CLASS 9 NOTES
ATOMS AND
MOLECULES
⚛️Atoms & Molecules
📌 Key Laws
Law of Conservation of Mass – Matter is neither created nor destroyed in a chemical
reaction
Law of Constant Proportions – A compound always contains same elements in fixed
ratio by mass
Atom
Smallest particle of an element
Represented by symbols (H, O, C, Na, etc.)
Atomic Mass = mass of one atom (in u / amu)
Element Symbol Atomic Mass
Hydrogen H 1u
Carbon C 12 u
Oxygen O 16 u
Nitrogen N 14 u
Sodium Na 23 u
📌 Molecule
Group of two or more atoms bonded together
H₂O (water), CO₂ (carbon dioxide), O₂ (oxygen)
📌 Mole Concept ⭐ Most Important
1 Mole = 6.022 × 10²³ particles (Avogadro's Number)
Molar Mass = Atomic/Molecular mass in grams
ATOMS AND
MOLECULES
⚛️Atoms & Molecules
📌 Key Laws
Law of Conservation of Mass – Matter is neither created nor destroyed in a chemical
reaction
Law of Constant Proportions – A compound always contains same elements in fixed
ratio by mass
Atom
Smallest particle of an element
Represented by symbols (H, O, C, Na, etc.)
Atomic Mass = mass of one atom (in u / amu)
Element Symbol Atomic Mass
Hydrogen H 1u
Carbon C 12 u
Oxygen O 16 u
Nitrogen N 14 u
Sodium Na 23 u
📌 Molecule
Group of two or more atoms bonded together
H₂O (water), CO₂ (carbon dioxide), O₂ (oxygen)
📌 Mole Concept ⭐ Most Important
1 Mole = 6.022 × 10²³ particles (Avogadro's Number)
Molar Mass = Atomic/Molecular mass in grams
