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Lessons
- Chapter 1 – Matter in our surroundings
- Chapter 2 – Is matter around us pure
- Class 9 chapter 3 – Atoms & Molecules
- Class 9 Chapter 4 – Structure of the Atom
- Class 9 Chapter 5 – The fundamental unit of life
- Class 9 Chapter 6 – Tissues
- Class 9 Chapter 7 – Diversity in living organisms
- Class 9 Chapter 8 – Motion
- Class 9 Chapter 9 – Force and laws of motion
- Class 9 Chapter 10 – Gravitation
- Class 9 Chapter 11 – Work & Energy
- Class 9 Chapter 12 – Sound
- Class 9 Chapter 13 – Why do we fall ill
- Class 9 Chapter 14 – Natural Resources
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Question Bank
- Class 9 Question Bank Chapter 1 – Matter in our surrounding
- Class 9 Question Bank Chapter 2 – Is matter around us pure
- Class 9 Question Bank Chapter 3 – Atoms and molecules
- Class 9 Question Bank Chapter 4 – Structure of the Atom
- Class 9 Question Bank Chapter 5 – The fundamental unit of life
- Class 9 Question Bank Chapter 6 – Tissues
- Class 9 Question Bank Chapter 7 – Diversity in living Organism
- Class 9 Question Bank Chapter 8 – Motion
- Class 9 Question Bank Chapter 9 – Force and laws of Motion
- Class 9 Question Bank Chapter 10 – Gravitation
- Class 9 Question Bank Chapter 11 – Work and energy
- Class 9 Question Bank Chapter 12 – Sound
- Class 9 Question Bank Chapter 13 – Why do we fall ill
- Class 9 Question Bank Chapter 14 – Natural Resources
Class 9 chapter 3 – Atoms & Molecules
LAWS OF CHEMICAL COMBINATION
According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.
- The relative atomic mass of the atom of an element is defined as the average mass of the atom.
- The number of atoms constituting a molecule is known as its atomicity.
- A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
- Atoms of the same element or of different elements can join together to form molecules.
Compounds composed of metals and non-metals contain charged species. The charged species are known as ions
MOLECULAR MASS
CHEMICAL FORMULAE
- It is therefore the relative mass of a molecule expressed in atomic mass units (u).
- The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 1023. This number is called the Avogadro Constant or Avogadro Number.
- Avogadro Number is named in honour of the Italian scientist, Amedeo Avogadro.
- The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
- Molar mass of atoms is also known as gram atomic mass.
MOLE CONCEPT
ISOTOPES
The atoms of same element, having same atomic number but different mass numbers. e.g. hydrogen 11H, deuterium 1 2H and tritium 1 3H