Video Transcript
A nonmetal, X, has five electrons
in the outer shell. What is the chemical formula of the
ionic compound formed when X reacts with sodium?
Let’s start by reminding ourselves
what an ionic compound is. An ionic compound is a neutral
compound composed of positively and negatively charged ions. An example is sodium chloride,
whose chemical formula is NaCl. This ionic compound is composed of
positively charged sodium metal ions, Na+, and negatively charged nonmetal chloride
ions, Cl−.
Notice that the compound is neutral
overall as there are equal numbers of positive and negative charges. In general, in an ionic compound,
the positively charged ion comes from a metal atom, which loses one or more outer
electrons. And the negatively charged ion
comes from a nonmetal atom, which gains one or more outer electrons. These gained electrons cause the
outer electron shell to be filled up with eight electrons. A strong ionic bond exists between
the positively and negatively charged ions. This is an electrostatic
attraction.
In this question, we are told that
a nonmetal X reacts with sodium, which is a metal, forming an ionic compound. The metal sodium will lose an
electron when it reacts with the nonmetal X. A sodium ion forms. It has a charge of one plus. Remember, we can leave off the
number one and just write the positive sign. Now, since nonmetal atoms can gain
an electron or electrons when they react and form negative ions, we can deduce that
X will form the negatively charged ion. We do not know the charge value on
the nonmetal ion. If we can determine what this value
is, it will help us determine what the chemical formula is of this ionic
compound.
The question gives us some extra
information to help us. It says that X has five electrons
in its outer shell. Nonmetal atoms tend to have five,
six, or seven electrons in their outer shell. This nonmetal atom has five
electrons in its outer shell. Note that we do not know which
outer shell it is. But it does not matter, since we
know that nonmetal atoms tend to gain electrons and end up with eight outer
electrons. Therefore, we can deduce that this
atom will gain three electrons into its outer shell. Since the neutral atom gained three
negatively charged electrons, we can deduce that the charge on the nonmetal ion is
three minus.
Now we are ready to determine the
chemical formula of the ionic compound composed of sodium ions and X3− ions. Remember, ionic compounds are
neutral overall, so the number of positive charges must equal the number of negative
charges. To balance out the negative three
charge on X, we will need three positive sodium ions. In other words, we need three Na+
ions for every one X3− ion. We can write this ratio as a simple
chemical formula, Na3X. The subscript of three tells us
there are three sodium ions. And we need not write a subscript
of one next to X, as the number one is implied.
Note that there is another way to
solve this question. We can find the valence of sodium,
which is one. And we can deduce that the valence
of X is three, since it gains three electrons when it reacts. We can cross over the valence
values, simplify them if possible, and make them the subscripts of the opposite
chemical symbol. This gives us Na3X.
Let’s conclude this question. What is the chemical formula of the
ionic compound formed when X reacts with sodium? The answer is Na3X.
