Video Transcript
In this video, we will learn how to describe the formation of covalent bonds in simple molecules.
First, what is a chemical bond? Well, our entire world is made up of atoms. Atoms are the smallest unit of ordinary matter that forms a chemical element. Atoms interact with each other to form many different types of matter and countless compounds. The forces that hold atoms together are called chemical bonds.
Chemical bonds often allow atoms to be more stable together than when they are apart. The type of chemical bond formed between atoms depends on what type of elements the atoms are. The known elements can be categorized as metals, nonmetals, metalloids, and noble gases. When metal atoms transfer electrons to nonmetal atoms during a chemical reaction, an ionic bond is formed. A covalent bond is formed when two nonmetal atoms share electrons. This forms a discrete unit called a molecule. Let’s take a closer look at covalent bonds and how covalent molecules are formed.
We can define a covalent bond as a chemical bond that is formed when two nonmetal atoms share one or more pairs of electrons. Let’s take a look at how two fluorine atoms share electrons. Each fluorine atom has seven electrons in its outermost electron shell. Electrons found in the outermost shell are called valence electrons. The fluorine molecule forms when two fluorine atoms share a pair of valence electrons. The pair of electrons shared between the atoms is the covalent bond, shown here as the electrons in the overlap of the electron shells. In other diagrams, a single line represents the shared pair of electrons.
When the fluorine atoms share a pair of electrons, each then has eight outer electrons. This can be explained using the octet rule. The octet rule states that atoms tend to share enough electrons to have eight valence electrons. This gives the atoms the same stable electron configuration as an atom of a noble gas.
While the octet rule is useful, let’s have a look at another molecule where we’ll see an exception to the octet rule. Hydrogen atoms only have one valence electron that is found in the first electron shell. An atom of hydrogen and an atom of fluorine can form a covalent bond by sharing a pair of electrons. In this molecule, fluorine has eight valence electrons and satisfies the octet rule. But the hydrogen atom only has two valence electrons. This does not obey the octet rule. But with two valence electrons, hydrogen’s valence shell is full. And it does have the same stable electron configuration as the noble gas helium.
So far, we’ve seen molecules that share just one pair of electrons between two atoms. Let’s look more closely at molecules that have multiple shared pairs.
Water has the chemical formula H2O. The chemical formula indicates that a molecule of water has one oxygen atom and two hydrogen atoms. The oxygen atom has six valence electrons, and each hydrogen atom has one valence electron. When one hydrogen atom forms a covalent bond with the oxygen atom, the hydrogen atom will have two valence electrons. And the oxygen atom will have seven valence electrons. This allows hydrogen to acquire a noble gas configuration.
However, the oxygen atom, with only seven valence electrons, is not yet stable. When the second hydrogen atom forms a covalent bond with the same oxygen atom, all three atoms will have a stable noble gas electron configuration. Each of the covalent bonds in a molecule of water are single bonds. A single covalent bond is formed when one pair of electrons is shared between two atoms. So a water molecule has two single covalent bonds.
Another way to represent the water molecule is by drawing the chemical symbols with a line connecting each hydrogen atom to the oxygen atom. Each of these lines represents a single bond and one shared pair of electrons. Lots of other molecules contain more than one single covalent bond. Examples include ammonia, which contains three single bonds, and methane, which contains four single bonds. In all of these bonds, a single pair of electrons is shared between two atoms. However, it is possible for two atoms to share more than one pair of electrons.
To help us understand the bonding between atoms, we can use dot-and-cross diagrams. A dot-and-cross diagram assigns dots to one atom’s electrons and crosses to another atom’s electrons to more clearly see which electrons are shared. Let’s use dot-and-cross diagrams to examine the bonding in a molecule of oxygen.
A molecule of oxygen contains two oxygen atoms that each have six valence electrons. If the atoms share one pair of electrons, then each oxygen atom has seven valence electrons. However, with only seven valence electrons, the oxygen atoms are not stable. But if the oxygen atoms share two pairs of electrons, then each oxygen atom will have eight valence electrons and a stable noble gas electron configuration. When two atoms share two pairs of electrons, a double covalent bond is formed. A double bond can be represented by drawing two lines between the atoms’ chemical symbols.
Now let’s take a look at a molecule of nitrogen. A molecule of nitrogen contains two nitrogen atoms that each have five valence electrons. When three pairs of electrons are shared between the atoms, then each nitrogen atom has eight valence electrons. When two atoms share three pairs of electrons, a triple covalent bond is formed. A triple bond can be represented by drawing three lines between the atoms’ chemical symbols.
For these molecules, the dot-and-cross diagrams helped us to distinguish between each atom’s valence electrons. In later lessons, determining which atoms contribute electrons to shared pairs will tell us further information about the molecules produced.
Oxides are just one of the many different types of covalent compounds. Oxides are compounds that contain oxygen and another element. In a covalently bonded oxide, the other element must be a nonmetal. Thus, these compounds are called nonmetal oxides. Examples of nonmetal oxides include carbon dioxide and sulfur trioxide.
Nonmetal oxides are formed when a nonmetal element reacts with oxygen. Let’s take a closer look at the reaction between carbon and oxygen. Carbon has four outer shell valence electrons. And as we have seen, an oxygen molecule contains a double covalent bond. When these two substances react, the double bond is broken. New double covalent bonds are formed between the carbon atom and each oxygen atom to create the nonmetal oxide carbon dioxide.
Now that we’ve seen examples of covalent bonds and covalent compounds, let’s take a look at some questions.
Which of the following is not a covalent molecule? (A) CO2, (B) HCl, (C) H2O, (D) SO3, (E) MgO.
A covalent molecule is a molecule composed of nonmetal atoms joined by one or more covalent bonds. To determine which of the chemical formulas given is not a covalent molecule, we can use the periodic table to see which does not contain two nonmetal elements.
CO2 contains carbon and oxygen. Both of these elements are nonmetals. So they are covalently bonded in the molecule CO2. The question asks which of the answer choices is not a covalent molecule. So we can eliminate answer choice (A). HCl contains the nonmetals hydrogen and chlorine. So HCl is a covalent molecule and is not the answer to the question. We’ve already seen that hydrogen and oxygen are nonmetals. Therefore, H2O is a covalent molecule and is not the answer to the question. SO3 contains sulfur and oxygen. This is a covalent molecule, since sulfur and oxygen are nonmetals. MgO contains the metal magnesium and the nonmetal oxygen. Since this formula contains a metal and a nonmetal, MgO is likely an ionic compound, not a covalent molecule.
So the chemical formula that does not represent a covalent molecule is answer choice (E), MgO.
Although there are some rare exceptions, the number of atoms of an element in a covalent compound depends on the number of bonds that need to be formed to create full outer shells in all the atoms. The electronic structures of two elements are shown in the diagram. What is the likely chemical formula of the covalent molecule formed between A and B? (A) A3B2, (B) AB2, (C) A3B, (D) AB, (E) A2B.
This question asks us to determine the chemical formula of a covalent molecule. Covalent molecules contain covalent bonds. A covalent bond is a bond formed when two atoms share one or more pairs of electrons. We are told in the question that the atoms of A and B will form a number of bonds in order to fill both atoms’ outer shells.
Atom A has one valence electron in a shell that is full when it contains two electrons. Atom B has six electrons in a shell that is full when it contains eight electrons. We can deduce the number of covalent bonds needed between atoms of A and B by using a dot-and-cross diagram. Here we have assigned crosses to represent the electrons of atom A and dots to represent the electrons of atom B.
An atom of A and an atom of B can form a covalent bond by sharing a pair of electrons. With two electrons, atom A has a full outer shell. However, atom B only has seven valence electrons and needs one more electron to have a full outer shell. Atom B can form another covalent bond with a second atom of A. Now, with the second shared pair of electrons, atom B has eight valence electrons and a full outer shell.
So, in order for all of the atoms in the covalent molecule to have full outer shells, the molecule must contain two atoms of A and one atom of B. Therefore, the most likely chemical formula of the covalent molecule formed between A and B is answer choice (E), A2B.
Now let’s review what we’ve learned. Covalent bonds form when two nonmetal atoms share electrons. A single covalent bond is formed when one pair of electrons is shared between two atoms. A double covalent bond is formed when two pairs of electrons are shared between two atoms. And a triple covalent bond is formed when three pairs of electrons are shared between two atoms.
The octet rule allows us to predict how bonds will be formed. It states that atoms tend to share enough electrons to have eight valence electrons and the same electron configuration as a noble gas atom. We can use dot-and-cross diagrams to distinguish between the electrons of different atoms. Nonmetal oxides, like carbon dioxide, are formed when oxygen reacts with another nonmetal.
