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CBSE | Class 9 | science
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Summary Note
Key concept recap
Ancient Indian philosophers like Maharishi Kanad proposed that matter is made up of tiny indivisible particles called 'Parmanu.'
Similarly, Greek philosophers such as Democritus and Leucippus coined the term 'atom' (meaning indivisible) based on philosophical reasoning.
These early ideas laid the foundation for the scientific study of matter that emerged much later with experimentation.
Summary Note
Key concept recap
Antoine Lavoisier established the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.
Joseph Proust proposed the Law of Constant Proportions, stating that elements combine in fixed mass ratios to form compounds.
These laws helped guide further research into the nature of atoms and molecules.
Summary Note
Key concept recap
John Dalton postulated that all matter is composed of tiny, indivisible atoms.
Atoms of the same element are identical, while atoms of different elements vary in mass and properties.
Atoms combine in simple whole-number ratios to form compounds, and chemical reactions involve rearrangement of atoms.
Dalton's theory explained the laws of conservation of mass and definite proportions.
Summary Note
Key concept recap
Atoms are the fundamental building blocks of matter and are extremely small in size.
They cannot usually exist independently and form molecules or ions to make visible matter.
Atomic radius is measured in nanometres, and modern techniques allow visualization of atoms using magnified images.
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Key concept recap
Dalton introduced symbols for elements; modern symbols are based on English or Latin names and follow IUPAC conventions.
The first letter is always capitalized; if two letters are used, the second is lowercase (e.g., H for Hydrogen, Na for Sodium).
Symbols help in representing elements and their atomic masses concisely.
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Key concept recap
Atomic mass is the mass of one atom of an element relative to 1/12th the mass of a carbon-12 atom.
The atomic mass unit is denoted as ‘u’.
It helps in calculating the masses of atoms and molecules involved in chemical reactions.
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Key concept recap
Molecules are groups of atoms chemically bonded together and are the smallest units showing all properties of a substance.
Molecules of elements contain the same type of atoms (e.g., O₂, N₂), while molecules of compounds contain different atoms (e.g., H₂O, CO₂).
Ions are charged particles – cations are positively charged, and anions are negatively charged. They form ionic compounds like NaCl.
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Key concept recap
The chemical formula of a compound represents the types and numbers of atoms in it.
Valency is the combining capacity of an element and is used to determine the ratio in which elements combine.
Formulae are written using criss-cross method to balance the charges or valencies of ions.
Summary Note
Key concept recap
Molecular mass is the sum of atomic masses of all atoms in a molecule (e.g., H₂O = 2×1 + 16 = 18u).
Formula unit mass applies to ionic compounds and is calculated similarly using atomic masses (e.g., NaCl = 23 + 35.5 = 58.5u).
These concepts help in stoichiometric calculations in chemistry.
Summary Note
Key concept recap
Atom – Smallest particle of an element involved in chemical reactions.
Molecule – Smallest particle of an element/compound showing all properties.
Ion – Charged particle formed by gain or loss of electrons.
Valency – Combining capacity of an element.
Chemical Formula – Representation of elements and their ratio in a compound.
Molecular Mass – Sum of atomic masses in a molecule.
Atomic Mass Unit – 1/12 the mass of a carbon-12 atom (symbol: u).
Law of Conservation of Mass – Mass remains unchanged in a chemical reaction.
Law of Constant Proportions – Elements in a compound are in fixed mass ratio.
Polyatomic Ion – Ion made of multiple atoms with net charge (e.g., SO₄²⁻).
