Question Video: Determining the Chemical Formula for an Ionic Compound Composed of Sodium and an Unknown Nonmetal | Nagwa Question Video: Determining the Chemical Formula for an Ionic Compound Composed of Sodium and an Unknown Nonmetal | Nagwa

Question Video: Determining the Chemical Formula for an Ionic Compound Composed of Sodium and an Unknown Nonmetal Science

A nonmetal, X, has 5 electrons in the outer shell. What is the chemical formula of the ionic compound formed when X reacts with sodium?

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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.

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