Covalent Bonds: Types of Chemical Formulas
There are several types of chemical formulas that you can use to represent chemical bonds. These include empirical formulas, molecular (or true) formulas, and structural formulas.
You can predict the formula of an ionic compound based on the loss and gain of electrons, to reach a noble gas configuration. However, you really can’t make that type of prediction with covalent compounds, because they can combine in many ways, and many different possible covalent compounds may result.
Most of the time, you have to know the formula of the molecule you’re studying. But you may have several different types of formulas, and each gives a slightly different amount of information.
Empirical formula: Just the elements
The empirical formula indicates the different types of elements in a molecule and the lowest whole-number ratio of each kind of atom in the molecule. For example, suppose that you have a compound with the empirical formula:
Three different kinds of atoms are in the compound, C, H, and O, and they’re in the lowest whole-number ratio of 2 C to 6 H to 1 O. So the actual formula (called the molecular formula or true formula) may be any of the following, or another multiple of 2:6:1.
Molecular or true formula: Inside the numbers
The molecular formula, or true formula, tells you the kinds of atoms in the compound and the actual number of each atom.
You may determine, for example, that the following empirical formula is actually the molecular formula, too, meaning that there are actually two carbon atoms, six hydrogen atoms, and one oxygen atom in the compound:
For ionic compounds, this formula is enough to fully identify the compound, but it’s not enough to identify covalent compounds. To write a formula that stands for the exact compound you have in mind, you often must write the structural formula instead of the molecular formula.
Structural formula: Add the bonding pattern
The structural formula shows the elements in the compound, the exact number of each atom in the compound, and the bonding pattern for the compound. The electron-dot formula and Lewis formula are examples of structural formulas.
Look at the Lewis formulas presented in the following figure.
Both compounds in the figure have two carbon atoms, six hydrogen atoms, and one oxygen atom. The difference is in the way the atoms are bonded, or what’s bonded to what. These are two entirely different compounds with two entirely different sets of properties:
The formula on the left represents dimethyl ether. This compound is used in some refrigeration units and is highly flammable.
The formula on the right represents ethyl alcohol, the drinking variety of alcohol.
Simply knowing the molecular formula isn’t enough to distinguish between the two compounds.
Compounds that have the same molecular formula but different structures are called isomers of each other. To identify the exact covalent compound, you need its structural formula.
Chemistry Glossary
Archimedes Principle
A principle discovered by the Greek mathematician Archimedes which states that the volume of a solid is equal to the volume of water it displaces.
Chemistry Glossary
atomic number
The number of protons in the nucleus of an atom.
Chemistry Glossary
Bohr model
A model of atomic structure developed by Niels Bohr, a Danish scientist. In this model, electrons occur in orbits of differing energy levels around the nucleus of an atom.
Chemistry Glossary
condensation
The change in the physical state of matter from a gaseous state to a liquid state.
Chemistry Glossary
deposition
The change in the physical state of matter from a gaseous state to a solid state without ever becoming a liquid. The reverse of sublimation.
Chemistry Glossary
electrolytes
Substances that can conduct electricity either in the molten state or when dissolved in water.
Chemistry Glossary
electron configuration notation
A method used by chemists to represent electrons in bonding and chemical reactions.
Chemistry Glossary
electronegativity
A measure of an atom’s strength to attract a bonding pair of electrons to itself.
Chemistry Glossary
energy level diagram
A method used by chemists to diagram the electrons for an atom (including orbitals and subshells) in bonding and chemical reactions.
Chemistry Glossary
heterogeneous mixture
A mixture whose composition varies from position to position within a sample.
Chemistry Glossary
homogeneous mixture
A mixture whose composition is the same from position to position within a sample.
Chemistry Glossary
isotopes
Atoms of the same element that have varying numbers of neutrons.
Chemistry Glossary
mass number
The sum of the protons and neutrons in a particular isotope; also called atomic weight.
Chemistry Glossary
nonelectrolytes
Substances that do not conduct electricity in the molten state or when dissolved in water.
Chemistry Glossary
nuclear fission
A nuclear reaction in which an atom’s nucleus splits into smaller parts.
Chemistry Glossary
nuclear fusion
A process in which lighter nuclei of atoms join together into a heavier nucleus; essentially the opposite of nuclear fission.
Chemistry Glossary
nuclear reaction
Any reaction that involves a change in nuclear structure.
Chemistry Glossary
periodic table
A table that displays all known chemical elements in an arrangement that is based on the properties of the elements; changes over time as new elements are discovered.
Chemistry Glossary
quantum mechanical model
A model of atomic structure that is based on mathematics and can be used to explain observations made on complex atoms.
Chemistry Glossary
radioactivity
The spontaneous decay of an unstable nucleus in an atom.
Chemistry Glossary
SI system
A worldwide measurement system that is based on the older metric system. The SI comes from the French Systeme International.
Chemistry Glossary
sublimation
The change in the physical state of matter from a solid state to a gaseous state without ever becoming a liquid (such as dry ice).
Chemistry Glossary
valence electrons
The electrons in the outermost energy level of an atom, the farthest away from the nucleus.
