Question

Notes of Atoms and molecules class 9 part 3

Answer 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  Hydrogen�H  Helium�He  Carobon�C  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.

Answer 2

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