Unit 16: Covalent Nomenclature

 

Unit Overview:

In the last unit, you examined the chemical nomenclature of ionic compounds, learning how to write ionic formulas and name them.  In the laboratory, you conducted a laboratory to identify the components of unknown ionic compounds.

 

In this unit, you will engage in the same learning process, examining the chemical nomenclature of covalent molecules.

 

How is a covalent molecule different from an ionic compound?

An ionic compound is a substance that is held together by an ionic bond, which refers to the transfer of electrons between atoms. It is formed between positive and negative ions, which are formed by metals and nonmetals.  The chemical formula is a ratio formula that shows the ratio of ions that result in a neutral compound.

 

An covalent molecule is a substance that is held together by a covalent bond, which refers to the sharing of electrons between atoms. Because electrons are shared, not transferred, no ions are formed; instead, a singular structure in which elements overlap electron clouds is formed. Molecules form between two or more nonmetals. The chemical formula is an actual formula that shows how many atoms share electrons within that structure.

 

 

What are the Greek Prefixes?

Covalent nomenclature utilizes Greek prefixes to indicate the numbers of elements involved in a molecule.

The number of atoms that are involved in the molecule is indicated by Greek Prefixes:

 

Number of Atoms

Greek Prefix

1

mono-

2

di-

3

tri-

4

tetra-

5

penta-

6

hexa-

7

hepta-

8

octa-

9

nona-

10

deca-

 

 Some Special NOTES:

1.)  The prefix mono- is omitted from the description of the first element in the molecule.

2.)  The final vowel in a prefix is often dropped before a vowel in a stem name.

        Example:  CO is carbon monoxide, NOT monocarbon monooxide!

 

How is a covalent formula written?

In covalent nomenclature, by convention, the less electronegative atom is placed first, followed by the more electronegative atom.  Remember that electronegativity describes the pull of electrons within a bond. So let’s review the periodic trends of electronegativity that you explored in unit 11. . .

        Within a family, the electronegativity decreases as you move down the family.

        Within a period, the electronegativity increases as you move across a period.

 

 

To write formulas, given the name:
  1. Convert the names of elements to symbols.
  2. Convert the prefixes to subscripts.
  3. Write the formula.

 

Let’s Try Some!     Write the formula for each of the following covalent molecules:

1.   nitrogen dioxide

Step 1:  Convert the names of the elements to symbols.

 

nitrogen is the symbol N.

 

0xide is the element oxygen, with the symbol O.

Step 2:  Convert the prefixes to subscripts.

 

No prefix in front of nitrogen means that there is 1 nitrogen atom in the molecule.

 

The prefix di- in front of oxide means that there are 2 oxygen atoms in the molecule.

Step 3: Write the formula.

NO2

 

 

2.   carbon disulfide

Step 1:  Convert the names of the elements to symbols.

 

carbon is the symbol C.

 

sulfide is the element sulfur, with the symbol S.

Step 2:  Convert the prefixes to subscripts.

 

No prefix in front of carbon means that there is 1 carbon atom in the molecule.

 

The prefix di- in front of sulfide means that there are 2 sulfur atoms in the molecule.

Step 3: Write the formula.

CS2

 

 

3.   silicon trichloride

Step 1:  Convert the names of the elements to symbols.

 

silicon is the symbol Si.

 

chloride is the element chlorine, with the symbol Cl.

Step 2:  Convert the prefixes to subscripts.

 

No prefix in front of silicon means that there is 1 silicon atom in the molecule.

 

The prefix tri- in front of chloride means that there are 3 chlorine atoms in the molecule.

Step 3: Write the formula.

SiCl3

 

 

4.   diphosphorus pentoxide

Step 1:  Convert the names of the elements to symbols.

 

phosphorus is the symbol P.

 

Oxide is the element oxygen, with the symbol O.

Step 2:  Convert the prefixes to subscripts.

 

The prefix di- in front of phosphorus means that there are 2 phosphorus atoms in the molecule.

 

The prefix pent(a)- in front of oxide means that there are 5 oxygen atoms in the molecule.

Step 3: Write the formula.

P2O5

 

 

 

 

To name compounds, given the formula:
  1. Convert the subscripts to prefixes.
  2. Convert the symbols to names.
    1. 1st atom: prefix + name of element
    2. 2nd atom: prefix + root name of element + -ide ending
  3. Write the name.

 

Let’s Try Some!   Write the name of each of the following covalent compounds:

1.     N2O4

1st Atom: Convert subscript to prefix, a symbol to name.

●     Subscript 2 is prefix di-

●     Symbol N is nitrogen

2nd Atom: Convert subscript to prefix, symbol to a name, with -ide ending.

●     Subscript 4 is prefix tetra-

●     Symbol O is oxygen → oxide.

The name is dinitrogen tetroxide.

 

2.     CI2

1st Atom: Convert subscript to prefix, a symbol to name.

●     No subscript is an understood 1, which means the prefix mono-, but this prefix is dropped from the first element.

●     Symbol C is carbon.

2nd Atom: Convert subscript to prefix, a symbol to name, with -ide ending.

●     Subscript 2 is prefix di-

●     The symbol I is iodine → iodide.

The name is carbon diiodide.

 

3.     CCl4

1st Atom: Convert subscript to prefix, a symbol to name.

●     No subscript is an understood 1, which means the prefix mono-, but this prefix is dropped from the first element.

●     Symbol C is carbon.

2nd Atom: Convert subscript to prefix, a symbol to name, with -ide ending.

●     Subscript 4 is prefix tetra-

●     Symbol Cl is chlorine → chloride.

The name is carbon tetrachloride.

 

4.     BF3

1st Atom: Convert subscript to prefix, a symbol to name.

●     No subscript is an understood 1, which means the prefix mono-, but this prefix is dropped from the first element.

●     Symbol B is boron.

2nd Atom: Convert subscript to prefix, a symbol to name, with -ide ending.

●     Subscript 3 is prefix tri-

●     Symbol F is fluorine → fluoride.

The name is boron trifluoride.

 

Watch the following video for a verbal and visual description of writing formulas and naming of covalent molecules:  Covalent Nomenclature

Practice:  Complete this online practice quiz of naming and writing covalent molecules.

 

 

ChemLab: Identifying Nutrients

 

Overview:

In this lab, you will not directly explore covalent nomenclature.  Instead, you will analyze food samples in order to determine if the food is a carbohydrate, a protein, or a lipid, all of which are held together by covalent bonds. You will perform multiple tests to identify these nutrients.

 

Directions:

1.   Download the Student Exploration and Vocabulary sheets for the Identifying Nutrients.

2.   Familiarize yourself with the words on the vocabulary sheet.

3.   Log-in to your Explore Learning account.

4.   Click on “Identifying Nutrients” and launch the gizmo.

5.   Answer the Prior Knowledge Question.

6.   Practice using the Gizmo, using the Gizmo warm-up instructions.

7.   After you are comfortable using the Gizmo, begin the activity. Use the lab sheet as a guide to complete the 2 activities:

a.   Activity A: Identifying Nutrients

b.   Activity B: Nutrients and Food Types