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Naming & Writing Formulas for Binary Molecular Compounds

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  • 0:00 What Is A Binary…
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  • 4:23 Writing Formulas
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, you will learn what it means to be a binary molecular compound and how to properly name these types of chemicals. You'll practice naming compounds, learn how to write formulas and finish by testing yourself with a short quiz.

What is a Binary Molecular Compound?

Let's begin by dissecting the term binary molecular compound. Binary refers to a substance comprised of only two different kinds of materials. In this case, we're referring to atoms. Molecular refers to the sharing of electrons between non-metal atoms. Sharing of electrons is known as covalent bonding. A compound is a chemical made of at least two atoms bonded together.

If we put all those terms together, we have a definition: a binary molecular compound is made of two different types of nonmetal atoms covalently bonded together through the sharing of electrons.

For example, carbon dioxide (CO2) is comprised of one carbon element and two oxygen elements. There are just two types of elements present: carbon and oxygen. Both of these elements are nonmetals. Other binary molecular compounds include nitrogen monoxide (NO2) and sulfur dioxide (SO2).

Naming Binary Molecular Compounds

There are four main rules to naming binary molecular compounds and four easy steps to the naming process.

Rule 1: If both elements in the compound are in the same period on the periodic table, the element with the smaller group number is listed first. The only exception to this rule is when oxygen and halogen, or group 7A elements like fluorine and chlorine, are in a compound. Then the halogen is listed first.

Rule 2: If the elements are in two different periods, the element in the larger period will be listed first. Correct formulas should list the elements in their proper order. Thus, when naming compounds based on formulas, the elements are already in the correct order!

Rule 3: Greek prefixes are used to indicate the number of each element in the compound. They are simply attached in front of name of the element that they modify.

In case you're not familiar, here are the Greek prefixes for numbers 1-10.

Number Greek Prefix
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-

The only exception to Rule 3 occurs when there is only one of the first element. In this case, no prefix is needed.

Rule 4: The ending of the second element is dropped and replaced with '-ide'.

Now let's use these steps to name P2Br5.

  1. First ensure that the elements in the compound are correctly ordered, as required by Rules 1 and 2. As phosphorus is in a lower group number (5A) than bromine (7A), the order is correct.
  2. Next, list the number and type of each element that's present in the compound: 2 phosphorus and 5 bromine.
  3. Next, replace the number with a Greek prefix, and add it to the element it modifies. So, 2 phosphorus becomes diphosphorus, and 5 bromine becomes pentabromine
  4. Now, let's remove the ending of the second element and replace it with '-ide': diphosphorus pentabromide.

Here's another example: NO2. This compound follows all the naming rules with one exception.

  1. Make sure the elements in the compound are ordered correctly. As nitrogen is located in group 5A, which is lower than oxygen's group (6A), the order is correct.
  2. Then, list the number and type of each element that's present in the compound: 1 nitrogen and 2 oxygen.
  3. Next, replace the number with a Greek prefix, and add it to the element it modifies. Ahoy! Here is the exception. Remember, when there is only one of the first element in the compound, it does not get a prefix. So, 1 nitrogen is shown as nitrogen, and 2 oxygen becomes dioxygen.
  4. The last step involves removing the ending of the second element and replacing it with '-ide': nitrogen dioxide.

When your prefix ends in a vowel and your element name begins with a vowel, the element's vowel trumps the prefix vowel. Thus, the vowel of the prefix is dropped and the vowel of the element is kept. For example, penta + oxide becomes pentoxide

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