Atoms and Molecules

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The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products in any chemical reaction.

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The Law of Constant Proportions states that in a chemical substance, the elements are always present in definite proportions by mass. For example, in water (H₂O), hydrogen and oxygen are always presen

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John Dalton proposed the atomic theory. Main postulates: (1) All matter is made of tiny particles called atoms (2) Atoms are indivisible and cannot be created or destroyed (3) Atoms of same element ar

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An atom is the smallest particle of an element that cannot usually exist independently and retains all the chemical properties of that element. Atoms are the basic building blocks of all matter.

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Atoms are extremely small - millions of atoms stacked together would make a layer barely as thick as a sheet of paper. Atomic radius is measured in nanometres (nm), where 1 nm = 10⁻⁹ m. For example, h

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Atomic mass unit (u) is the standard unit for measuring atomic masses. One atomic mass unit is defined as exactly 1/12th the mass of one atom of carbon-12. All atomic masses are measured relative to t

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Relative atomic mass of an element is the average mass of its atom compared to 1/12th the mass of one carbon-12 atom. It tells us how heavy an atom is compared to the carbon-12 standard.

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A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance. It is formed when two or more atoms are chemically bonded tog