NCERT Class 9 Science Chapter 3 Notes - Atoms And Molecules PDF Notes

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NCERT Notes for Class 9 Science Chapter 3

The NCERT Class 9 Science Chapter 3 Atoms And Molecules Notes contain all the topics in detail with appropriate examples. These notes provide a clear explanation of important concepts like laws of chemical combination, atomic mass, and molar mass in a simple manner. Students can use them as a handbook for learning and revision of the concepts. Scroll down to know more!

An Indian philosopher, Maharishi Kanad, said that if we go on dividing matter (padarth), we will get smaller and smaller particles of matter. Ultimately, we will get the smallest particle of matter, which cannot be divided any further. Based on this philosophy, Kanad was one of the first persons to propose that matter (or padarth) is made up of very small particles called ‘parmanu’. John Dalton called these particles by the name of an atom. The word ‘atom’ means ‘indivisible’; something which cannot be divided further.

Another Indian philosopher, Pakudha Katyayana went a step further and proposed that the particles of matter (atoms or parmanu) normally exist in a combined form and various combinations of particles give us various kinds of matter. This combined form of atoms is now called ‘molecules’.

We now know that all matter is made up of small particles called atoms and molecules. Different kinds of atoms and molecules have different properties due to which different kinds of matter also show different properties. Thus, the properties of matter depend on the properties of atoms and molecules from which it is made. Democritus explained that if we keep on dividing matter, we will end up with its smallest unit, which won’t be divisible further, and he named it an atom. Students can understand the concepts of this chapter more clearly with the help of NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules.

3.1 Laws of Chemical Combination

Antoine L. Lavoisier is called the father of chemistry, for he gave two important laws of chemical combination, which laid the foundation of chemical sciences, and the laws are as follows:

3.1.1 Law of Conservation of Mass

It states that “Mass is not created and not destroyed, in a chemical reaction”. The substances which combine together (or react) in a chemical reaction are known as 'reactants’, whereas the new substances formed (or produced) as a result of the chemical reaction are called ‘products’. There is no change in mass during a chemical reaction. Since there is no gain or loss in mass in a chemical reaction, the mass remains conserved.

For example,

$\underset{100 \mathrm{~g}}{\mathrm{CaCO}_3} \rightarrow \underset{56 \mathrm{~g}}{\mathrm{CaO}}+\underset{44 \mathrm{~g}}{\mathrm{CO}_2}$

3.1.2 Law of Constant Proportions

“The elements are present in definite proportions by mass in a chemical substance.” This law means that whatever be the source from which it is obtained (or the method by which it is prepared), a pure chemical compound is always made up of the same elements in the same mass percentage.

For example, water is a compound that always consists of the same two elements, hydrogen and oxygen, combined together in the same constant proportion of 1:8 by mass (1 part by mass of hydrogen and 8 parts by mass of oxygen). You can also download the Class 9 Science Chapter 3 Notes to revise such examples and understand the laws of chemical combination in a simple way.

Dalton’s Atomic Theory

The theory that ‘all matter is made up of very tiny indivisible particles (atoms)’ is called the atomic theory of matter. Dalton put forward his atomic theory of matter in 1808.

The postulates of the theory are given below:

1. “Matter is made of very small particles called atoms”.

2. “Atoms are not divisible particles, which are not created and not destroyed in a chemical reaction”.

3. “Atoms of the same element have identical mass and chemical properties”.

4. “The chemical properties of the atoms are different for different elements.”.

5. “Compounds are formed by combining atoms of different elements in ratios of small whole numbers”.

3.2 What is an Atom?

An atom is the smallest unit of matter that retains all of the chemical properties of an element. For example, a gold coin is simply a very large number of gold atoms molded into the shape of a coin, with small amounts of other, contaminating elements. Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold. A gold atom gets its properties from the tiny subatomic particles it's made up of.

An atom consists of two regions. The first is the tiny atomic nucleus, which is in the center of the atom and contains positively charged particles called protons and neutral, uncharged particles called neutrons. The second, much larger, region of the atom is a “cloud” of electrons, negatively charged particles that orbit around the nucleus. The attraction between the positively charged protons and negatively charged electrons holds the atom together.

The size of an atom is indicated by its radius, which is called ‘atomic radius’. Atomic radius is measured in ‘nanometres’ (which is a very small unit of measuring length). The symbol of a nanometre is nm.

$\begin{aligned} & 1 \text { nanometre }=\frac{1}{10^9} \text { metre } \\ & \text { or } 1 \mathrm{~nm}=10^{-9} \mathrm{~nm}\end{aligned}$

Hydrogen atom is the smallest atom of all. The atomic radius of a hydrogen atom is 0.037 nanometre (or 0.037 nm). If we express the radius of a hydrogen atom in metres, it will be 0.037 × 10^–9 metre which means 0.000000000037 metre ! It is really very, very small. The atomic radii of some common elements are given below:

The relative size of different matters:

3.2.1 WHAT ARE THE MODERN DAY SYMBOLS OF ATOMS OF DIFFERENT ELEMENTS?

Dalton was the first scientist to use the symbols for elements in a very specific sense.

Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) is an international scientific organisation which approves names of elements, symbols and units. Many of the symbols are the first one or two letters of the element’s name in English. Some elements and their symbols:

Atomic Number

It is the number of protons present in an element.

3.2.2 ATOMIC MASS

It is the number of protons and neutrons present in an element.

Carbon-12 isotope is the standard reference for calculating atomic masses of other elements. One atomic mass unit is a mass unit equal to one-twelfth (1/12th) the mass of one atom of carbon-12.

3.2.3 HOW DO ATOMS EXIST?

Atoms of most elements are not able to exist independently. Atoms form molecules and ions.

3.3 What is a Molecule?

A molecule is an electrically neutral group of two (or more) atoms chemically bonded together. The forces which hold the atoms together in a molecule are called covalent bonds. Thus, a combination of atoms is called a molecule. A molecule is the smallest particle of a substance (element or compound) that has the properties of that substance and can exist in the free state. Molecules can be formed either by the combination of atoms of the ‘same element’ or of ‘different elements.

Depending on this, there are two types of molecules : molecules of elements, and molecules of compounds. This is discussed below.

3.3.1 MOLECULES OF ELEMENTS

The molecule of an element contains two (or more) similar atoms chemically combined together. For example, a molecule of hydrogen element contains 2 hydrogen atoms combined together, and it is written as H₂. Some similar examples include nitrogen gas exists as N₂ molecules, oxygen gas as O₂ molecules and chlorine gas as Cl₂ molecules.

The molecule of a compound contains two (or more) different types of atoms chemically combined together. For example, hydrogen chloride(HCl) and water(H₂O).

It is a group of two or more elements held together in a chemical bond by attractive forces.

Atomicity

The number of atoms in a molecule is called the atomicity of the molecule.

• For example:, argon and helium are monoatomic,
• Oxygen and chlorine are diatomic.
• Phosphorus is tetra-atomic and
• Sulphur is polyatomic.

3.3.3 WHAT IS AN ION?

An ion is a charged particle; it can be positively charged or negatively charged. Positively charged ions is called ‘cation’ and negatively charged ion is called ‘anions’.

Sodium ion (Na⁺) and magnesium ion (Mg²⁺) are cations because they are positively charged ions. A cation is formed by the loss of one or more electrons by an atom. For example, sodium atom loses 1 electron to form a sodium ion, Na⁺, which is a cation :
$\underset{\text { Sodium atom }}{\mathrm{Na}} \xrightarrow{-1 \text { electron }} \underset{\begin{array}{c}\text { Sodium ion } \\ \text { (A cation) }\end{array}}{\mathrm{Na}^{+}}$

Similarly, Chloride ion (Cl^–) and oxide ion (O^2–) are anions because they are negatively charged ions. An anion is formed by the gain of one or more electrons by an atom.

3.4 Writing Chemical Formulae

It is a symbolic representation of a compound and its composition.

Given below are the rules for writing a chemical formula:

• The charges on the ions have to balance each other.

• If the compound contains a metal and a nonmetal, the metal is written first.

3.4.1 FORMULAE OF SIMPLE COMPOUNDS

For example: sodium chloride (NaCl), where chlorine is a non-metal, so written on the right, and sodium is a metal, so written on the left.

• In compounds with polyatomic ions, the ion is enclosed in a bracket and then the number of the ion is indicated. If number of polyatomic ion is one, the bracket is not required.

For example:

3.5 Molecular Mass

The molecular mass of a substance is equal to the sum of the atomic masses of all the atoms in that molecule.

3.5.1 FORMULA UNIT MASS

If the substance contains ions as its constituents, then the molecular mass is called Formula Unit Mass.

Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

For example, sodium chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be calculated as 1 × 23 + 1 × 35.5 = 58.5 u

Previous Year Questions of Class 9 Science Chapter 3

Students can refer to previous year questions of Science Class 9 Chapter 3 Atoms And Molecules that helps students understand important questions and frequently asked concepts, making revision more effective.

Question 1. Which of the following is the correct representation of one mole of a substance?
A) $6.022 \times 10^{22}$ atoms of oxygen
B) 12 g of carbon-12
C) 1 g of hydrogen gas
D) 22.4 g of nitrogen gas

Answer: B) 12 g of carbon-12
By definition, 1 mole $=12 \mathrm{~g}$ of C-12 $=6.022 \times 10^{23}$ particles.

Hence, the correct answer is option (2).

Question 2. What is the mass of 0.5 mole of $\mathrm{O}_2$ gas?
A) 8 g
B) 16 g
C) 32 g
D) 48 g

Answer: B) 16 g
Molar mass of $\mathrm{O}_2=32 \mathrm{~g} / \mathrm{mol}$
So, mass $=0.5 \times 32=16 \mathrm{~g}$

Hence, the correct answer is option (2).

Question 3. Which of the following is the correct chemical formula for magnesium chloride?
A. MgCl
B. $\mathrm{MgCl}_2$
C. $\mathrm{Mg}_2 \mathrm{Cl}$
D. $\mathrm{Mg}_2 \mathrm{Cl}_3$

Answer:

Magnesium has a valency of +2, and chlorine has a valency of -1, so two chloride ions are needed to balance one magnesium ion which will give the formula $\mathbf{M g C l}_2$.

Hence, the correct answer is option (2).

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