Learn about covalent bonds, how covalent compounds are formed and the properties inherent to covalent compounds, such as low melting and boiling points, in this lesson. Also, learn what rules to follow to name simple covalent compounds.
Before we begin, take a couple breaths. Think about what you are breathing in and breathing out – what is the stuff we inhale and exhale? If you are thinking of oxygen, carbon dioxide, water vapor or nitrogen gas, then you are thinking about this lesson’s topic: covalent compounds.
Covalent compounds are quite common, they are in the air we breathe, the gas we use to fuel our cars, the water we drink and even the food we eat. A covalent compound is made when two or more nonmetal atoms bond by sharing valence electrons. Let’s take a closer look at what that definition means.
Nonmetals are types of elements that lack metallic characteristics; they are usually gases at room temperature, they are poor conductors of heat and electricity and they gain electrons to form negative ions. Nonmetals are located to the far right of the periodic table, on the right side of the ‘staircase.’ Hydrogen is also a nonmetal, though it ‘lives’ in the top left of the periodic table.
Valence electrons are the outermost electrons of an atom. These electrons are involved in bonding. Elements want to have a full valence shell of eight valence electrons. According to the octet rule, atoms will lose, gain or share electrons to achieve a full valence shell. Hydrogen is an exception to this rule in that hydrogen only needs two electrons to have a full valence shell.
Needless to say, valence electrons play a critical role in the formation of covalent compounds. The shared valence electrons between two nonmetal atoms is called a covalent bond. Think about the term covalent. ‘Co’ means share and ‘valent’ refers to valence electrons.
Two shared electrons are known as a single covalent bond. Four shared electrons are known as a double bond and six shared electrons are known as a triple bond. The oxygen we breathe consists of two oxygen atoms sharing four electrons in a double bond. Methane, or natural gas, is a covalent compound made of one carbon bonded to four hydrogens via single bonds.
Formation of Covalent Compounds
Have you ever played with two magnets? The positive end of one is attracted to the negative end of the other, exerting a pull. A similar thing happens during the formation of a covalent bond.
An atom is made of a positive nucleus and the negatively charged electrons that surround it. Imagine two chlorine atoms, each with seven valence electrons. Each of these chlorine atoms is only in need of one more valence electron to complete its outer shell. The atoms bond by sharing two electrons, forming a single bond. These electrons are concentrated in between the two chlorine atoms, locked into position by magnetic attraction. The negative electrons are attracted to the positively charged nucleus of each atom, keeping the atoms from separating.
The bonded electrons between two chlorine atoms are shared evenly – each atom exerts the same pull on the shared electrons. This isn’t always the case however, as some atoms are more electronegative than others. Electronegativity is the ability of an atom to draw electrons to itself. If a covalent bond is made between one atom that is really electronegative and another that is not, the electrons will not be shared evenly in the bond. Uneven sharing results in the formation of what is called a dipole, or the separation of charges between two covalently bonded atoms.
Hydrogen fluoride is an example of a simple covalent compound that has a dipole. The fluorine atom is extremely electronegative compared to the hydrogen. The shared electrons spend most of their time swarming the fluorine atom, resulting in an uneven distribution of charge. In this image, you can see that fluorine has a partial negative charge and hydrogen has a partial positive charge.
An example of a simple covalent bond with a dipole
A dipole covalent bond
Properties of Covalent Compounds
Next time you’re in the kitchen and have a moment to spare, sprinkle some sugar on a dark surface and make some observations about this common, tasty covalent compound. Sugar is made of covalent bonds between nonmetals – carbon, hydrogen and oxygen. It exhibits some classic properties of covalent compounds; it is a brittle solid, it has a relatively low melting point and is a poor conductor of heat and electricity.
Other properties of covalent compounds not exhibited by sugar include low boiling points, variously colored compounds and high vapor pressure in liquid phase. Some covalent compounds have incredibly low boiling points: nitrogen (N2) boils at -320 degrees Fahrenheit; carbon dioxide (CO2) at -70.5 degrees Fahrenheit. Many covalent compounds have high vapor pressure, which makes them volatile and great as fuels. Methane, propane and gasoline are all covalent compounds that readily undergo combustion, producing energy as a bi-product.
Naming Simple Covalent Compounds
Naming simple covalent compounds is easy and it makes you sound really smart. Simple covalent compounds consist of only two types of nonmetals bonded together. CO2, H2 O and CCl4 are all types of simple covalent compounds. Before we get started naming, let’s look at some simple covalent compound names and their formulas at the same time. Look at the name, and then look at the formula. Does the compound name give you any clues about the formula?
Compound Name Formula
Carbon dioxide CO2
Dinitrogen tetrachloride N2 Cl4
Phosphorus trihydride PH3
You probably noticed that the name of each element appears in the name of the compound, but that the name may be slightly modified. Also, you probably noticed that there are prefixes attached to some of the element names. If your Greek is really sharp, you probably noticed that these are Greek prefixes and that they indicate the number of each element present in the compound. For example, we know that carbon dioxide has two oxygens because the prefix ‘di’ modifies the element oxygen. Here is a list of the prefixes for numbers one through ten:
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