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CBSE · Class 9 · Science

Chapter 9: Atomic Foundations of Matter

Laws of chemical combination, Dalton's theory, covalent and ionic bonds, chemical formulae, molecular mass and formula unit mass.

Notes

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Law of conservation of mass
  • Physical changes (dissolving salt in water): the mass of the solution equals the sum of the masses of salt and water.
  • Chemical changes (vinegar + baking soda): in a closed system the total mass of reactants equals the total mass of products. In an open flask the reading falls because the gas (carbon dioxide) escapes.
  • Law of Conservation of Mass (Lavoisier, 1789): matter can neither be created nor destroyed in a chemical reaction.
  • Activity 9.3: sodium sulfate + barium chloride gives a white precipitate of barium sulfate and sodium chloride; the total mass is unchanged.
Law of constant proportions
  • Proust: in any compound the elements are present in a fixed ratio by mass, whatever its source or method of preparation.
  • Water always contains hydrogen and oxygen in the mass ratio 1 : 8 (9 g of water gives 1 g H and 8 g O).
  • It is also called the Law of Definite Proportions or Proust's Law. It does not hold for mixtures, which can have any composition.
Dalton's atomic theory
  • All matter is made of very tiny particles called atoms.
  • Atoms are indivisible and cannot be created or destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass and chemical properties; atoms of different elements differ.
  • Atoms combine in the ratio of simple whole numbers to form compounds.
  • The relative number and kinds of atoms in a compound are constant.
Molecules and chemical bonds
  • A molecule is an electrically neutral entity of more than one atom that can exist independently and shows the properties of the substance. Helium exists only as atoms.
  • Atoms combine to become stable (complete octet; duplet for the K shell). The force holding atoms together is a chemical bond. The energy of the bonded system is lower than that of the separate atoms.
  • Two ways: sharing of electrons (covalent bond) and transfer of electrons (ionic bond).
Covalent bond
  • A bond formed by a shared pair of electrons. Single bond: H2 (H–H), Cl2 (Cl–Cl), HCl (H–Cl). Double bond: O2 (O=O). Triple bond: N2 (N≡N).
  • Water: oxygen shares one electron with each of two hydrogen atoms (H2O).
  • Naming: prefixes mono-, di-, tri-, tetra-, penta-, hexa-. Examples: CO carbon monoxide, CO2 carbon dioxide, CS2 carbon disulfide, PCl3 phosphorus trichloride, SF6 sulfur hexafluoride, N2O4 dinitrogen tetroxide, N2O5 dinitrogen pentoxide. No prefix before hydrogen (H2S is hydrogen sulfide).
Ionic bond
  • A metal atom loses valence electrons to form a cation; a non-metal atom gains electrons to form an anion. The electrostatic attraction between the oppositely charged ions is the ionic bond.
  • Na (2,8,1) loses one electron to become Na+; Cl (2,8,7) gains one to become Cl−; NaCl results. Sulfur gains two electrons to form S2−; oxygen forms O2−.
  • Ionic compounds form 3-D crystal lattices (each Na+ is surrounded by six Cl−), not molecules.
  • Naming: cation first, then anion ending in -ide (sodium chloride, magnesium sulfide). Polyatomic ions such as OH−, NO3−, CO32−, SO42−, NH4+, HCO3− usually do not end in -ide.
Writing chemical formulae
  • Covalent compounds: write the symbols, write the valencies, crossover the valencies as subscripts (H2S, CCl4, HCl).
  • Ionic compounds: write the cation first, then the anion; crossover the charges (numbers only); divide by a common factor to get the simplest ratio: CaCl2, Al2O3, MgO (not Mg2O2), CaCO3, Mg(OH)2, Al(OH)3, Al2(SO4)3.
  • Use brackets when two or more polyatomic ions of the same kind are present.
Properties of ionic and covalent compounds
  • Ionic (NaCl, CuSO4): usually soluble in water, insoluble in kerosene and petrol; do not conduct in the solid state (ions fixed) but conduct when dissolved in water or molten (ions free); high melting and boiling points.
  • Covalent (camphor, naphthalene): usually insoluble in water but soluble in kerosene and petrol; most do not conduct (no ions), even sugar, which dissolves in water but gives no ions; low melting and boiling points.
Molecular mass and formula unit mass
  • Molecular mass (covalent compounds) = sum of the atomic masses of all atoms in the molecule: H2O = 18 u, CO2 = 44 u.
  • Formula unit mass (ionic compounds) = sum of the atomic masses in the simplest ratio of ions: Na2O = 62 u, Ca(NO3)2 = 164 u.
At a glance
  • Conservation of mass and constant proportions led to Dalton's theory.
  • Covalent bond: sharing. Ionic bond: transfer.
  • Formulae are written by crossing over valencies or charges.