Atoms and Molecules
Ancient Idea of Matter
• Indian Philosophers: Maharishi Kanad proposed matter is divisible until reaching the
smallest particle called "Parmanu"[1].
• Greek Philosophers: Democritus and Leucippus named the indivisible particle
"atom"[1].
• Early theories were philosophical, not experimental[1].
Laws of Chemical Combination
Law of Conservation of Mass
• Statement: Mass can neither be created nor destroyed during a chemical reaction[1].
• Experimental Proof: Mixing chemicals and comparing mass before and after reaction
shows no mass change[1].
Law of Constant Proportions (Definite Proportions)
• Statement: Chemical compounds always contain the same elements in the same
proportion by mass, regardless of source or method of preparation[1].
• Example: In water (H₂O), hydrogen and oxygen are always in the mass ratio 1:8[1].
Dalton’s Atomic Theory
• All matter is made of tiny particles called atoms[1].
• Atoms are indivisible and can't be created or destroyed in chemical reactions[1].
• Atoms of an element are identical in mass and chemical properties[1].
• Atoms of different elements have different masses and properties[1].
• Atoms combine in small whole number ratios to form compounds[1].
• In a compound, the type and number of atoms are constant[1].
What is an Atom?
• Atom: Smallest particle of an element that cannot usually exist independently[1].
• Size: Measured in nanometres; extremely small—millions of atoms fit in a thin paper
layer[1].
Symbols for Elements
• Dalton first used symbols for elements; Berzilius suggested using one or two letters of
element names[1].
• IUPAC decides the symbols and names now[1].
• Example: H for hydrogen, Na for sodium (from Latin), Fe for iron (from Latin ferrum)[1].
Atomic Mass
• Definition: The relative atomic mass is the mass of an atom compared to one-twelfth
of the mass of a carbon-12 atom[1].
• Earlier, 1/16th of oxygen’s mass was used; now, 1/12th of carbon-12 is standard[1].
• Atomic Mass Unit (u): Defined as one-twelfth the mass of a carbon-12 atom[1].
Molecules and Atomicity
• Molecule: Group of two or more atoms chemically bonded together[1].
• Molecules show all properties of the substance[1].
• Atomicity: Number of atoms in a molecule[1].
• Examples: O2 (atomicity 2), O3 (atomicity 3), S8 (atomicity 8)[1].
Molecules of Compounds
• Molecules formed by atoms of different elements in fixed proportions[1].
• Example (Water): Hydrogen and oxygen combine in ratio 2:1 by number of atoms[1].
Ions
• Ion: Charged particle formed by loss or gain of electrons—can be single atoms or
groups (polyatomic ions)[1].
• Cation: Positive ion (e.g., Na+)[1].
• Anion: Negative ion (e.g., Cl-)[1].
• Polyatomic ion example: SO₄²⁻ (sulphate)[1].
Writing Chemical Formulae
• Chemical formula shows element symbols and number/ratio of atoms[1].
• Valency: Combining power of an element; used to determine formula[1].
• For ionic compounds, balance charges to write chemical formula (use brackets for
multiple polyatomic ions)[1].
Example Formulas
• HCl: Hydrogen chloride[1]
• H₂S: Hydrogen sulphide[1]
• CCl₄: Carbon tetrachloride[1]
• MgCl₂: Magnesium chloride[1]
• Al₂O₃: Aluminium oxide[1]
• Ca(OH)₂: Calcium hydroxide[1]
• Na₂CO₃: Sodium carbonate[1]
• (NH₄)₂SO₄: Ammonium sulphate[1]
Molecular Mass and Formula Unit Mass
• Molecular Mass: Sum of atomic masses of all atoms in a molecule[1].
• Example (H₂O): $$2\times 1 + 16 = 18$$ u[1].
• Formula Unit Mass: Sum of atomic masses in a formula unit (used for ionic
compounds)[1].
• Example (NaCl): $$23 + 35.5 = 58.5$$ u[1].
Key Terminologies
• Law of Conservation of Mass: Mass of reactants = Mass of products in a chemical
reaction[1].
• Law of Definite Proportions: Elements in a compound are present in fixed proportions
by mass[1].
• Atom: Smallest particle, usually can't exist alone, retains properties[1].
• Molecule: Smallest particle of element/compound capable of independent
existence[1].
• Ions: Atoms/groups with a charge due to loss/gain of electrons[1].
• Chemical formula: Shows composition of a compound using element symbols and
subscripts[1].
• Valency: Capacity of atom to combine with others, used to balance formulae[1].
• Polyatomic ions: Group of atoms with a fixed charge behaving as a single unit[1].
