Atoms and Molecules Class 9 Science — Revision Notes
This chapter covers atomic structure, molecular composition, and chemical formulas. Students learn about atomic mass unit, molar mass, and relationship bet
TL;DR: This chapter covers atomic structure, molecular composition, and chemical formulas. Students learn about atomic mass unit, molar mass, and relationshi…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
This chapter covers atomic structure, molecular composition, and chemical formulas. Students learn about atomic mass unit, molar mass, and relationship bet
Atoms and Elements
- Atom is smallest unit of element retaining chemical properties
- Atoms cannot be created or destroyed in chemical reaction
- Atomic mass unit (amu) is 1/12 mass of carbon-12 atom
- Atoms of same element are identical in properties
Molecules
- Molecule is smallest unit of compound
- Molecules formed by atoms bonded together
- Diatomic molecule: two atoms (O2, H2, N2, Cl2)
- Polyatomic molecule: more than two atoms (H2O, CO2, NH3)
Chemical Formula
- Chemical formula shows symbols and number of atoms in molecule
- H2O: 2 hydrogen atoms and 1 oxygen atom
- Subscript indicates number of atoms (no subscript means 1 atom)
- Molecular formula gives actual number of atoms
Mole Concept
- Mole is unit for amount of substance
- 1 mole of substance = 6.022 x 10^23 particles (Avogadros number)
- Molar mass = sum of atomic masses in grams per mole
- Number of moles = mass / molar mass
Key Terms
- Atom: Smallest unit of element retaining chemical properties of element
- Molecule: Smallest unit of compound formed by atoms bonded together
- Mole: Unit of amount of substance equal to 6.022 x 10^23 particles
Frequently Asked Questions
What is Avogadros number?
Avogadros number is 6.022 x 10^23. It represents number of particles (atoms, molecules, ions) in one mole of any substance.
How do you find number of atoms in a molecule?
Count the number of atoms of each element from chemical formula. For example, H2SO4 has 2 hydrogen, 1 sulfur, and 4 oxygen atoms, total 7 atoms.
More Class 9 Science Revision Notes
- Matter in Our Surroundings
- Is Matter Around Us Pure
- Structure of the Atom
- The Fundamental Unit of Life
- Tissues
- Motion
🤖 Stuck on any of these? Ask Syllab's free AI Tutor to explain step by step →