What is the difference between a molecular formula and a structural formula and electron dot formula

The key difference between molecular formula and structural formula is that molecular formula gives the chemical elements present in a compound and their numbers, whereas structural formula gives the drawing of the structure of the molecule.

The terms molecular formula and structural formula are closely related terms that are often confused with each other. However, they are two different terms.

CONTENTS

1. Overview and Key Difference
2. What is Molecular Formula 
3. What is Structural Formula
4. Molecular Formula vs Structural Formula in Tabular Form
5. Summary – Molecular Formula vs Structural Formula 

What is Molecular Formula?

Molecular formula is a formula showing the type of atoms and the number of each atom connected in the molecule. Typically, the atoms are depicted by their symbols, which are shown in the periodic table. Moreover, we should write the numbers of atoms as subscripts. Some molecular formulas are neutral (no charge), but if there is a charge, we can show it on the right side of it as a superscript.

Figure 01: Molecular Formula

Generally, we can use the molecular formula in chemical reactions or when documenting any chemical details. By just looking at the molecular formula, we can get a lot of information about the molecule. For example, we can calculate the molecular mass. Moreover, if it is an ionic compound, we can predict what the ions are and how many of them will be released when it is dissolved in water.

Furthermore, we can predict the oxidation numbers of each atom, how they are going to react in a reaction, and the resultant products using molecular formulas. However, we cannot predict the exact molecular arrangement only from the molecular formula. This is because sometimes, there are several structural formulas for a single molecular formula. These are “isomers.” Isomers have the same molecular formula but can differ from the connectivity of atoms (constitutional isomers) or the spatial arrangement of atoms (stereoisomers). Therefore, by looking at the molecular formula, we can write all the possible isomers for a molecule.

What is Structural Formula?

Structural formula of a chemical compound is an important way of showing the types of atoms present in a particular compound and the connectivity of these atoms. In other words, the structural formula shows us how the atoms are bonded to each other and the type of bonds in the molecule (e.g. single bonds, double bonds, and triple bonds).

Figure 02: Structural Formulas of Different Chemical Compounds

Generally, a structural formula gives the location of chemical bonds between the atoms using small, short lines that represent the chemical bond. These lines are only used to show covalent bonds. We can show ionic bonds in the structural formula. But sometimes, plus and minus signs (+ and -) are used to show the ions in an ionic compound. When using short lines, single lines show single covalent bonds, two parallel lines show double bonds, and three parallel lines show triple covalent bonds.

The atoms are written using the chemical symbol as given in the periodic table. The short line that represents the chemical bond is drawn between two atoms. Sometimes, to avoid complexities, the group of atoms is attached through short lines, e.g. functional groups can be written as a group of atoms.

What is the Difference Between Molecular Formula and Structural Formula?

A molecular formula is important as the first glance at a chemical compound, but a structural formula is important because it shows the arrangement of the molecule as well. The key difference between molecular formula and the structural formula is that a molecular formula gives the chemical elements present in a compound and their numbers, whereas a structural formula gives the drawing of the structure of the molecule.

There are many important uses of molecular formulas and structural formulas, including the determination of the properties of the compounds. The key difference between molecular formula and the structural formula is that molecular formula gives the chemical elements present in a compound and their numbers, whereas structural formula gives the drawing of the structure of the molecule.

Reference:

1. “Structural Formula.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.

Image Courtesy:

1. “Naming-and-Writing-Formulas-of-Covalent-Compounds” By Zappys Technology Solutions (CC BY 2.0) via Flickr
2. “The structure formulas of different types of pheromones” By Encyclopedia2 – The Free Dictionary (CC BY-SA 4.0) via Commons Wikimedia

Molecular formula and structural formula of compounds represent atoms of different chemical elements present in the compounds. The molecular formula gives the ratio between these elements. Structural formula gives the relative positions of these atoms. The main difference between molecular formula and structural formula is that molecular formula cannot indicate the relative positions of the atoms whereas structural formula indicates the relative positions of the atoms.

Key Areas Covered

1. What is Molecular Formula
     – Definition, Finding the Molecular Formula
2. What is Structural Formula
    – Definition, Uses
3. What is the Difference Between Molecular and Structural Formula
     – Comparison of Key Differences

Key Terms: Condensed Formula, Empirical Formula, Lewis Structure, Molecular Formula, Structural Formula

What is Molecular Formula

Molecular formula or chemical formula of a compound is the representation of the types of atoms and their ratios present in that compound. The molecular formula is given by symbols of chemical elements and the numbers representing the ratio between them. This ratio is the actual whole number of atoms (the simplest whole number is given by the empirical formula).

The molecular formula can be used in the nomenclature of simple molecules, but not for complex molecules. A molecular formula of a compound can be used to determine whether the compound is a binary compound, ternary compound, quaternary compound or has even more elements.

The molecular formula of a compound can be found when the required details are given. The following examples show how to find the molecular formula.

Question:

Find the molecular formula of a compound having C=40%, H=6.72% the rest is oxygen, and the molecular weight of the compound is found as 180 g/mol.

Answer:

Elements present are;

C = 40%H = 6.72%

O = (100-{40+6.72}) % = 53.28%

The number of moles of each element present in 100g of the compound;

C = 40g/12gmol-1 = 3.33 molH = 6.72g/1gmol-1 = 6.72 mol

O = 53.28g/16gmol-1 = 3.33 mol

The simplest ratio between these elements are found by dividing each value by the largest value among them.

C = 3.33 mol / 6.72 mol = ½ H = 6.72 mol / 6.72 mol = 1

O = 3.33 mol / 6.72 mol = ½

Find the simplest whole number ratio between these atoms;

C = ½ x 2 = 1H = 1 x 2 = 2

O = ½ x 2 = 1

Hence, the empirical formula of the compound is CH2O.

To find the molecular formula using this empirical formula, we first need to find the number of empirical formula units present in the compound.

Molar mass of empirical formula unit = CH2O = (12 g/mol) + (1g/mol x 2) + (16g/mol)
                                                                     = 30 g/mol.

The number of empirical formula units present in the compound = 180 gmol-1 / 30 gmol-1 = 6
Then the molecular formula of the compound can be obtained by multiplying the empirical formula by 6.

Molecular formula = (CH2O) x 6 = C6H12O6

Structural formula of a compound is the representation of the arrangement of atoms in the compound. This structural formula gives many details about the molecule and the properties of the compound also can be predicted using these details.

The structural formula can be used for the nomenclature of the molecule. This is because the structural formula shows all the functional groups present in the compound and their relative positions. The structural formula can also be used to predict the chemical properties of compound (such as polarity) and physical properties (such as boiling point).

There are few different ways that are used to give the structural formula. Lewis structures show the connectivity of atoms and the lone pairs or unpaired electron in the compound. The condensed formula is another type of structural formula, which gives the relative positions of atoms, but this is not much helpful (Ex: CH3CH2OH is the condensed formula of ethanol).

Figure 1: Lewis Structure of Water Molecule

An important type of structural formula in organic chemistry are skeletal formulas. Most complex organic molecules can be given via skeletal formulas. This type of formulas show the functional groups and their positions. But the hydrogen atoms attached to the carbon atoms are not indicated there.

Figure 2: Ethylene Glycol Skeletal Formula

In stereochemistry, there are many forms of structural formulas. For example, the structural formula indicates the cis-trans geometry of the compound, Newman projection and sawhorse projection of organic compounds, Fischer projection of sugar molecules, Haworth projection, etc.

Definition

Molecular Formula: Molecular formula or chemical formula of a compound is the representation of the types of atoms and their ratios present in that compound.

Structural Formula: Structural formula of a compound is the representation of the arrangement of atoms in the compound.

Details Given

Molecular Formula: Molecular formula gives the ratio between atoms present in the compound.

Structural Formula: Structural formal gives the arrangement of atoms and the relative positions of the functional groups in the compound.

Uses

Molecular Formula: Molecular formula can be used for the nomenclature of simple molecules, to determine whether the compound is a binary compound, ternary compound, quaternary compound or has even more elements, etc.

Structural Formula: Structural formula can be used for the nomenclature of complex molecules and to predict the chemical properties of compound (such as polarity) and physical properties (such as boiling point).

Conclusion

Molecular formula and structural formula are very important in determining the actual structure of a compound. The main difference between molecular formula and structural formula is that molecular formula cannot indicate the relative positions of the atoms whereas structural formula indicates the relative positions of the atoms.

Reference:

1. Helmenstine, Anne Marie. “Calculating Compound Empirical & Molecular Formula.” ThoughtCo, Aug. 4, 2017, Available here.
2. “2.1: Structural Formulas.” Chemistry LibreTexts, Libretexts, 22 June 2017, Available here.

Image Courtesy:

1. “Water-2D-flat” (Public Domain) via Commons Wikimedia
2. “Ethylene glycol” By Sander de Jong – Own work (Public Domain) via Commons Wikimedia