the two given compounds? We find sodium on the periodic
table in group one. And we find magnesium in group
two. And we find chlorine in group
17. For some elements in the periodic
table, we can use the group number to determine the number of valence electrons an
atom of that element has. Sodium in group one has one valence
electron, and magnesium in group two has two valence electrons. Chlorine, which is in group 17, has
seven valence electrons. For groups 13 to 18, the second
digit of the group number tells us the number of valence electrons.
We can take this information and
draw simple structures to help us determine the formulas of the compounds in the
question. We can represent the number of
valence electrons in an atom of sodium and an atom of chlorine using dots. These diagrams are called Lewis
structures. Notice that sodium has one dot for
its one valence electron and chlorine has seven dots. We place the seven dots around
chlorine in pairs. Notice that the last electron of
chlorine is unpaired.
For many elements on the periodic
table, we can use the octet rule as a guide to understand how some atoms react. The octet rule is a general rule of
thumb used for main group elements. It says that main group elements
bond in such a way as to obtain a full valence shell. That is, they gain or lose
electrons to end up with eight electrons in their outermost shell.
So when a sodium atom reacts with a
chlorine atom, it donates its one valence electron to chlorine. A sodium one-plus ion forms, and a
chloride one-minus ion forms. We say the chloride ion has a full
octet, or eight electrons, in its valence shell. It is now stable. And the sodium ion also has a full
octet, or full valence shell, although this Lewis diagram does not show the eight
electrons in the valence shell. However, we can deduce that the ion
of sodium has a full octet like this.
The sodium atom had one electron in
its valence shell. This electron was donated to
chlorine, forming a sodium ion. This is now the valence, or outer
shell, of the sodium ion. And it has eight electrons, a full
octet. And so the sodium ion is
stable. For every one sodium ion, there is
one chloride ion. And the compound is specifically
neutral. And so the formula of sodium
chloride is Na1Cl1, which we simplify to NaCl.
It’s a bit different when magnesium
and chlorine react. Two chlorine atoms per magnesium
atom are needed to obtain stable ions. Magnesium donates one of its
valence electrons to one of the chlorine atoms and its other valence electron to the
other chlorine atom. A magnesium two-plus ion forms and
two chloride ions Cl− ions form. The resulting compound is
electrically neutral. And it consists of one magnesium
ion and two chloride ions.
The ions all have full octets. We can see each chloride ion has
eight valence electrons. The magnesium ion obtains a full
valence shell in a similar way to the sodium ion, although we have not shown this
here.
Finally, why do the subscripts for
the chloride ion in the formulas for sodium chloride and magnesium chloride
differ? The answer is (E). Magnesium forms two-plus ions, so
it requires two chloride ions to form an electrically neutral compound.
