CBSE NCERT Notes for Class 9 Science Chapter 3- Atoms and Molecules

Chapter 3 -CBSE Atoms And Molecules Class 9 Notes

Atoms and molecules are responsible for formation of everything from microscopic sand particles to enormous black holes. The atom is the basic unit of matter, constituting everything visible in our environment. It is incredibly small, measuring between 0.1 to 0.5 nanometers in diameter.

CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

These CBSE Class 9 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.

Laws of Chemical Combination

When reactants join to generate products or when components combine to form a complex, certain rules apply. These are referred to as chemical combination laws.

Antoine L. Lavoisier set the foundation for chemical sciences by developing the following two fundamental rules of chemical combination:

Law Of Conservation Of Mass

According to the article,’ mass cannot be generated or destroyed during a chemical process.’

This indicates that the entire mass of the reactants is equal to the total mass of the products in any chemical reaction, and there is no mass change throughout the process.

Law Of Constant Proportions/ Law Of Definite Proportions

According to this law, the elements are always present in specified quantities (or ratios) by mass in a chemical element (or compound).

In a compound such as water, the mass ratio of hydrogen to oxygen is always 1: 8, regardless of the source of the water. Thus, when nine grammes of water are destroyed, one gramme of hydrogen and eight grammes of oxygen are always obtained.

Carbon dioxide (CO2), on the other hand, always includes a 3:8 ratio of carbon to oxygen. If a CO 2 sample has 36 grammes of carbon, it must also include 96 grammes of oxygen.

Explanation of Laws of Chemical Combination: Dalton’s Atomic Theory

Dalton’s atomic theory explained how the law of chemical combination works. According to Dalton’s atomic theory, all matter is made up of small particles called atoms, whether it is an element, a compound, or a combination.

The Main Postulates Of Dalton’s Atomic Theory

  • Every substance is composed of very tiny particles known as atoms.
  • Atoms are self-contained particles that cannot be generated or destroyed during a chemical process.
  • Atoms of the same element have the same mass and chemical property.
  • Different elements contain atoms with various masses and chemical characteristics.
  • Compounds are formed when atoms join in a ratio of tiny whole numbers.
  • The relative amounts and types of atoms in a particular compound remain constant.


Atoms are the element’s tiniest particles, which may or may not exist independently but participate in chemical reactions. These are the basic elements of all matter.

Hydrogen, oxygen, and nitrogen atoms, for example, are not capable of self-existence, but helium, neon, and other atoms can.

Size Of Atoms

Atoms are very tiny, with a radius of just a few nanometres. The hydrogen atom is the tiniest atom, with a radius of 0.1 nm.

Modern Day Symbols Of Atoms Of Different Elements

In chemistry, symbols are used to represent elements. Chromium Mercury Cr Hg Uranium U It is more convenient to use the symbol for an element than to write the element’s full name.

Dalton was the scientist who developed the use of symbols to denote components.

Because Dalton’s symbols for the elements were difficult to design and difficult to use, J J Berzelius devised contemporary symbols for the elements. These are characterised as “a shortcut for the element’s name.”

Initially, the names of elements were determined from the location of their discovery. The IUPAC (International Union of Pure and Applied Chemistry) now accepts the element names and symbols. Numerous symbols match the first one or two letters of the element’s English name.

A symbol’s first letter is usually written in capital letters, whereas the second letter is always written in tiny letters.

chlorofluorocarbons (Cl), zinc (Zn), and aluminium (Al).

The symbols for certain elements were taken from their names in various languages such as Latin, German, and Greek.

CobaltCo from Kobold (German name)
IronFe from Ferrum (Latin name)
GoldAu from Aurum (Latin name)
PotassiumK from Kalium (Latin name)
ChlorineCl from Chloros (Greek name)
SodiumNa from Natrium (Latin name)

Atomic Mass

Each element, according to Dalton’s, has a unique atomic mass. However, owing to the atom’s very tiny size, measuring its mass was a somewhat challenging task.

As a result, their respective atomic masses were estimated using chemical combination rules and the resulting compounds.

Initially, 1/16 of the mass of an atom of naturally occurring oxygen was chosen as the reference unit for this purpose for two reasons:

  1. Oxygen interacted with a broad range of other elements, forming compounds.
  2. This unit expressed the masses of the majority of elements as whole figures.

However, in 1961, carbon (the C-12 isotope) was adopted as the international standard reference for atomic mass calculations.

Relative Atomic Mass : It is defined as the number of times an atom is heavier than 1/12th the mass of a carbon-12 (C-12) atom or as the atom’s average mass in relation to 1/12th the mass of a carbon-12 atom.

Atomic Mass Unit

It is defined as the mass unit equal to approximately 1/12th the mass of a single C-12 isotope atom. Previously abbreviated as AMU, it is now written as ‘u’– unified mass, in accordance with IUPAC’s most recent standards.


A molecule is the smallest particle of an element or compound that is capable of self-existence and has all of the attributes of that molecule. In general, a molecule is a collection of two or more chemically bound atoms. Molecules are formed when atoms of the same or different elements combine.

Molecules Of Elements

Each element’s molecules include the same sort of atoms. Numerous elements have molecules composed of just one atom of that element. For example, noble gases such as argon (Ar), helium (He), and others. The molecules of the majority of non-metals include more than one atom.A molecule of oxygen (O2) is composed of two oxygen atoms and is referred to as a diatomic molecule; a molecule of ozone (O3) is composed of three oxygen atoms and is referred to as a triatomic molecule.


It is the number of atoms contained inside a molecule. Molecules may be classified into the following categories based on their atomicity:

  • Diatomic molecules
  • Triatomic molecules
  • Monoatomic molecules
  • Polyatomic molecules
  • Tetra-atomic molecules

Molecules Of Compounds

Atoms of various elements combine in specific ratios to create molecules of compounds.

Prediction Of Number Of Atoms From Mass Ratio

To calculate the number of atoms from the mass ratio, divide the provided mass of each element by the atomic mass of the opposite element and find the simplest ratio between the resultant moles.

For example, we know that the mass ratio of nitrogen to hydrogen in ammonia is 14: 3. The amount of nitrogen and hydrogen atoms contained in an ammonia molecule may be estimated as,

ElementsMass RatioAtomic massMole RatioSimplest Ratio


The charge occurs when atoms, groups of atoms, or molecules lose or collect electrons. Ions are these charged species. In general, atoms in solution exist in the form of ions. These may be negatively or positively charged and hence fall into one of two categories.

Cations : Cations are positively charged ions. For example, Na-, K+,Ca2+,Al3+. These are generated when an element’s electrons are lost. Metals often create cations.

Anions : Anions are negatively charged ions. For example, Cl-,Br-,N3-. These are generated as a result of an element gaining electrons. Non-metals often create anions.

Poly-Atomic Ion

A polyatomic ion is a collection of charged atoms that function as a single entity. It is subject to a flat fee. for instance, NO3 (ion nitrate), CO32- (ion carbonate), and SO42 (ion sulphate), among others.


The valency of an element refers to its ability to combine (or capacity to combine). Valency may be used to determine how one element’s atoms will mix with those of another element to produce a chemical combination. An ion’s valency is equal to its charge.

Writing Chemical Formulae

The chemical formula is the simplest method to express a compound using symbols and the valency of elements.

A compound’s chemical formula identifies the elements and the number of atoms in each combining element. The charge on each ion is used to calculate the chemical formula of an ionic molecule.

  1. The ion’s valencies or charges must be balanced.
  2. When a compound contains both a metal and a non-metal, the metal’s symbol is written first and to the left, while the non-symbol metal is written to the right.

For example, calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), and copper oxide (CuO), among others, where oxygen, chlorine, and sulphur are non-metals and are written on the right, while calcium, sodium, iron, and copper are metals and are written on the left.

When polyatomic ions are used to make a compound, the ion is surrounded in a bracket before the number to denote the ratio. Ca(OH)2  is an example. The bracket is not necessary if the number of polyatomic ions is one. For instance, NaOH.

Formulae of Simple Compounds

To write the chemical formula for a simple compound in the following manner:

  1. As indicated below, write the symbols for the constituent elements and their valencies.
  2. Begin with the cation sign and end with the anion symbol.
  3. The formula is obtained by crisscrossing their charges or valencies.
  4. Positive and negative charges must be balanced, and the overall structure should be neutral.
  • Formula of hydrogen chloride
  • Formula of hydrogen sulphide
  • Formula of carbon tetrachloride
  • Formula of magnesium chloride
  • Formula for aluminium oxide
  • Formula for calcium oxide
  • Formula of sodium nitrate
  • Formula of calcium hydroxide

Molecular Mass

masses of all its atoms in a molecule. Thus, it is the molecule’s relative mass represented in atomic mass units (u). For example, water (H2O) has a relative molecular mass of 18 u, which can be computed as

the atomic mass of hydrogen = 1 u 

the atomic mass of oxygen = 16 u

Two hydrogen atoms and one oxygen atom make up H2O. As a result, the molecular mass of water is equal to 2 x 1 + 1 x 16 = 18 u.

Formula Unit Mass

It is the total atomic masses of all the atoms in a compound’s formula unit. The formula unit mass is computed in the same way that the molecular mass is calculated. For sodium chloride (NaCI), for example, the formula unit mass is

1 x 23 + 1 x 35.5 = 58.5 u.

Mole Concept

The molecular weight of a substance is equal to the number of particles (atoms/ions/molecules/formula units, etc.) contained in 12 g of C-12. Thus, a mole of any species (atoms, molecules, ions, or particles) is the amount in number that has the same mass as its atomic or molecular mass expressed in grammes.

12 g is 1 mole of carbon (C, atomic mass = 12).

1 mole of oxygen (O2 , molecular mass = 2 x 16) is equal to 32 g.

1 mole of water (H2O, molecular mass = 2 x 1+ I x 16) is equal to 18g.

Each mole of any material has the same and set number of particles, which is equal to 6.022 x 1023.

This is the Avogadro constant, or Avogadro number (NA), named after the Italian physicist Amedeo Avogadro. Thus, the term “mole” may also refer to the number of particles equal to the Avogadro constant, Na (6.022 x 1023).

1 mole = 6.022 x 1023 particles, in number.

Molar Mass And Moles

A mole of a substance has the same mass as its relative atomic or molecular mass in grammes. The atomic mass of an element indicates the mass of one of its atoms in atomic mass units (u).

Thus, in order to get the mass of one mole of that element, we must use the same numerical number but change the units from ‘u.’ to ‘g’. The atomic mass of a molecule is often referred to as the gramme atomic mass. To determine the gramme molecular mass or molar mass, we maintain the numerical value but change the units from ‘u’ to ‘g’.

we know the molecular mass of water (H2O) is 18 u,

18 u water = one molecule of water.

18 g water = one mole of water

= 6.022 x 1023 molecules of water

The number of moles can be calculated directly by using the formula.


NCERT questions & answers from Atoms And Molecules

In a reaction, 5.3 g of sodium carbonate reacted with 6.0 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. (CBSE 2012)


The chemical reaction leading to products is :

sodium carbonate + ethanoic acid ———–> sodium ethanoate + carbon dioxide + water.

Mass of reactants = (5.3 + 6.0) = 11.3 g

Mass of products = (8.2 + 2.2 + 0.9) = 11.3 g.

The mass of the reactants and products are identical. This indicates that no mass was lost during the reaction. As a result, the data is consistent with the law of mass conservation.

Which postulate of Dalton’s Atomic theory is the basis of law of conservation of mass? (CBSE 2012)

Answer: The law of conservation of mass is based on Dalton’s Atomic theory’s following assumption.

“Atoms cannot be generated or destroyed by physical or chemical changes.”

Which postulate of Dalton’s Atomic theory can explain the law of definite proportions?

Answer: The law of definite proportions is based on Dalton’s Atomic theory’s following assumption.

“All of an element’s atoms are identical in every way. This means they have the same mass, size, and chemical properties.”

Define atomic mass unit.

Answer: The atomic mass unit is defined as the mass of one-twelfth (1/12) of the mass of a single carbon atom, expressed in units of 12 u. It is denoted by the symbol 1 u. (unified mass).

Why is not possible to see an atom with naked eye?

Answer: Due to the atom’s exceedingly small size, it cannot be seen with the human eye. For instance, the radius of a hydrogen atom is around 10-10 m. In fact, an atom is considered to be a minuscule particle. These minuscule particles are invisible to the naked eye.

What is meant by the term chemical formula?

Answer: A molecule is a group of two or more atoms (same or dissimilar) that are chemically linked and held closely together by strong attraction forces. Chemical formulas are used to represent molecules in terms of the symbols that represent the atoms.

What is the mass of :

(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium
= 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?


(a) 1 mole of nitrogen atoms

Mass of 1 mole of nitrogen (N) atoms = 14 u

(b) 4 moles of aluminium atoms

Mass of 1 mole of aluminium (Al) atoms = 27 u

Mass of 4 moles of aluminium (Al) atoms = 4 x 27 = 108 u

(c) 10 moles of sodium sulphite (Na2SO3)

Molar mass of Na2SO3 = 2 x Atomic mass of Na + Atomic mass of S + 3 x Atomic mass of O

= 2 x 23 + 32 + 3 x 16 = 126 u

1 mole of sodium sulphite has mass = 126 u

10 moles of sodium sulphite have mass = 10 x 126 = 1260 u.

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