This chapter explains that all matter is made up of extremely small particles called atoms, which combine to form molecules. It traces the development of the idea of atoms from ancient philosophers to John Dalton’s atomic theory. The chapter introduces the laws of chemical combination, which explain how substances react and combine in fixed ways. It also explains important concepts such as atomic mass, molecular mass, ions, valency, and chemical formulae, helping students understand the basic building blocks of matter and chemical reactions.
Key Points
Matter is made up of very tiny particles called atoms.
Ancient Indian and Greek philosophers proposed the idea of indivisible particles.
Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction.
Law of Constant Proportions states that elements in a compound are always present in fixed mass ratios.
John Dalton explained these laws using his atomic theory.
According to Dalton’s atomic theory, atoms are indivisible and identical for a given element.
Atoms of different elements have different masses and properties.
Atoms combine in simple whole number ratios to form compounds.
Atoms are extremely small and cannot be seen with naked eyes.
Each element has a chemical symbol, approved by IUPAC.
Atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom.
Atoms usually do not exist independently.
Atoms combine to form molecules or ions.
A molecule is the smallest particle that can exist independently.
Atomicity is the number of atoms present in a molecule.
Ions are charged particles formed by loss or gain of electrons.
Cations are positively charged ions, while anions are negatively charged ions.
Valency is the combining capacity of an element.
Chemical formulae represent the composition of compounds.
Molecular mass is the sum of atomic masses of all atoms in a molecule.
Formula unit mass is used for ionic compounds.
👉 👉Understanding atoms and molecules helps us realise that all matter is structured and organised at a microscopic level. This knowledge forms the foundation of chemistry and allows us to explain chemical reactions, write formulae correctly, and apply science meaningfully in daily life and technology.