Scribd
Upload
11 views4 pages

Atoms and Molecules

The document provides notes on the laws of chemical combination, including the Law of Conservation of Mass and Dalton's Atomic Theory, which outlines the nature of atoms and molecules. It defines key concepts such as atoms, molecules, atomicity, atomic number, and mass number, along with examples and Dalton's symbols for various elements. The information is aimed at IX grade chemistry students studying atoms and molecules.

Uploaded by

A&S YouTube life and vlogs
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
11 views4 pages

Atoms and Molecules

The document provides notes on the laws of chemical combination, including the Law of Conservation of Mass and Dalton's Atomic Theory, which outlines the nature of atoms and molecules. It defines key concepts such as atoms, molecules, atomicity, atomic number, and mass number, along with examples and Dalton's symbols for various elements. The information is aimed at IX grade chemistry students studying atoms and molecules.

Uploaded by

A&S YouTube life and vlogs
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
You are on page 1/ 4

Subject: CHEMISTRY Class: IX

Chapter – 3
Atoms and Molecules
Notes (Part – 1)

Laws of Chemical Combination


 Law Of Conservation Of Mass
Law of conservation of mass states that mass can neither be created nor
destroyed in a chemical reaction.
 Law Of Constant Proportions

”In a chemical substance the elements are always present in definite proportions
by mass”

 Dalton’s Atomic Theory

(i) All matter is made of very tiny particles called atoms, which participate in
chemical reactions.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a
chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in each compound.
Question Answer
Atom:
An atom is defined as the smallest particle of an element which may or may not be
capable of free existence. However, it is the smallest particle that takes part in a
chemical reaction.
Example : Free state – Helium, Neon, Argon
Combined state: Hydrogen – H2 (dihydrogen), N2 (dinitrogen)
Combined with other elements: HCl, MgCl2
Molecule:
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. e.g., molecule of oxygen is O2, ozone is O3, phosphorus is P4, sulphur is S8

 Molecules of an element constitute the same type of atoms.


 Molecules may be monoatomic, diatomic or polyatomic.
 Molecules of compounds combine together in definite proportions and
constitute a different type of atoms

Atomicity:

The number of atoms constituting a Molecule is known as its atomicity.


Monoatomic – He, Ar
Diatomic – O2, H2, N2, Cl2
Tetra-atomic - P4
Poly-atomic – S6

Atomic number and Mass number:

The atomic number abbreviated “Z,” of an element indicates how many protons it has.
The symbol “A” stands for an element’s mass number, which is equal to the total of its
neutron and proton. Specific symbols are used to represent an element’s mass number
and atomic number.

Atomic mass is the total mass of a single atom. The atomic mass unit serves as a
representation for the measurement of atomic mass (amu).
Atomic mass unit =1/12 th of the mass of a carbon-12 atom
The atomic masses of all other elements are determined by comparing the
mass of their atom with the mass of a carbon-12 atom.
DALTON’S SYMBOLS OF ELEMENTS:

 Dalton was the first scientist to use symbols to represent elements in a short
way.

Elements/ Symbol Elements/ Symbol


Hydrogen - H Iron - Fe
Carbon - C Copper - Cu
Oxygen - O Silver - Ag
Phosphorus - P Gold - Au
Sulphur - S Lead - Pb
Platinum - Pt Mercury - Hg

***********************************************************

You might also like