Class 9 Science Chapter 3 Manipur Board: Atoms and Molecules

Class 9 Science Chapter 3 Manipur Board

Class 9 Science Chapter 3 Manipur Board

Class 9 Science Chapter 3 Manipur Board Notes and Q&A is a comprehensive study guide that covers all the topics covered in your science textbook as per Board of Secondary Education Manipur(BOSEM) Chapter 2: Pure Substances and Mixtures.

The Class 9 Science Chapter 3 Manipur board notes are prepared for you in an easy-to-understand with animations wherever applicable. The notes provided below will help you understand the core concept of the nature of matter.

Atoms

Atoms can be defined as the smallest particles of an element which can take part in a chemical reaction.

Atoms are building blocks of matter. Everything around us such as air, water, and solid objects is made up of atoms. Atoms cannot be seen with our naked eyes due to its size.

An atom consists of electrons, protons and neutrons. These particles are held together in the centre of the atom, called the nucleus.

The electron orbits around the nucleus. Atoms combine with other atoms to form molecules, which make up all the substances we encounter in our daily lives.

Laws of Chemical Combination

  1. Laws of conservation of mass
  2. Laws of constant proportion

Laws of conservation of mass (Antonie L. Lavoisier, 1774)

This law states that mass can neither be created nor destroyed in a chemical reaction.

Imagine you have a sealed container with some baking soda (sodium bicarbonate) and vinegar. When you mix them together, they react to produce carbon dioxide gas, water, and a salt called sodium acetate.

If you were to weigh the container before the reaction and then again after the reaction, you would find that the total mass of the container and its contents remains the same. Even though new substances (carbon dioxide, water, and sodium acetate) were formed during the reaction, the total mass before and after the reaction remains constant.

This is a clear demonstration of the law of conservation of mass.

Law of Constant Proportion (Proust,1799)

This law states that a pure compound always contains the same elements combined in the same proportions by mass e.g. hydrogen and oxygen combine together in the same proportion of 1:8 by mass.

Dalton’s Atomic Theory (1808)

Dalton’s Atomic theory states that

  • Matter is made up of indivisible particles known as atoms.
  • Atoms are neither created nor destroyed
  • Atoms of one element are all identical. They have the same mass and the same properties.
  • Atoms of different elements combine to form compounds, the atoms of these elements unite in simple whole-number ratios to form compound atoms.
  • The relative number and kinds of atoms are constant in a given compound.

Molecules

  • Molecules are the smallest particles of an element or a compound which can exist independently and show all the properties of that substance.
  • Molecules of an element are composed of the same type of atoms.
  • Molecules may be monoatomic, di-atomic, polyatomic.
  • Molecules of compounds join together in definite proportions and constitute different types of atoms.

Atomicity: Atomicity may be defined as the number of atoms present in one molecule of the element or compound.

Types of elementsElementAtomicity
 Non-metal Helium 1 (Mono-atomic)
Neon 1 (Mono-atomic)
Argon 1 (Mono-atomic)
Hydrogen 2 (Di-atomic)
Oxygen 2 (Di-atomic)
Nitrogen 2 (Di-atomic)
Chlorine 2 (Di-atomic)
Ozone 3 (Tri-atomic)
Phosphorus 4 (Tetra-atomic)
Metals  Iron 1 (Mono-atomic)
Copper 1 (Mono-atomic)
Silver 1 (Mono-atomic)
Table: Atomicity of some common elements

Atomic radii: Atomic radii is a size of an element i.e. distance from the nucleus to the outermost shell of an atom. It is measured in nanometres (nm)

1nm = 1/109 meter

ElementAtomic Radius
Hydrogen0.037 nm
Carbon0.077 nm
Oxygen0.073 nm
Sulphur0.104 nm
Nitrogen0.074 nm
Table: Atomic radii of some common elements.

Atomic Mass

It is the average relative mass of an atom of an element as compared to the mass of an atom of carbon (C-12 isotope) taken as 12(u)

Atomic mass = Mass of an atom of an element/ 1/2(half) of the mass of an atom of C-12

Atomic Mass Unit (amu)

The atomic mass unit has been defined as 1/12 (one-twelfth) the mass of carbon-12 atom taken as 1 u.

Nowadays, the atomic mass unit which was abbreviated as amu, is written as “u” (unified mass), according to the IUPAC(International Union of Pure and Applied Chemistry).

Thus, 1u = 1/12 the mass of carbon – 12

ElementAtomic mass(u)
Hydrogen1
Carbon12
Oxygen16
Nitrogen14
Chlorine35.5
Table: Atomic mass for some common elements

Gram Molecule Mass: The atomic mass of an element expressed in grams is known as gram atomic mass.

Example:

The atomic mass of oxygen = 16u

Therefore, the gram atomic mass of oxygen = 16g.

Class 9 Science Chapter 3 Manipur Board: Question and answers

  1. Name any two laws of chemical combination.

    Answer:
    The two laws of chemical combination are
    1. Law of conservation of mass
    2. Law of constant proportions.

  2. What law of chemical combination was given by (a) Lavoisier and (b) Proust?

    Answer:
    Lavoisier gave the law of conservation of mass.
    Proust gave the law of constant proportion.

  3. 10.6g of sodium carbonate reacted with 7.3g of hydrochloric acid producing 11.7g of sodium chloride, 1.8g of water and 4.4g of carbon dioxide gas. Show that this data verifies the law of conservation of mass.

    Answer:
    Mass of the reactant = Mass of sodium carbonate+Mass of hydrochloric acid
    =10.6g + 7.3g
    =17.9g
    Mass of the product = Mass of sodium chloride + Mass of water + Mass of carbon dioxide
    =11.7g + 1.8g + 4.4g
    =17.9g
    Here,
    the total mass of reactant = the total mass of the product.
    Hence, this data verified the law of conservation of mass.

  4. A pure sample of calcium oxide contains calcium and oxygen in a ratio of 5:2 by mass. How many grams of calcium will be required to combine with 32g of oxygen to form calcium oxide?

    Answer:
    2g of oxygen combines with 5g of calcium by mass.
    ∴ 1g of oxygen combined with 5/2g of calcium by mass.
    Hence, 32g of oxygen combines with 5/2 x 32g of calcium by mass.
    =80g of calcium by mass

  5. Which postulates of Dalton’s atomic theory can explain:
    a). Law of conservation of mass and
    b). Law of constant proportions?

    Answer:
    a). Atoms are neither created nor destroyed explains the Law of Conservation of mass.
    b). The postulates
    i) The relative number and kinds of atoms that are constant in a given compound can explain the law of constant proportions.
    ii) Atoms of one element are all identical. They have the same mass and the same properties.
    iii) Atoms of different elements combine in fixed ratios to form compounds.
    iv) When elements combine to form compounds, the atoms of these elements unite in simple whole-number ratios to form compound compound atoms.

  6. Define an atom.

    Answer:
    An atom can be defined as the smallest particle of an element which can take part in a chemical reaction.

  7. In what unit the radius of an atom is usually expressed?

    Answer:
    The radius of an atom is usually expressed in nanometres.

  8. What is a molecule? Explain with an example.

    Answer:
    A molecule is the smallest particle of an element or a compound which can exist independently and shows all the properties of that substance.
    Example:
    Au(Gold) molecule exists as a monoatomic and it possesses all the properties of the gold metal.

  9. What is the difference between the molecule of an element and the molecule of a compound?
    Illustrate with one example.

    Answer:
    Molecules of an element are composed of atoms of the same kind.
    Example: A molecule of Cl2 consists of two chlorine atoms.
    While a molecule of a compound consists of two or more different atoms.
    Example: Carbon dioxide (CO2) is a compound whose molecules are made up of one atom of carbon and two atoms of oxygen.

  10. What is meant by atomicity? Give example.

    Answer:
    Atomicity is the number of atoms present in one molecule of the element.
    Example: A molecule of oxygen consists of two atoms of oxygen and hence its atomicity is 2(diatomic).

  11. Define atomic mass. What is meant by saying that the atomic mass of calcium is 40?

    Answer:
    The atomic mass is defined as the average relative mass of an atom of an element as compared to the mass of an atom of carbon taken as 12u or amu.

    Atomic mass = Mass of one atom of an element/1/12 of the mass of an atom of C-12.

    It means that calcium has 40 times 1/12th the mass of the carbon-12 (C-12) atom in its one atom.

  12. What are the postulates of Dalton’s atomic theory?

    Answer:
    The postulates of Dalton’s atomic theory are as follows:

    1) Matter is made up of indivisible particles known as atoms.
    2) Atoms are neither created nor destroyed.
    3) Atoms of one element are all identical. They have the same mass and the same properties.
    4) Atoms of different elements combine in a fixed ratio to form compounds.
    5) When elements combine to form compounds, the atoms of these elements unite in simple whole-number ratios to form compound atoms.

  13. 3g of magnesium combines with 2g of oxygen to produce 5g of magnesium oxide compound. If 6g of magnesium is allowed to react with 20g of oxygen, what mass of magnesium oxide will be produced in the reaction? Name the law which governs your answer.

    Answer:
    Magnesium combines with oxygen in the ratio of 3:2 by mass to produce 5g of magnesium oxide. Therefore 6g of magnesium will react with only 4g of oxygen to give 10g of magnesium oxide(ratio 6:4). Here, the remaining (20 – 4) = 16g of oxygen will remain unreacted.

    It governs the law of constant proportion but it does not obey the law of conservation of mass.

  14. Distinguish between the atom and molecule of an element.

    Answer:
     An atom is the smallest particle of an element which can take part in a chemical reaction.

    A molecule is the smallest particle of an element or a compound which can exist independently and shows all the properties of that substance.

  15. Define atomic mass unit.

    Answer:
    The atomic mass unit is the average mass of an atom of an element as compared to 1/12 of the mass of an atom of C-12.

  16. Why are atoms regarded as building blocks of all matter?

    Answer:
    Atoms are regarded as the building blocks of all matter because all the matter is constituted by molecules and all the molecules are constituted by atoms.

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