8th Class Science Materials : Metals and Non-metals NCERT Summary - Elements

NCERT Summary - Elements

Category : 8th Class






  • An element is a substance which can neither be broken nor built from two or more simple substances by any physical or chemical method. Or in other words an elements is a pure substance which contains only one kind of atom, e.g., Fe (Iron), Na (Sodium), etc.


Types of Elements

(a) Metals

(b) Non-Metals

(c) Metalloids



  • A compound is a pure substance which contains more than one kind of elements or atom in fixed proportion by weight e.g., NaCl (Sulphur dioxide), (Sodium chloride),\[S{{O}_{2}}\]etc.


Types of Compounds

(a) Organic compounds

(b) Inorganic compounds

  • The properties of a compound are completely different from those of its constituents.



  • A material containing two or more elements or compounds in any proportion is a mixture. It can be seperated into constituents, e.g., air, milk, paints, cements, etc.


Types of Mixture

(a) Homogeneous

(b)  Heterogeneous

(a) A mixture is said to be homogeneous if its composition is uniform throughout.

(b) A mixure is said to be heterogeneous if its composition is not uniform.

  • The properties of a mixture are the properties of its constituents.
    • A mixture with definite boiling point is known as azeotropic mixture.


Separation of Mixtures

(a) Sublimation: In this process, a solid substance passes directly into its vapours on application of heat, and when vapours are cooled, they give back the original substance, e.g., iodine, napthalene, benzoic acid, etc.

(b) Sedimentation and decantation: This method is used when one component is liquid and the other is insoluble solid.

(c) Crystallisation: This method is based on the difference in solubility of the various compounds in a solvent, e.g., mixture of \[KN{{O}_{3}}\] and \[NaCl\] can be separated by this process.

(d) Filtration: It is used for quick and complete removal of solid suspended particles from a liquid (or gas) by passing the suspension through a filter.

(e) Evaporation: In this method the solution is heated so that the solvent vapourises to give the solute (solid substance), e.g., salt can be obtained from salt solution.

(f)   Distillation: A mixture of two substances, only one of which is volatile, can be separated by this process, e.g., \[NaCI\]can be separated from water by distillation.

(g) Fractional distillation: This process is used if both the components of a mixture are volatile. It is based on the difference of boiling points, e.g., the various fractions of crude petroleum can be separated by this process.

(h) Steam distillation: It is used to separate a liquid (should be immiscible with water) from a mixture by heating with steam, e.g., lemon oil, toluene, nitrobenzene, etc.

(i)   Mechanical separation: Two immiscible liquids can be separated by using a separately funnel, e.g., oil and water.

(j)   Magnetic separation: Two solids one of which is magnetic substance can be separated by this method.

(k) Atmolysis: In this process a mixture of gases can be separated based on their rates of diffusion, e.g., Isotopes of uranium \[({{U}_{235}}\,and\,{{U}_{238}})\] are separated by converting them into gaseous \[U{{F}_{6}}\].

(I) Chromatography: This is the most versatile separation method which can be applied to solid, liquid or gas. In this method the components of a mixture are adsorbed on a suitable adsorbent at different rates and thus get separated, e.g. to separate coloured materials from dyes, green vegetables, ink, etc.



  • It is the smallest particle of an element which does not always exist independently, for example, atoms of hydrogen do not exist independently. Instead, they always occur in pairs. Two atoms of hydrogen form a molecule of hydrogen which exists independently. Atom is the smallest particle of an element that can take part in a chemical change.



  • It is the number of atoms present in a molecule of an element. Atomicity of hydrogen, oxygen, nitrogen and chlorine is 2, for ozone, it is 3 and for phosphorus, it is 4.



  • It is the smallest part of an element or compound that can normally exist separately.


Atomic Weight (or Atomic Mass)

  • The atomic mass of an elements is the number of times its atom is heavier the l/12th of the mass of carbon atom. The unit used to measure atomic mass is called atomic mass unit, i.e., amu. In this unit, atomic mass of hydrogen is 1 amu, and that of oxygen is 16 amu.

\[\therefore \] Atomic wt. of an element

= Average wt. of one atom of the element\[\times \]12 wt. of one atom of \[{{C}_{12}}\]


Atomic Mass of Some Common Elements


Atomic Mass


Atomic Mass


























Molecular Weight

  • The molecular weight of an element or compound is the number of times its molecule is heavier than 1/12th of mass of carbon atom.

\[\therefore \] Molecular weight of an element or compound

= Average weight of one atom of the element or compound \[\times \] 12 wt. of one atom of \[{{C}_{12}}\]


Mole (Or Mol)

  • One mole is equal to the number of atoms present in 12 gram \[{{C}_{12,}}\,i.e.,\]

\[1\,mole\,=\,6.022\times {{10}^{23}}amu,\]

            Here, \[6.022\times {{10}^{23}}\] is known as Avogadro’s number

  • Relation between gram and amu

= 1 gram = \[6.022\times {{10}^{23}}\] amu.



  • It is the number of the parts of a substance by weight, which combine with or displace directly or indirectly 1.008 part by weight of H, 8 parts of weight of O or 35.5 parts by weight of Cl.



Other Topics

Notes - Elements

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