Lesson Plan - Get It!
What kinds of things do you share with your friends? How do atoms make friends?
It can be easy to share things with friends because you are connected and enjoy being with one another.
Some elements share electrons easily in a chemical bond, much like friendship. When two elements are sharing electrons, it is called a covalent bond.
Before continuing, if you skipped or need to review the first two Related Lessons in this Chemical Bonding and Reactions series, find them in the right-hand sidebar. Also, if you need a copy of the periodic table, find one under Downloadable Resources.
By Apostoloff [Public domain], from Wikimedia Commons
Notice above how hydrogen and chlorine are sharing the two electrons located between them.
- How many valence electrons does chlorine have?
It has seven, because it is located in group 7A. Hydrogen has one valence electron, and is located in group 1A. Chlorine needs that one electron to satisfy the octet rule, the magic number of eight electrons in the outer layer. When chlorine bonds with hydrogen, it is able to satisfy that requirement.
Covalent bonds do not occur between every type of element. Covalent bonds form between two non-metals. Non-metals are found on the right side of the periodic table, shown in green, yellow, orange, and red in the image below:
When two elements enter into a covalent bond, they form a molecule. Since most non-metals are liquids or gases to start with, most covalent molecules are also liquids or gases. These molecules generally have low melting and boiling points and do not conduct electricity.
There are many useful covalent molecules, including water, carbon dioxide, and glucose. These molecules are drawn using the Lewis dot structure to communicate how the electrons are distributed and shared between atoms. Lewis dot structures for molecules are a little more complicated than those for a single element.
First, count the total number of valence electrons for all elements in the molecule. Water will be our example: H2O includes two atoms of hydrogen and one atom of oxygen. Hydrogen has one valence electron, but water has two atoms of hydrogen. Oxygen has six valence electrons, which comes to a total of eight valence electrons for the molecule.
Second, designate a central atom. In molecules with carbon, it is always the central atom. If the molecule does not have carbon, choose the element that you only have one atom of. In water, it is oxygen.
Place oxygen in the middle and write an H on either side, it should look like H O H. Now, you are ready to place electron dots. Start by placing the dots around the central atom; in this case, the oxygen. Place two electrons on each side until the central atom is satisfied:
By Riccio [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
Oxygen has six electrons, and hydrogen has one each, as seen in the image above from WikiMedia Commons. Each atom in the bond has the right number of electrons. To make the image more efficient, pairs of electrons might be written as lines, as shown in the image below by WikiMedia Commons:
From Wikimedia Commons, the free media repository
The lines are written at a slant in the image because of the shape of water. The shape of water is called "bent;" notice how the angle of the hydrogen bonds creates a non-linear shape. Notice also that there are two sets of electrons on the top of the oxygen atom. These are unbonded electrons.
- So, how do you know if a molecule is covalent based on the name?
Well, you would use your periodic table to identify the elements found in the bond.
- Which two elements do you think make up carbon dioxide?
The answer is, "carbon and oxygen." The di- prefix indicates that you have two oxygens. The chemical formula for carbon dioxide is CO2. Notice that the number of atoms is written as a subscript attached to the element that it describes. Carbon doesn't have a subscript, because the one atom of carbon is implied.
Covalent compounds follow a simple naming structure. The name of the first element doesn't change. A prefix is added to the first element if there is more than one atom of the element in the molecule. The second element gets a prefix to show how many atoms of the element are found in the molecule, even if it is only one. The ending of the second element changes to –ide.
In the example, carbon dioxide, carbon doesn't change. There is only one carbon atom, so it doesn't get a prefix. Oxygen changes to oxide, and a prefix is added to show that there are two oxygen atoms.
The prefixes are based on the following values:
Write the formula for selenium hexachloride. You should have SeCl6. You have one atom of selenium, and six of chlorine.
Writing the names from the formula can be a little more challenging. Try to name N2O. Be sure to use a periodic table to identify the elements!
N2O is dinitrogen monoxide, because there are two atoms of nitrogen and one atom of oxygen. Remember that the first element does get a prefix if there is more than one atom, and the second element always gets a prefix!
Covalent molecules form between two non-metals. These molecules have lower melting and boiling points because they are made up of mostly liquids and gases. Scientists use Lewis dot structures to model covalent compounds, representing the bonded electrons as dots or lines drawn between the atoms. Covalent molecules use a naming system that relies on prefixes to communicate the number of atoms in each bond. Use a table like the one below to summarize what you learned about covalent molecules:
In the Got It? section, practice naming and identifying covalent compounds with an online activity.