This particular resource used the following sources:. Skip to main content. Atoms, Molecules, and Ions. Search for:. Molecular Formulas. Learning Objective Identify the molecular formula of a compound given either its name or structural formula.
Key Points A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. The empirical formula represents the simplest whole-integer ratio of atoms in a compound. A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. Water is composed of hydrogen and oxygen in a ratio. Sulfuric acid is one of the most widely produced chemicals in the United States and is composed of the elements hydrogen, sulfur, and oxygen.
Certain groups of atoms are bonded together to form what is called a polyatomic ion that acts as a single unit. Polyatomic ions are discussed in more detail in Section 5. Polyatomic ions are enclosed in parenthesis followed by a subscript if more than one of the same ion exist in a chemical formula. To count the total number of atoms for formulas with polyatomic ions enclosed in parenthesis, use the subscript as a multiplier for each atom or number of atoms.
Molecular formulas are also used as abbreviations for the names of compounds. The structural formula for a compound gives the same information as its molecular formula the types and numbers of atoms in the molecule but also shows how the atoms are connected in the molecule.
The structural formula for methane contains symbols for one C atom and four H atoms, indicating the number of atoms in the molecule Figure 1. The lines represent bonds that hold the atoms together.
A chemical bond is an attraction between atoms or ions that holds them together in a molecule or a crystal. We will discuss chemical bonds and see how to predict the arrangement of atoms in a molecule later.
For now, simply know that the lines are an indication of how the atoms are connected in a molecule. A ball-and-stick model shows the geometric arrangement of the atoms with atomic sizes not to scale, and a space-filling model shows the relative sizes of the atoms. Although many elements consist of discrete, individual atoms, some exist as molecules made up of two or more atoms of the element chemically bonded together.
For example, most samples of the elements hydrogen, oxygen, and nitrogen are composed of molecules that contain two atoms each called diatomic molecules and thus have the molecular formulas H 2 , O 2 , and N 2 , respectively. Other elements commonly found as diatomic molecules are fluorine F 2 , chlorine Cl 2 , bromine Br 2 , and iodine I 2. The most common form of the element sulfur is composed of molecules that consist of eight atoms of sulfur; its molecular formula is S 8 Figure 2.
It is important to note that a subscript following a symbol and a number in front of a symbol do not represent the same thing; for example, H 2 and 2H represent distinctly different species. H 2 is a molecular formula; it represents a diatomic molecule of hydrogen, consisting of two atoms of the element that are chemically bonded together. The expression 2H, on the other hand, indicates two separate hydrogen atoms that are not combined as a unit.
The expression 2H 2 represents two molecules of diatomic hydrogen Figure 3. Compounds are formed when two or more elements chemically combine, resulting in the formation of bonds. For example, hydrogen and oxygen can react to form water, and sodium and chlorine can react to form table salt. We sometimes describe the composition of these compounds with an empirical formula , which indicates the types of atoms present and the simplest whole-number ratio of the number of atoms or ions in the compound.
For example, titanium dioxide used as pigment in white paint and in the thick, white, blocking type of sunscreen has an empirical formula of TiO 2. These can get more complicated, but the basic rules are easy to understand. Understanding Chemical Formulas. How to Write a Chemical Compound Formula. How to Name Covalent Compounds. What is the Formula For Sulfurous Acid?
How to Find the Number of Ions in a Compound. How to Remember the Charges of Polyatomic Ions. How to Memorize Ionic Compounds. How to Count Particles in Chemical Formulas. How to Find Relative Mass. How to Calculate Equivalent Weight.
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