CBSE Class 9 Chemistry, Atoms and Molecules

Class IX Science
Notes for Atoms and Molecules
Syllabus
Particle nature, Basic units, Atoms and molecules, Law of constant proportions, Atomic and molecular masses.
Facts that Matter
•  Law of Chemical Combination
      Given by Lavoisier and Joseph L. Proust as follows:
      (i) Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction.
           e.g.,A+B-C+D
           Reactants R Products
           Mass of reactants = Mass of products
      (ii) Law of constant proportion: In a chemical substance the elements are always present in definite proportions by mass.
           E.g., in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively.
           These laws lacked explanation. Hence, John Dalton gave his theory about the matter. He said that the smallest particle of matter is called ‘atom’.
•  Dalton’s Atomic Theory
      1. Every matter is made up of very small or tiny particles called atoms.
      2. Atoms are not divisible and cannot be created or destroyed in a chemical reaction.
      3. All atoms of a given element it are same in size, “mass and chemical properties.
      4. Atoms of different elements are different in size, mass and chemical properties.
      5. Atoms combine in the ratio of small whole number to form compounds.
      6. The relative number and kinds of atoms are constant in a given compound. 9
•  Atom
      Atoms are the smallest particles of an element which can take reaction.
      Size of an atom: Atomic radius is measured in nanometres.
      
      Atomic radii of hydrogen atom = 1 × 10–10 m.
      Symbols of atoms:
      (a) Symbols for some elements as proposed by Dalton:
      (b) Symbols of some common elements:
  Name of the elementLatin nameSymbol
  HydrogenH
  HeliumHe
  CarobonC
  CopperCuprumCu
  CobaltCo
  ChlorineCl
  CadmiumCd
  BoronB
  BariumBa
  BromineBr
  BismuthBi
  SodiumNatriumNa
  PotassiumKaliumK
  IronFerrumFe
  GoldAurumAu
  SilverArgentumAg
  MercuryHydragyrumHg
•  Molecule
      It is the smallest particle of an eleme4It- or a dolnpound which can wxist independently.
      • Molecules of an element constitutes same type of atoms. 1.w
      • Molecules may be monoatomic, di-atomic or polyatomic. IT .
      • Molecules of compounds join together in defmite proportionsrand constitutes different type of atoms.
•  Atomicity
      The number of atoms constituting a Molecule is known as its atomicity.
  Name of the elementAtomicityMolecules formula
  HeliumMonoatomicHe
  NeonMonoatomicNe
  ArgonMonoatomicAr
  SodiumMonoatomicNa
  IronMonoatomicFe
  AluminiumMonoatomicAl
  HydrogenDi-atomicH2
  OxygenDi-atomicO2
  ChlorineDi-atomicCl2
  NitrogenDi-atomicN2
  PhosphorusPolyatomic (Tetra)P4
  SulphurPolyatomic (Octa)S8
•  Ions
      The charged particles (atoms) are called ions, they charge or negative charge on it:
      Negatively charged ionis called anion (C1).
      Positively charge ion is called cation (Na+).
•  Valency
      The combining capacity of an element is known as its valency: Valency is used to fmd out how atom of an element will combine with the atom of another element to form a chemical compound.
      (Every atom want, to become stable, to do so it may loose, gain or share electrongs.
      (i) If an atom consists of 1, 2 or 3 electrons in its valgncesI ell then its valency is 1, 2 or 3 respectively,
      (ii) If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron respectively and its valency will be 3, 2 or 1 respectively.
      (ii) If an atom has 4 electrons in the outermost shell than it will she this electron and hence its valency will be 4.
      (iv) If an atom has 8 electrons in the outermost shell then its valency is 0.
•  Chemical Formulae
      Rules: (i) The valencies or charges on the ion must balance.
      (ii) A metal and non-metal compound should show the name or symbols of the metal first.
           e.g., Na+ Cl → NaCl
      (iii) If a compound consist of polyatomic ions. The ion before writing the number to indicate the ratio.
           e.g., [SO4]2– → polyatomic radical
           H1+ SO42– → H2SO4.
      Chemical formula of some simple compounds
      (a) Calcium hydroxid
           
      (b) Aluminium oxide
           
•  Molecular Mass
      It is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass unit (u).
      e.g., 2H+ + O2 H2O [H = 1, 0 = 16]
      1 × 2 + 16 = 18 u
•  Formula Unit Mass
      It is the sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles are ions.
      e.g., Na+ + Cl– → NaCl
      1 × 23 + 1 × 35.5 = 58.5 u
•  Mole Concept
      Definition of mole: It is defined as one mole of any species (atom, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
                  1 mole = 6.022 × 1023 in number
      Molar mass = mass of 1 mole → is is always expressed in r gram, and is also known as gram atomic mass.
      1u of hydrogen has → 1 atom of hydrogen
      lg of hydrogen has → 1 mole of hydrogen
      = 6.022 × 1023 atoms of hydrogens