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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.

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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 pentoxideWithout looking ahead, can you name H2O and CO yourself? Pause this video for a moment to determine the names for these molecules.H2O is dihydrogen monoxide (aka water) and CO is carbon monoxide.

Writing Formulas

Writing formulas for binary molecular compounds is like decoding a message. If you can name a compound from its formula, then it’s easy to do the reverse. We will rely on some of the same rules and use essentially the same four steps.Rule 1 and 2 remain unchanged. Rule 3, which has to do with numbers, states that the number of the element will be noted as a subscript immediately following the element symbol.

Elements that only occur once do not require a 1. There is no rule 4!Let’s find the formula for the compound sulfur dichloride.

  1. The first step is to make sure the elements are in the correct order. Sulfur is in a lower group (6A) than chlorine (7A), so the order is correct.

  2. Next, determine the name and number of each element present: 1 sulfur and 2 chlorine.
  3. Replace the element names with their symbols: 1S and 2Cl.
  4. Lastly, turn numbers into subscripts, and place them to the bottom right of the element they modify. Since the number 1 is never used in chemical formulas, we drop it. Our formula becomes SCl2.

Here’s another example: carbon tetrafluoride.

  1. Ensure that the elements are in the correct order. Carbon is in a lower group (4A) than fluorine (7A), so the order is correct.
  2. Determine the name and number of each element present: 1 carbon and 4 fluorine.
  3. Next, replace the element names with their symbols: 1 C and 4 F.
  4. Turn numbers into subscripts, and place them to the bottom right of the element they modify: CF4.

Lesson Summary

A binary molecular compound consists of two different types of nonmetal atoms that share electrons and form a covalent bond. The proper naming of binary molecular compounds is based upon four rules and steps:Rule 1: List the element with the smaller group number first if both elements in the compound are in the same period.Rule 2 If the elements are in two different periods, list the one in the larger period first.Rule 3 Use one of the Greek prefixes to indicate the number of each element.Rule 4 Replace the ending of the second element with ‘ide’.Additionally, you can use the names of binary molecular compounds to write their formulas by reversing the process and eliminating the fourth step.

Some binary molecular compounds include carbon dioxide (CO2), nitrogen monoxide (NO2) and sulfur dioxide (SO2).

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