NCERT Solutions for Class 9 Science Chapter 3: Atoms and Molecules

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The NCERT Solutions for Class 9 Science are provided in this article. Some of the important topics in Chapter 3 Atoms and Molecules include:

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NCERT Solutions for Class 9 Science Chapter 3

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Class 9 Science Chapter 3 Atoms and Molecules – Important Topics

Atom is the smallest constituent unit of matter that possess the properties of the chemical element. Atoms don’t exist independently, instead, they form ions and molecules which further combine in large numbers to form matter that we see, feel and touch.

Dalton’s Atomic Theory

Dalton’s Atomic Theory hypothesized and further claimed that the law of conservation of mass and the law of definite proportions could possibly de elaborated by using the idea of atoms. According to him, all substances are indestructible building units.

Example: What are two postulates of Dalton’s Atomic Theory?

Solution: The two postulates of Dalton’s Atomic Theory are:

  • The substances around us are composed of matter, which are tiny, indestructible particles called atoms.
  • Every atom of a particular element are identical according to their mass, size, and several other properties. 

Atomic Mass

The complete mass of one atom of a distinct element can be otherwise called as its atomic mass. The unit it has is known as unified atomic mass unit, that is represented by the letter ‘u.’

Example: Calculate the atomic mass of an oxygen isotope that has 9 neutrons.

Solution: To calculate the atomic mass of an oxygen isotope, we need to know the number of protons and neutrons it has.
Number of protons in an oxygen atom = 8
As per the question, the Number of neutrons in oxygen atom = 9
Hence, the Atomic mass of given oxygen atom = number of protons + number of neutrons = 8 + 9 = 17

Mole Concept

Mole concept can be defined as the atomic mass of a mole which is usually measured in grams. Ideally, the gram atomic mass of an element is otherwise referred to as a mole. 

Example: What is the formula of Mole Concept?

Solution: The number of units responsible to compose mole can be evaluated by 6.022 ×10²³. The fundamental constant is called the Avogadro's number (NA) or Avogadro constant. It can be represented using an explicit unit termed per mole. Thus, to calculate Mole Concept,
Number of Moles = (Mass of the Sample)/(Molar Mass)

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CBSE X Related Questions

1.
Draw the structure of a neuron and explain its function.

      2.
      Explain the following terms with one example each. 
      (a) Corrosion 
      (b) Rancidity

          3.
          Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 108 m s−1.

              4.
              Write the balanced chemical equations for the following reactions. 
              (a) Calcium hydroxide + Carbon dioxide \(→\) Calcium carbonate + Water 
              (b) Zinc + Silver nitrate \(→\) Zinc nitrate + Silver 
              (c) Aluminium + Copper chloride \(→\) Aluminium chloride + Copper 
              (d) Barium chloride + Potassium sulphate \(→\) Barium sulphate + Potassium chloride

                  5.

                  What are the differences between aerobic and anaerobic respiration? Name some organisms that use the anaerobic mode of respiration.

                      6.
                      Which of the statements about the reaction below are incorrect?
                      \(\text{ 2PbO(s) + C(s) → 2Pb(s) + C}O_2\text{(g)}\)
                      (a) Lead is getting reduced. 
                      (b) Carbon dioxide is getting oxidized. 
                      (c) Carbon is getting oxidized. 
                      (d) Lead oxide is getting reduced.

                        • (a) and (b)

                        • (a) and (c)

                        • (a), (b) and (c)
                        • all

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