CBSE Class 9 Chemistry, Atoms and Molecules

Particle nature, Basic units, Atoms and molecules, Law of constant proportions, Atomic and molecular masses.

      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’.

      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

      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:

      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.

      The number of atoms constituting a Molecule is known as its atomicity.

      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⁺).

      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.

      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⁺ SO₄^2– → H₂SO₄.

      Chemical formula of some simple compounds

      (a) Calcium hydroxid

           

      (b) Aluminium oxide

           

      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

      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

      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