This document provides an overview of atoms and molecules. It defines key terms like atom, molecule, ion, and discusses Dalton's atomic theory and its postulates. The document explains that atoms are the smallest particles that make up matter and combine to form molecules or ions. It discusses how elements are represented by symbols and how atomic mass is measured in atomic mass units relative to carbon-12. The document also summarizes laws of chemical combination and provides examples of writing chemical formulas based on valencies of elements.

1. E-Content for Class IX (Science)
Chapter: Atoms and Molecules
BY
SANTOSH KUMAR CHAURASIA, PGT CHEMISTRY
JNV JHABUA-2 MP
NAVODAYA VIDYALAYA SAMITI, NOIDA

3. CHAPTER-3
ATOMS AND MOLECULES

4. We come across different things around us
like chair, table, etc. and all the things that
surround us have mass and weight. They all
constitute matter. Matter is anything that
occupies space and has mass. Matter is made
up of small particles called atoms.

5. LEARNING OUTCOMES AND GOALS
• LAW OF CONSERVATION OF MASS,
• LAW OF CONSTANT PROPORTIONS,
• POSTULATES OF DALTON’S ATOMIC THEORY,
• DEFINE AN ATOM,
• IDENTIFY THE SYMBOLS OF ELEMENTS,
• IDENTIFY THE ATOMIC MASS OF ELEMENTS,
• DEFINE MOLECULE,
• DEFINE AN ION,
• DEFINE VALENCY,
• RULES FOR WRITING A CHEMICAL FORMULA,
• MOLECULAR MASS,
• MOLE CONCEPT.

6. The idea of divisibility by Indian philosophers-
• Maharishi Kanad – He postulated that if we
keep on dividing the matter (called as
‘padarth’) we will get smaller and smaller
particles. And soon we will achieve the
smallest of particles (called as ‘parmanu’)
which may not divide further.

7. Pakudha Katyayama – He said that
these particles normally exist in a
combined form which gives us various
forms of matter.

8. The idea of divisibility by Greek philosophers-
• Ancient Greek philosophers – Democritus and Leucippus
suggested that if we go on dividing matter, a stage will come when
particles obtained cannot be divided further. Democritus called
these indivisible particles atoms (meaning indivisible).

9. We also know that a lot of chemical reactions take place in our day to day
life like making of tea, changing milk to curd, making cheese from milk and
lots more. All the chemical reactions taking place obey certain set of laws.
Let us study these chemical reactions and the rules that they obey.

10. Chemical reaction
• The process by which some substances react to form a new
substance. The substances which react are called reactants and
the new ones formed are called products. For example:
A+B🡪C+D
In this, A and B are reactants and C and D are called products.
• For example: when we make tea, we add sugar, tea, milk and
water. We mix them and heat the mixture. The result is that we
get a new substance that is tea.

11. Laws of Chemical Combination
There are two main laws of Chemical
Combination as established by
Lavoisier and Joseph L. Proust-
(a)Law of Conservation of Mass
(b) Law of constant or definite
proportion

12. Law of Conservation of Mass
• For example:-
2H2 + O2 🡪 2 H2O
(Reactants) (Product)
=> 4 + 2 x 16=32 🡪 2 x 18
=> 36 g 🡪 36 g.
• It states that mass cannot be created or destroyed in chemical
reaction. So mass of reactants is mass of the Product.
A + B -> C +D
• Mass of Reactants=Mass of (A+B)
Mass of Products = Mass of (C+D)
Mass of Reactants = Mass of Products

13. Law of constant or definite proportion
For example,
• Hydrogen and oxygen are present in water in a ratio of 1:8.
So if we decompose 9g of water we will obtain 1g of hydrogen and
8g of oxygen.
•
It states that elements combine in their definite proportion of
mass to give compounds. or In a chemical substance the
elements are always present in definite proportions by
mass

14. The Dalton’s Atomic Theory
John Dalton proposed an atomic theory
which acted as an explanation of the above
two laws. As per the theory, all matter
whether it is an element, a compound or a
mixture consists of tiny invisible particles
called ‘atoms’.

15. The postulates of the atomic theory by John
Dalton -
• All matter is made of very tiny particles called atoms.
• Atoms are indivisible particles
• Atoms can neither be created nor be destroyed
• Atoms of same elements are similar.
• Atoms of different elements are different.
• Atoms combine in the ratio of small whole numbers to form
compounds.
• The relative number and kinds of atoms are constant in a given
compound.

17. Drawbacks of Dalton’s Atomic Theory
Drawbacks of Dalton’s Atomic Theory were as follows-
According to Dalton, an atom is indivisible but later on it was
proved that atom can be subdivided into electrons, protons and
neutrons.
Atoms of an element can somehow differ from each other.
So, these drawbacks led to the failure of Dalton’s theory of an
atom.

18. What is an Atom
• Matter is made up of Atoms
• Atom are the smallest particle of elements
• Atoms are very small, they are smaller than
anything that we can imagine or compare
with
• The size of the atoms is measured by the
Atomic Radius .Atomic radius is measured in
nanometres ( 1 nm=10−9 m)
• Atomic Radius of the Hydrogen is 10−10 m
And it is the smallest of all

19. Atomic radius is measured in nanometers
1/109 = 1nm
1m = 109 nm
Radii (in m) Example
10-10 Atom of hydrogen
10-9 Molecule of water
10-8 Molecule of hemoglobin
10-4 Grain of Sand
10-2 Ant

20. Symbols of Atoms or Elements
Dalton proposed the below symbols for the atoms

21. IUPAC and Atomic symbols
IUPAC - International Union Of Pure & Applied Chemistry
• According to IUPAC-
• Symbol of an element is either the first letter of the name of the
element or the first two letters of the name of an element.
• In case there are two letters, then the first letter is in a capital case and
the second letter is in small case.

24. Sometimes, the symbol is derived from the Latin name of an element.

25. The Atomic Mass
• The Dalton’s Atomic Theory suggested that each element has a
distinguishing atomic mass. With this theory, the law of constant
proportions could be explained easily.
• But it is indeed difficult to evaluate the mass of an atom since the
size of an atom is relatively small.
• Therefore scientists started evaluating the mass of an atom
by comparing it with the mass of a standard atom.

26. Earlier 1/16 of the mass of an oxygen atom was used as a
standard for calculating the mass of other elements. Now,
carbon - 12 is considered a standard atom for calculating the
mass. Its atomic mass is 12u (12 atomic mass units).
• We define One atomic mass unit is a mass unit equal to exactly
one-twelth (1/12th) the mass of one atom of carbon-12. relative
atomic masses of all elements have been found with respect to an
atom of carbon-12.
• So, Atomic mass of atom is measured in amu. amu is written as 'u

27. • In simple language, atomic mass is the number of times an
atom of an element is heavier than 1/12th of a Carbon
atom.
For example: If we say that the atomic mass of Sodium is
23, it means that sodium is 23 times heavier than 1/12th of
a carbon atom.

29. Can atoms exist independently?
• Atoms cannot survive independently. So,
atoms join together and form molecules
or ions.

30. Molecule
• A molecule is a collection of various
atoms that combine chemically with
each other.
• These atoms are bound together by
certain forces of attraction.
• Atoms of the same elements or
different elements can bind together
to form molecules.
• Therefore, a molecule is the smallest
particle of a substance that can exist
independently and shows all the
properties of that substance

31. Molecules of Elements -Generally atoms of most of
the elements exists as molecules Like Oxygen exists
as O2, Hydrogen exists as H2.
Molecules of Compounds- The molecule of
compounds contains two or more different atoms
chemical bonded together.
Example- HCl ,H20

33. Atomicity – the number of atoms in a molecule
of an element is called its atomicity. For
example, helium is monoatomic and oxygen is
diatomic
• .
• Monoatomic – when an element comprises
of a single atom. Example – all metals
• Diatomic – when an element comprises of
two atoms. Example – all gases

34. • Triatomic – when an element comprises of three atoms
• Tetra-atomic – when an element comprises of four atoms
• Poly-atomic – when an element comprises of more than two
atoms

36. Molecules of Compounds
• Molecules of compounds
constitute atoms of
different elements that
combine together in a fixed
proportion. For example,
water comprises of two
atoms of hydrogen and one
atom of oxygen.

37. Ions
• Compounds composed of
metals and non-metals contain
charged species
• The charged species are known
as ions.
• An ion is a charged particle and
can be negatively or positively
charged.

38. Types of ion-
1-Cation 2- Anion
• Cation: they are positively
charged ions. They are formed
when an atom loses a negative
charge. Therefore, they always
have negative charges less
than normal atoms.
• Example: Na +1 positive
charge 🡪Na+ (cation)

39. • Anion: they are negatively
charged ions and are formed
when an atom gains a negative
charge. Therefore, they have
negative charges more than
normal atoms.
Example: Cl -1 negative
charges 🡪 Cl- (anion)

40. Valency
• The combining power
(or capacity) of an
element is known as its
valency. For ions, the
charge indicates the
valency of the ions

41. Rules of writing a Chemical Formula
• Valencies of on the ions must balance.
• In a case where both metal and non-metal substances are
present in a compound, the name of the metal is always
written first in the chemical formula. For example, Sodium
Chloride is written as NaCl
• In case of polyatomic ions, the ion is written in brackets
before writing the number of ions associated to it. In case
of a single ion, there is no need to mention the ion in
brackets

42. Writing the Formulae of Simple Compounds
Binary compounds – compounds that consist of two different
elements
How to write a Formula of a Compound
• Write the symbols of the corresponding elements of the
compound as explained above
• Write the valencies of the elements of the compound
• Crossover the valencies of the elements

43. Here are a few examples of writing the
chemical formula

44. Molecular Mass
• Molecular Mass – summation of all the atomic masses in a
molecule
• Molecular mass is expressed in atomic mass units (amu).
• For example, the molecular mass of HNO3 can be calculated
as:
• Atomic mass of H =1u
• Atomic mass of N =14u
• Atomic mass of O =16u
• Molecular mass of HNO3 = 1 + 14 + (16*3) = 63u

45. Formula Unit Mass
• The sum of atomic masses of all
atoms in a formula unit of a
compound is called as its
formula unit mass. The formula
unit mass is used in case of
substances that constitute ions.
For example, formula unit mass
of Sodium Chloride (NaCl) can be
calculated as: (1*23) + (1*35.5)
= 58.5u

46. Mole Concept
How do we interpret a chemical equation?
2C + O2 2CO2
• We say that two molecules of carbon combine with one
molecule of oxygen to form two molecules of carbon dioxide.
• We can also say that 24u of Carbon molecules combine with 32u
of oxygen molecules to form 56u of carbon dioxide molecules.
• Therefore, we can characterize the quantity of a substance by its
mass or by its number of molecules.
• A chemical equation directly indicates the number of molecules
participating in the reaction. Thus, it is convenient for us to refer
to the number of substances in a chemical reaction as numbers
of molecules or atoms.
•

47. • A mole of a substance is that amount of the substance which contains
the same number of particles (atoms, molecules or ions) that are
present in 12g of Carbon – 12.
• The number of particles (atoms) present in 12g of Carbon – 12 is
6.022 x 1023 . This number is called Avogadro Number or Avogadro
Constant.
• A mole represents two things :-
• i) It represents a definite number of particles (atoms, molecules or
ions) equal to 6.022 x 1023 .
• ii) It represents a definite mass of a substance equal to the gram
atomic mass of an element or the gram molecular mass of a
compound.

49. • Gram atomic mass of an element :- is its atomic mass
expressed in grams.
• Eg :- Gram atomic mass of Oxygen = O2 = 16 x 2 = 32g.
• Gram molecular mass of a compound :- is its molecular mass
expressed in grams.
• Eg :- Gram molecular mass of water = H2O = 1 x 2 + 16 = 2 + 16
= 18g.

50. Mole
• Mole is a numerical quantity that has a mass equal to the
atomic or molecular mass of species (atoms, molecules, ions
or particles).
• 1 mole of any substance = 6.022 X 1023 number of particles
(atoms, ions or molecules)
• This is called the Avogadro number or Avogadro
Constant which is represented as N0
• The mass of 1 mole of a substance is the same as that its
atomic mass or molecular mass expressed in grams.

51. • Gram atomic mass of a substance – the atomic mass of a
substance when expressed in grams is known as its gram
atomic mass.
• Gram molecular mass of a substance – the molecular mass
of a substance when expressed in grams is known as its
gram molecular mass.
• For example, the atomic mass of Sulphur is 32u. Gram
atomic mass of Sulphur is 32g.
• Also, 32u of Sulphur has 1 atom of Sulphur. 32g of Sulphur
has 1 mole atoms, that is, 6.022 X 1023 atoms of Sulphur.

52. Formula

53. • 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)?

54. Weighing

55. A quick review of how mole, Avogadro
number and Mass are related to each other –

56. RECAPITULATION
During a chemical reaction, the sum of the masses of the
reactants and products remains unchanged. This is known
as the Law of Conservation of Mass.
• In a pure chemical compound, elements are always present
in a definite proportion by mass. This is known as the Law
of Definite Proportions.
• An atom is the smallest particle of the element that cannot
usually exist independently and retain all its chemical
properties.
• A molecule is the smallest particle of an element or a
compound capable of independent existence under
ordinary conditions. It shows all the properties of the
substance.

57. HOME ASSIGNMENTS
Q 1.What is meant by the term chemical formula?
Q 2.State the postulates of Dalton’s atomic theory which indicates
the law of constant proportions?
Q3.Name two scientists who established the laws of chemical
combination?
Q 4.Give two drawbacks of Dalton’s atomic theory?
Q 5.What is meant by a molecule? Give examples?
Q 6.What formed the basis for John Dalton to formulate Dalton’s
atomic theory?
Q7.State the law of conservation of mass. Is this law applicable to
the chemical reactions? Elaborate your answer with the help of an
example?
Q8.How to write a chemical formula of a binary compund
Q9. Define mole/ Avogadro number.