As mentioned in class 9 Atoms and Molecules NCERT, atoms were, for a long time, believed to be the smallest constituent that made up any object in the universe. They are the fundamental building blocks for everything in the universe, and their size is so small, that they are impossible to be observed by the naked eye.

They can only be seen by specialised, high-power microscopes. The study of atoms is very important as their behaviour helps us to understand the behaviour of all objects that we see every day.

Atomic Mass

Atomic mass is a way of classifying elements based on their properties. Since atoms cannot be weighed on a regular scale, their weight is measured in terms of “Atomic Mass Units” or amu. Although nowadays, “u” is also widely accepted.

NCERT class 9 science chapter Atoms and Molecules mentions that atomic masses were measured using an oxygen atom as a reference as it combined with a large range of elements, with a single oxygen atom weighing 16 amu, and using this to measure other elements. 

But today, we use Carbon-12 as an element to use for comparing the masses of other elements. This is because carbon also makes a big number of compounds with other elements. 

Hence, today an amu is defined as 1/12 mass of a carbon-12 atom, as mentioned in atoms and molecules class 9th NCERT. Therefore, all other elements will be measured using the 1/12 mass of a carbon-12 atom as a unit mass.

Occurrence

Most atoms cannot exist on their own. They usually exist as molecules or as free-flowing ions.

A molecule is a group of two or more atoms that have bonded together to become more stable, and exist collectively with each other. It can also be defined as the smallest unit of a substance that has the ability to exist independently. A molecule can be formed by combining the atoms of either the same element or different elements.

Molecules of Elements

Molecules of elements are composed of a single type of atom. Various noble elements, such as Helium, Neon, and Argon, do not exist as molecules, but as individual atoms. 

But there are also various elements such as Nitrogen, Fluorine and Oxygen, which exist as molecules by combining with another atom of the same element. Hence, Nitrogen exists naturally as \( N_2 \). Such a molecule is known as a diatomic molecule.

It is also worth noting that one element can have different compounds that contain the said element. For example, oxygen exists naturally as both \( O_2 \) and \( O_3 \), which are both stable molecules and can be found in nature with widely different properties.

Molecules of Compounds

Apart from atoms of the same element, atoms from different elements also combine together in definite proportions for forming various compounds. The most common example of this is atoms of hydrogen and oxygen combining in a 2:1 ratio to form water.

Ions are charged species that make up various compounds that are made up of metals and non-metals. The charge on an ion can be negative or positive depending on the atom that holds the charge.

An atom that holds a negative charge is called an ‘anion’, whereas an atom that holds a positive charge is called a ‘cation’.

A chemical formula is a simple and systematic way of writing a chemical compound. It uses the symbols of the elements in the compound as well as their proportions to create an easily recognisable formula for the same.

Valency

Before we understand how chemical formulae are, we need to understand valency. In simple terms, valency is the number of atoms that an atom needs to combine with to form a stable compound.

For example, Aluminium has a valency of +3. This means that to become stable, an aluminium atom will need to combine with 3 other atoms of different elements.

Writing a formula

To write a formula created by combining two elements, we first write the element and their valencies below them. Next, we cross-multiply the elements and the valencies. The result thus obtained gives us the chemical formula of the final compound.

Molecular mass, similar to atomic mass, is the mass of a molecule. To find the mass of a molecule, we need to add the atomic mass of all the atoms present in the molecule.

Taking an example, if we need to find the molecular mass of water, we need to add the mass of all the atoms present in a water molecule. A water molecule consists of 1 oxygen atom and 2 hydrogen atoms, each of which has a mass of 16 amu and 1 amu, respectively.

Hence, the molecular mass of water is

1+16+1=18 amu

1. The elements Hydrogen and Oxygen combine in a ratio of 1:8 by mass. This forms water. Find out what mass of oxygen is required to react completely with 3g of hydrogen gas?

Solution

The ratio of H:O by mass in water could be found by:

Hydrogen : Oxygen \( \rightarrow H_2 O \)

which means,

1:8 = 3:x

x = 8 x 3

x = 24 g, which means that 24g of oxygen are required to react with 3g of hydrogen gas.

2. A reaction contains 5.3g of sodium carbonate that reacts with 6 g of acetic acid. The products created from the reaction
include 2.2g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Depict these reactions with the law of
conservation of mass.

Sodium carbonate + acetic acid \( \rightarrow \) Sodium acetate + carbon dioxide + water

Solution

Sodium carbonate + acetic acid \( \rightarrow \)Sodium acetate + carbon dioxide + water, 

Where

Sodium carbonate = 5.3g

Acetic acid = 6g

Sodium Acetate = 8.2g

Carbon dioxide = 2.2g

Water = 0.9g

The law of conservation of mass says the total mass of reactants is equal to the total mass of products.

Which means, 

LHS = RHS,

Where

5.3g + 6g = 2.2g + 0.9 g + 8.2 g = 11.3 g

3. How many atoms does an (a)H2S molecule (b) \( PO_4^3- \) ion have?

Solution

(a) H2S --> 3 atoms

(b) PO43- --> 5 atoms

4. What are the formula unit masses of ZnO, Na2O, K2C03? Note that the given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Solution

(a) Zno = 65u + 16 u = 81 u

(b) Na2O = (23u 2x) + 16 u = 62 u

(c)K2C03 = (39 u x 2) + 12 u + 16 u x 3 = 138 u

Atoms and molecules are the fundamental units of the objects we see in our day to day lives. They will always have a crucial role to play in the chemistry and science fields. Thus, it is imperative that we learn as much as we can about them as seen in atoms and molecules. In this guide, we also gave you class 9th Atoms and Molecules NCERT solutions.

The advances in chemistry do not seem to stop with new innovations taking place almost every day. Innovations that will help us better improve our knowledge about the basic building blocks of all matter.

The 3rd Chapter, Atoms and Molecules, does justice to this crucial information which shall greatly help students in the future, not just in NCERT class 9th atoms and molecules but even beyond.