Q1
A molecule is formed when:
Atwo or more atoms combine chemically
Ban atom splits into pieces
Can atom loses its nucleus
Dprotons are removed from an atom
CBSEClass 9 (CBSE)ScienceCh 3 · Topic 2
Atoms, Molecules and Atomic Mass
Learning about atoms, molecules, and how their masses are represented.
Core concept
Molecular mass is calculated by adding up the atomic masses of all atoms present in a molecule — for example, the molecular mass of water (H₂O) is calculated as (2 × 1) + 16 = 18u, using hydrogen's atomic mass of 1u and oxygen's atomic mass of 16u. Avogadro's number (approximately 6.022 × 10²³) represents the number of particles (atoms or molecules) present in one mole of any substance, a crucial concept for calculating quantities in chemical reactions.
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• An atom is the smallest particle of an element; a molecule forms when atoms combine chemically.
• Atomic mass is measured in atomic mass units (u), based on carbon-12 as the reference.
• Molecular mass = sum of atomic masses of all atoms in the molecule (e.g., water = 18u).
• Avogadro's number (6.022 × 10²³) is the number of particles in one mole of a substance.