Chemical Formulas

Molecular Formula. Empirical Formula. Ball Stick Model. Space Filling Model. Naming Chemical Formulas. Naming Ionic Compounds. Naming Transition Metals.

In chemistry, we usually use chemical formulas to be more specific and comprehensive.A chemical formula is used to express the composition of molecules and ionic compounds using chemical symbols. The definite proportion of atoms of a molecule are also indicated in a chemical formula.Here we are mainly concerned with two types of formulas. They, are molecular and empirical formulas.

To be clear and concise, let us define the molecular formula.Molecular formula shows the exact number of atoms of each element as the smallest unit of a substance. Thus, CO₂ is the molecular formula of carbon dioxide. O₂ is the molecular formula of oxygen, and H₂O is water. What does the subscript mean?.

The subscript indicates the number of atoms of an element present. There is no subscript for C in CO₂. This means there is only one atom of carbon in a molecule of carbon dioxide, so the number “one” is omitted from the formula.Alright! Chemical formulas are used to express the ratios in which atoms are combined.How about the structure?.Can we illustrate the structure of an atom?.

There is another way to illustrate atoms.An effective way to represent molecular structure is to use molecular models. There are two standard types of molecular models use for visualization. One is the ball-and-stick model.The other one is the space-filling model.

Look at the figure. In ball-and-stick model, the atoms are Styrofoam balls or plastic balls with holes in them. A rod represents chemical bonds. In space-filling model, atoms are represented by shortened balls held together, so that the bonds are not visible. A structural formula shows how atoms are bonded to one another in a molecule.Is there another way express the composition of molecules?.

Let us proceed to empirical formulas which are the simplest chemical formula. They are written by reducing the subscripts in the molecular formulas to the smallest whole-number ratio of their atoms. Hence, molecular formulas are the true formulas of molecules. The molecular formula is the basis for empirical formula.How do reduce the subscript in molecular formula?.

As an example, consider the compound hydrazine, written as N₂H₄. Did you know hydrazine is used as rocket fuel? In writing the empirical formula, the subscript in the molecular formula must be converted to the smallest possible whole numbers.There are two nitrogen atoms and four hydrogen atoms in hydrazine. Dividing the subscript by two, we will get the empirical formula for hydrazine which is NH₂.

Let’s look at the chemical formula of an ionic compound.The chemical formula of anionic compound is almost the same as the empirical formula. This is because an ionic compound does not contain molecular units. Why don’t they have molecular units?.For ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero.

What do we do if the charges on the cation and anion are numerically different?.On such instances we apply the following rule to make the formula electrically neutral. The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation.

Here are some examples of the chemical formula of ionic compounds.Sodium chloride. The sodium cation Na+ and the bromine anion Cl⁻ combine to form sodium chloride. The sum of their electrical charges is (+1) + (-1) = 0. Therefore they don’t need a subscript.The formula is NaCl.

Let’s take magnesium bromide as our next example. The Mg²⁺ and the Br⁻ combine to form magnesium bromide. The sum of their charges is (+2) + (-1) = +1. To make the charges add up to zero we multiply the +1 charge of the anion by 2 and add the subscript “2” to the symbol for bromine. Therefore, the formula for magnesium bromide is MgBr₂.

Next, let’s look at Aluminum Oxide. The cation is Al³⁺ and the oxygen anion is O²⁻. The sum of the charges is 2×(+3) + 3×(-2) = 0. Thus, the formula for aluminum oxide is Al₂O₃.

After we come up with the chemical formula, how do we name them?.Chemists have developed a system of naming chemical formulas concerning their composition. These are classified into three. ionic compounds, molecular compounds, and acids and bases. Using these classifications, we apply some rules to derive the chemical name.

In the previous slides, ionic compounds are made up of ions such as cations and anions. Almost all the cations are metal atoms while anions are nonmetal atoms.Metal cations take their names from the elements. For example, Na⁺ is called Sodium Ion, Ca²⁺ is called Calcium Ion, Al³⁺ Aluminum Ion.How about anions?.

The anion is named by taking the first part of the element name and adding ide. Here are some examples. O²⁻ is named oxide. Cl⁻ is named chloride. N³⁻ is named nitride. H⁻ is named hydride.

The -ide ending is also used for some anion groups containing different elements. One example is hydroxide represented as OH⁻. Another example is cyanide represented as CN⁻. Thus, the compounds LiOH and KCN are named lithium hydroxide and potassium cyanide.

How about the transition metals?.Transition metals are found mostly at the center of the periodic table of elements. They can form more than one variation of cations. Take the element Iron as an example. The element Iron represented by Fe can form two cations. They are Fe²⁺ and Fe³⁺ ions.

The accepted naming procedure for different cations of the same element is to use Roman numerals. The Roman numeral I is used for one positive charge, II for two positive charges, and so on. This is called the Stock System.

With this, Fe²⁺ ion is named named Iron (II). Fe³⁺ ion is named Iron (III). FeCl₂, having the Fe²⁺ ion is called Iron (II) Chloride. FeCl₃. having the Fe³⁺ ion, is called Iron (III) Chloride.

Let us now proceed to the molecular compounds. Molecular compounds contain molecular units. They are usually composed of nonmetallic elements. Naming molecular compounds is almost the same as naming ionic compounds. We place the name of the first element in the formula first. Then the next element is named by adding -ide to the end of the element name. So what is the difference between the two?.

In these cases, confusion in naming the compounds is avoided using Greek prefixes. They are used to denote the number of atoms of each element present. Consider these examples. NH2.NO4.

These guidelines will help you name compounds with prefixes.The prefix mono can be omitted for the first element. For example, NH2 is named nitrogen dihydride, not mononitrogen dihydride. Thus, the absence of a prefix for the first element usually means that only one atom of that element is found in the molecule.

For oxides, the ending a in the prefix is sometimes omitted. For example, N2O4 can be called dinitrogen tetroxide rather than dinitrogen tetra oxide.

This figure summarizes the steps for naming ionic and molecular compounds. It shows how naming can be first separated as ionic and molecular compounds. Ionic compounds have different naming for cations and for naming anions.Retake this lesson so that you can better understand this naming. See you in the next lesson.