CLASS IX - SCIENCE
ATOMS AND
MOLECULES
PRASHANT KIRAD
PK HITS
Formula making
Molecular mass
Laws of chemical reaction
HISTORY
Around 500 B.C., Indian philosopher Antoine L. Lavoisier laid the
Maharishi Kanad, postulated the theory Ancient Greek philosophers foundation of chemical
if we go on dividing matter (padarth), – Democritus and sciences by establishing two
we will obtain smallest particle beyond Leucippus called these important laws of chemical
which further division can't be possible particles atoms. combination.
which is known as 'parmanu'.
ATOMS
Atoms are building blocks of all matter.
According to modern atomic theory, an atom is the smallest
particle of an element which takes part in chemical reaction.
Atoms are very small and which can’t be seen even through very
powerful microscope.
Laws of Chemical Combination
Law of Conservation Law of Constant
of Mass Proportions
Lavoisier and Joseph L. Proust.
LAW OF CONSERVATION OF MASS
The Law of Conservation of Mass states that mass is
neither created nor destroyed in a chemical reaction.
The total mass of the reactants equals the total mass
of the products.
Total Mass of Reactants =
Total Mass of Products
NCERT ACTIVITY
Solution Y in flask, Solution X in ignition tube.
Weigh flask with both solutions.
Tilt flask to mix X and Y.
Re-weigh the flask after mixing.
Check for any change in mass.
Set X: Copper sulphate (or barium
Mass remains constant
chloride, or lead nitrate)
Set Y: Sodium carbonate (or sodium (Law of Conservation of Mass).
sulphate, or sodium chloride)
LAW OF CONSTANT PROPORTIONS
The Law of Constant Proportions (also known as the Law of Definite Proportions) is a
fundamental chemical law that states a chemical compound always contains the
same elements in the same fixed proportion by mass, regardless of the source,
method of preparation, or the amount of the compound.
For example,
(i) 18 gm of H₂O = 2 gm of hydrogen + 16 gm of oxygen
mass of hydrogen : mass of oxygen = 2:16 = 1:8
(ii) 36 gm of H₂O = 4 gm of hydrogen + 32 gm of oxygen
mass of hydrogen : mass of oxygen = 4:32 = 1:8
(iii) In water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8
respectively.
DALTON’S ATOMIC THEORY
In 1808 John Dalton, was the first
scientist to describe the nature and
behavior of atoms.
Dalton’s theory laid the foundation for
modern chemistry and it helps us
understand the basic structure and
properties of matter.
Key Postulates of Dalton’s Atomic Theory are:
All matter is made of very tiny particles called atoms.
Atoms are indivisible particles, which cannot be created or destroyed in a chemical
reaction.
Atoms of a given element are identical in mass and chemical properties. (Law of
conservation of mass)
Atoms of different elements have different masses and chemical properties.
Atoms combine in the ratio of small whole numbers to form compounds. (Law of
constant proportion)
The relative number and kinds of atoms are constant in a given compound.
Drawbacks of Dalton’s Atomic Theory:
No Subatomic Particles: Dalton's theory said atoms were indivisible, but we now
know about electrons, protons, and neutrons.
Isotopes Not Defined: Dalton stated all atoms of an element have the same mass,
but isotopes of elements have different masses.
No Isobars: Dalton said atoms of different elements have different masses, but
isobars have the same mass number.
No Whole-Number Ratios Always: Complex compounds like sugar (C12H22O11)) do
not always follow simple whole-number ratios.
No Allotropes Defined: Allotropes like graphite and diamond have different
properties that Dalton's theory can't explain.
SYMBOLS OF ELEMENTS
John Dalton (1808), in his atomic theory, attempted to create a
set of symbols for elements based on simple circles with
different patterns or dots to represent different atoms.
Dalton: First scientist to use symbols for elements.
Berzelius: Suggested using one or two letters from the element's name for
its symbol.
Element Naming: Initially, elements were named after their discovery
locations (e.g., Copper from Cyprus).
IUPAC: Now responsible for approving element names, symbols, and units.
Symbols typically use one or two letters from the element's English name
(e.g., H for Hydrogen, Al for Aluminium).
Special Cases: Some symbols are derived from Latin, German, or Greek
names (e.g., Fe for Ferrum, Na for Natrium, K for Kalium).
MODERN SYMBOLS OF ELEMENTS
First Letter - Capital IUPAC : International Union of
Steps to Write Symbols:
Second Letter - Small Pure and Applied Chemistry
Trick to Rememeber First 20 Elements of Periodic Table
1. Hydrogen (H) 11. Sodium (Na)
2. Helium (He) 12. Magnesium (Mg)
3. Lithium (Li) 13. Aluminum (Al)
4. Beryllium (Be) 14. Silicon (Si)
5. Boron (B) 15. Phosphorus (P)
6. Carbon (C) 16. Sulfur (S)
7. Nitrogen (N) 17. Chlorine (Cl)
8. Oxygen (O) 18. Argon (Ar)
9. Fluorine (F) 19. Potassium (K)
10. Neon (Ne) 20. Calcium (Ca)
The standard reference element for measuring atomic mass is
Carbon-12
Carbon-12 (or C-12) is an isotope of carbon, where the nucleus contains 6 protons
and 6 neutrons, giving it an atomic mass of 12 atomic mass units (u).
1/12 of C-12
1 Atomic mass unit
(AMU)
ATOMIC MASS UNIT (amu)
Defined as 1/12th the mass of a carbon-12
atom.
This unit is often used to express atomic
masses.
12 Amu = Mass of a Carbon atom
ATOMS EXISTENCE
Most elements' atoms are highly reactive and do not exist freely.
Only noble gas atoms (He, Ne, Ar, Kr, Xe, Rn) are chemically inert and
can exist as single atoms.
Atoms of all other elements combine together to form molecules or
ions.
Ion (electrically charged)
Atom
Molecules (electrically neutral)
IONS
Ions are atoms or groups of atoms that carry an electric charge, either
positive or negative.
Ions are formed when atoms gain or lose electrons to achieve a stable
electron configuration
Type of Ion Charge Formation Examples
Formed when an atom loses Sodium ion (Na⁺),
Cations Positively charged ions
one or more electrons Calcium ion (Ca²⁺)
Negatively charged Formed when an atom gains Chloride ion (Cl⁻), Oxide
Anions
ions one or more electrons ion (O²⁻)
Ions
Simple Ions Polyatomic Ions
Ions made up of Ions made up of
single atoms. group of atoms.
EX: Na⁺, N³⁻, Ca²⁺ EX: NH₄⁺, OH⁻, SO₄²⁻
Names and symbols of some ions
MOLECULES
A molecule is the smallest particle of a substance that can exist
independently and retains the chemical properties of that substance.
Molecules are formed by two or more atoms bonded together, and
they can be of the same element or different elements.
Molecules
Molecules of Elements Molecules of Compounds
MOLECULES OF ELEMENTS
Molecules of elements are composed of two or more atoms of
the same element bonded together.
They represent the smallest independent unit of an element that
retains the element’s chemical properties.
These molecules are formed by atoms of the same element and
can exist as single atoms or bonded groups.
ATOMICITY
Atomicity is the number of atoms in a molecule of an element or
compound. It indicates how many atoms of the element are
bonded together in a single molecule.
For example, in an O₂ molecule, there are 2 atoms of oxygen, so
its atomicity is 2.
Achha to yeh baat hai!!!
Types of Atomicity:
Monoatomic – Molecules with a single atom .
Example: Helium (He), Neon (Ne), and Argon (Ar) have atomicity 1.
Diatomic – Molecules with two atoms of same element.
Example: Oxygen (O₂), Nitrogen (N₂), and Hydrogen (H₂) have atomicity 2.
Triatomic – Molecules with three atoms of same element.
Example: Ozone (O₃) has atomicity 3.
Tetratomic – Molecules with four atoms of same element.
Example: Phosphorus (P₄) has atomicity 4.
Polyatomic – Molecules with more than four atoms of same element.
Example: Sulfur (S₈) has atomicity 8.
Name of the Class Atomicity Examples
i) Noble gases: Helium (He), Argon (Ar), Neon
(Ne), Krypton (Kr)
Monatomic 1 ii) Metals: Sodium (Na), Magnesium (Mg),
Aluminium (Al)
iii) Carbon (C)
Hydrogen (H₂), Oxygen (O₂), Chlorine (Cl₂),
Diatomic 2
Fluorine (F₂), Nitrogen (N₂)
Triatomic 3 Ozone (O₃)
Tetratomic / Polyatomic 4 or more Phosphorus (P₄), Sulphur (S₈), Fullerenes (C₆₀)
Compound Combining Elements Atomicity Ratio by Mass
Hydrogen chloride (HCl) Hydrogen, Chlorine Diatomic 1 : 35.5
Water (H₂O) Hydrogen, Oxygen Triatomic 1:8
Ammonia (NH₃) Hydrogen, Nitrogen Tetratomic 1 : 4.67
Carbon dioxide (CO₂) Carbon, Oxygen Triatomic 1 : 2.67
Q1. Who proposed the Law of Conservation of Mass?
a) Dalton
b) Lavoisier
c) Democritus
d) Rutherford
Q2.What is the atomicity of phosphorus in P₄?
a) 2
b) 3
c) 4
d) 5
Q3. What is the atomic mass unit? (Amu) equal to ?
a)1/12 the mass of a carbon - 12atoms
b) 1/6 the mass of a carbon - 12atoms
c) 1/2 the mass of a carbon - 12atoms
d) 1 the mass of a carbon - 12atoms
Q4.Which of the following is the correct pair of atom and its atomic symbol?
(a) Sulphur – Su
(b) Potassium – P
(c) Phosphorus -P
(d) Sodium- S
Q5. Which of the following has the highest atomic mass?
a) Hydrogen
b) Carbon
c) Oxygen
d) Uranium
Q6. Which organisation approves the names of elements all ober the world? Write
symbol of gold?
Answer:
International union of Pure and applied chemistry (IUPAC)
Q7. What are the two laws of chemical combination? Explain with examples.
Answer:
Law of Conservation of Mass: Mass is neither created nor destroyed in a
chemical reaction. Example: In the reaction H₂ + Cl₂ 2HCl, the total mass
of reactants equals the total mass of products.
Law of Constant Proportions: A compound always contains the same
elements in the same ratio by mass. Example: Water (H₂O) always contains
hydrogen and oxygen in a 1:8 ratio by mass.
Q. A sample of magnesium oxide contains 2.4 g of magnesium and 1.6 g of
oxygen. Calculate the percentage composition of magnesium and oxygen in the
compound. If the Law of Constant Proportion is true, what weight of magnesium
and oxygen will be present in 5 g of magnesium oxide?
(Atomic masses: Mg = 24 u, O = 16 u)
Answer. Percentage composition: Mg=60%, O=40%
In 5 g of MgO, Mg=3.0 g and O=2.0 g
Q. A compound contains 2.4 g of magnesium and 3.2 g of oxygen. Show that this
data obeys the law of constant proportions.
Answer: Ratio of magnesium to oxygen = 2.4/3.2=3/4
This is consistent with the fixed ratio of Mg to O in MgO (3:4).
