Molecule

A molecule is a group of two or more atoms that are held or bonded with each other through chemical bonds. We can say that two or more atoms that are bonded to each other to make a unit that can exist independently in nature is called a molecule.

A single atom of an element is not a molecule. For example, a single oxygen atom 'C' is not a molecule, however, when it bonds to another oxygen atom such as O₂ or to a different atom such as CO₂ then molecules are formed. Besides this, when a molecule is made of atoms of the same element (O_2, N₂) then it is called a homonuclear molecule. On the other hand, if a molecule is made of different atoms such as H₂O, it is called a heteronuclear molecule.

A molecule is the smallest unit of a substance that can exist independently. Although an atom is the smallest particle in nature, it cannot exist independently, it is always bonded to other atoms to form molecules.

Does a molecule consist of same atoms or different atoms?

A molecule can be made of the same atoms or different atoms. It depends on whether a molecule belongs to an element or belongs to a compound. For example, a molecule of oxygen (O₂₎ has the same atoms or it belongs to element oxygen, whereas, a molecule of water (H₂O) has different atoms and it belongs to a compound.

It also shows that both element and compound contain molecules. Just like letters forms words and words form sentences, atoms form molecules and molecules form elements and compounds.

Difference between atom and element in tabular form

Properties of molecules:

• A molecule does not carry charge, so molecules are neutral.
• They may be made of non-metallic atoms of the same element such as O₂ or can be made of different elements such as with H₂
• The composition and geometry of molecules determine their chemical and physical properties.
• Isomers are also molecules that contain same atoms in different geometric arrangement, so they tend to have different chemical and physical properties.
• Molecules remain in motion which is more in gases. In solids and liquids they are tightly packed so as compared to gases, their movement is less in solids and liquids.

Molecules of an element

When we say molecules of an element it means one, two or more atoms of an element exists as one species in a free state. Furthermore, based on the number of atoms (one, two, three, etc.) present in a molecule of an element they are called monatomic, diatomic and triatomic molecules, which are described below;

• Monoatomic molecules: They have only one atom. For example, noble gases such as helium, neon, argon exist as single atoms such as He, Ne, Ar, etc. So, their molecules are called monoatomic molecules.
• Diatomic molecules: They are made of two atoms of the same element, such as Oxygen molecule (O₂), Hydrogen molecule (H₂), Nitrogen molecule (N₂).
• Triatomic molecule: They are made of three atoms of the same element such as ozone (O₃) that contains three atoms of oxygen.
• Tetratomic molecule: They are made of four atoms of same type such as P₄ in which four atoms of phosphorus makes one molecule of phosphorus.
• Polyatomic molecules: They are made of more than four atoms of the same element. For example, 8 atoms of sulphur are present in one molecule of sulphur (S₈).

Molecules of compound

The molecules of compound are made of different atoms or atoms of different element. So, if a molecule is made of different elements, it will form compound or it belongs to a compound.

So, if we talk about the molecule of the compound then the diatomic molecule of a compound contains atoms of different elements such as Hydrogen Chloride (HCl) and Carbon monoxide (CO) have different types of atoms.

Similarly, a triatomic molecule of compound contains different atoms such as the molecule of sulphur dioxide (SO₂) is triatomic as it has one atom of sulphur and two atoms of oxygen. The tetratomic molecule of a compound is made of four atoms of different elements such as hydrogen peroxide (H₂O₂) and ammonia (NH₃).

What is atomicity?

The number of atoms present in one molecule of a substance is called its atomicity. Let us take an example of elements Hydrogen and Helium. The symbol of hydrogen is H₂ and Helium is represented as He. Hydrogen (H) is written with 2 and Helium (He) is represented alone without any number. This is due to the difference in the atomicity of a molecule. Atomicity of hydrogen molecule is 2 as there are two atoms in H₂, similarly, the atomicity of Helium is 1 as its molecule has only one atom (He).

In case of some elements such as Helium, a Helium atom can exist alone or independently without forming a chemical bond with another Helium atom. Its outermost shell is complete so it does not need to react with other atoms. This is the reason that a helium molecule is made of single atom. This state of existence of a molecule is called monoatomic state. The other examples of such molecules are Argon (Ar), Neon (Ne), etc.

In case of hydrogen, there are two atoms in a molecule of hydrogen (H₂), so this state is called diatomic state. In this case, a molecule contains atoms of same element. Similarly, oxygen also exists in diatomic state, which is O_2.

Similarly, where molecules form compounds or in molecules of compounds atomicity is the number of atoms of different elements present in the molecule of a compound. For example, the atomicity of sulphur dioxide (SO₂) is 3 as it is a triatomic molecule with one atom of sulphur and two atoms of oxygen.

What is molecular mass?

It is the mass of one molecule of a substance in unified atomic mass units or u. A molecule is made of atoms, so we can say that it is the sum of masses of atoms present in the molecule of a substance. So, it is a number that is equal to the sum of the atomic masses of atoms present in a molecule.

Molecular mass gives the mass of a molecule relative to the atomic mass of C 12 atom. For example, molecular mass is relative to 1/12^th the mass of a single C-12 atom. It shows how many times the mass of one molecule of a substance is heavier than the 1/12^th of the mass of one C 12 atom.

Relative molecular mass = mass of one molecule of an element / 1/12^th of the mass of one carbon 12 atom

How to find the molecular mass?

We can find the molecular mass of a molecule by following the steps listed below;

i) You should have the chemical formula of a compound.

ii) From the formula, find out the number of atoms of each element in the formula or in one molecule.

iii) Multiply the atomic weight or mass of each element with its total number of atoms present in the chemical formula or molecule.

iv) Add the values calculated in the third step.

v) Add the unit to the value obtained in the fourth step. It will represent the molecular mass.

For example,

i) Molecular mass of water molecule whose chemical formula is H₂O

= mass of one oxygen atom + mass of two hydrogen atoms.

= 16 u + 2 x 1u = 18 u

ii) Molecular mass of Carbon Dioxide (CO₂):

= atomic mass of one carbon atom + atomic mass of two oxygen atoms

= 12 u + 2 x 16 u

= 44 u

iii) Molecular mass of one molecule of methane (CH₄):

= 12 u + 4 x 1 u

= 16 u

iv) Molecular mass of one molecule of ammonia (NH₃):

= 14 u + 3 x 1 u

= 17 u

Gram Molecular Mass

As the name suggests, the molecular mass or mass of one molecule in grams is known as gram molecular mass.

How to find the gram molecular mass?

i) Identify the relative atomic mass of each element present in the chemical formula of molecule.

ii) The number of atoms of each element are multiplied with atomic mass of the element. The number of atoms of an element is represented by the subscript after each element. If subscript is not present, there is only one atom of the element present in the given molecule.

iii) The values obtained in the above step are added and the final value is represented in grams.

For example, the molecular mass of Nitrogen molecule (N₂) is 28, so its gram molecular mass is 28 grams.

Similarly, gram molecular mass of carbon dioxide CO₂ is

= mass of one carbon atom + mass of two oxygen atoms

= 12 u + 2 x 16

= 12u + 32 u = 44 u

If we write this mass in grams such as 44 grams, it is called gram molecular mass or mass of one molecule of carbon dioxide in grams.

So, the gram molecular mass is numerically equal to the molecular weight or mass of a molecule. Furthermore, the gram molecular mass is the same as the molar mass. However, there is one difference which is for gram molecular mass the mass unit is required to be used. Besides this, the gram molecular mass may be represented in grams or grams per mole (g/mol). There are 6.023 x 10²³ molecules in one mole, so, when we express molecular mass in grams it means it is the mass of one mole (6.023 x 10²³ molecules) in grams. So, if the mass is given in grams, it is the mass of 6.023 x 10²³ molecules. If it is given in amu or u, it is the mass of one molecule. For example, see the table given below;

Let us see the difference between molecular mass and gram molecular mass (molar mass) in tabular form