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Atoms and Molecules
• A molecule is a group of atoms that are chemically bonded together (held together byattractive forces).
• A molecule can bedefined as the smallest particle of an element or 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 toform molecules.
• Law of conservation of mass: Law of conservation of mass states that mass can neither be created nor
destroyed in a chemical reaction.
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The postulates of John Dalton’s Atomic Theory
1. All matter is made of very tiny particles called atoms.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
3. Atoms of a given element are identical in mass and chemical properties.
4. Atomsofdifferent elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds
6. The relative number and kinds of atoms are constant in a given compound.
