Atoms and Molecules NCERT Questions



  1. Name the scientist who laid the foundation of chemical sciences. How?
  2. Define law of conservation of mass.
  3. Define law of constant proportion.
  4. Which organisation approves the names of elements all over the world? Write the symbol of gold.
  5. Write the symbols of tungsten and iron.
  6. Name the element which is used as the reference for atomic mass.
  7. ‘Atoms of most elements are not able to exist independently’. Name two atoms which exist as independent atoms.
  8. What is the number of electrons in Mg atom and Mg+2ion?
  9. Which of the following species is electrically neutral and why?
    Na+,  Cl, K+, Na, Ca+2
  10. Define atomicity.
  11. Write atomicity of the following:
    (i) Sulphur, (ii) Phosphorus
  12. What are polyatomic ions ? Give two examples.
  13. What is the atomicity of Argon ?
  14. Give one relevant reason why scientists choose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit.
  15. State the number of hydrogen atoms in 1 g of hydrogen.
  16. What is molar mass? What are its units?
  17. Define atomic mass unit.
  18. The relative atomic mass of oxygen atom is 16. Explain its meaning.
  19. Distinguish between molecular mass and molar mass.
  20. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass ?
  21. Which postulate of Dalton’s atomic theory can explain the law of definite proportions ?
  22. Why is it not possible to see an atom with naked eyes?
  23. What is meant by the term chemical formula?
  24. How many atoms are present in a (i) H2S molecule and (ii)PO43– ion ?


  1. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
  2. Give two drawbacks of Dalton’s atomic theory.
  3. Calculate molar mass of sulphuric acid.
  4. Give the electronic configuration of:
    (i) Al atom and its ion (ii) O atom and its ion
  5. Give one example each of (i) Monovalent cation (ii) Bivalent cation (iii) Monovalent anion (iv) Bivalent anion
  6. How would you differentiate between a molecule of element and a molecule of compound? Write one example of each type.
  7. What is meant by a chemical formula? Give examples.
  8. What is meant by a molecule ? Give examples.
  9. Define one mole, illustrate its relationship with Avogadro constant.
  10. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas ?
  11. Write down the formula of :
    (i) Sodium oxide (ii) Aluminium chloride (iii) Sodium sulphide (iv) Magnesium hydroxide
  12. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen ? Which law of chemical combination will govern your answer ?
  13. What are polyatomic ions ? Give examples.
  14. Give the names of the elements present in the following compounds:
    (a) Quicklime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate.
  15. What is the mass of:
    (a) 0.2 mole of oxygen atoms ?
    (b) 0.5 mole of water molecules ?


  1. Write the chemical symbols of two elements:
    (i) Which are formed from the first letter of the elements’ name ?
    (ii) Whose names have been taken from the names of the elements in Latin ?
    (iii) Which are formed from the first two letters of the elements’ name ?
  2. Write the correct symbols of the following elements which are written incorrectly.
    (i) FE (Iron) (ii) AL (Aluminium) (iii) CO (Cobalt) (iv) AG (silver) (v) NA (sodium) (vi) AR (Argon)
  3. State two examples in each case and write their chemical formulae:
    (a) Molecules having (same kind of atoms only)
    (b) Molecules having two different kinds of atoms.
    (c) Molecules having three different kinds of atoms.
  4. State what are ions. Write the formulae of two divalent cations and anions each.
  5. Classify the following compounds diatomic, triatomic and polyatomic molecules:
    HCl, H2, H2O, NH3, CH3OH, PCl5.
  6. Write the names of the following compounds:
    (a) NiS    (b) Mg (NO3)2   (c) Na2SO4   (d) Al (NO3)3  (e)  K3PO4  (f) Ca3N2
  7. Write the chemical names of the following compounds:
    (a) K2SO4  (b) Mg3(PO4)2  (c) NH4Cl (d) ZnS  (e) Na3N (f) AgBr
  8. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass,
    sodium carbonate + ethanoic acid  sodium ethanoate + carbon dioxide + water.
  9. Write down the names of compounds represented by the following formulae:
    (i) Al2(SO4)3    (ii) CaCl2   (iii) K2SO4    (iv) KNO3    (v) CaCO3   (vi) Ca (HCO3)2
  10. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65u, Na = 23u, K = 39 u, C =3 12 u, and O = 16 u.
  11. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
  12. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
  13. Write the chemical formulae of the following:
    (a) Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride
    (e) Calcium carbonate  (f) Ammonium hydroxide
  14. What is the mass of:
    (a) 1 mole of nitrogen atoms ?
    (b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27) ?
    (c) 10 moles of sodium sulphite (Na2SO3) ?
  15. Convert into mole. (a)12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide.
  16. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
  17. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u).
  18. When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of chemical combination will govern your answer ? State the law.
  19. State the law of conservation of mass. Is this law applicable to the chemical reactions? Elaborate your answer with the help of an example.
  20. A sample of vitamin C is known to contain 2.58 × 1024 oxygen atoms. How many moles of oxygen atoms are present in the sample ?
  21. Sidak took 5 moles of carbon atoms in a container and Sehaj also took 5 moles of sodium atoms in another container of the same weight, (a) Whose container is heavier? (b) Whose container has more number of atoms?
  22. Fill in the missing data in the table:
Species H2O CO2 Na atom MgCl2
No. of moles 2 0.5
No. of particles 3.011 ×1023
Mass 36 g 115g
  1. The visible universe is estimated to contain 1022 stars. How many moles of stars are present in the visible universe?
  2. The formula unit of mass of Na2O is 62 U. Calculate the atomic mass of sodium (Na).
    (Given atomic mass of oxygen = 16)
  3. What are ionic and molecular compounds ? Give examples.
  4. (a) Why does not atomic mass of an element represent the actual mass of its atom ?
    (b) “The atomic mass of an element is in fraction.” What does it mean ?
  5. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mote of Helium gas ?
  6. Why atoms form ions ?
  7. Write three points of difference between an atom and a molecule.
    What is the difference between an atom and a molecule ?
  8. Define formula unit mass. Calculate formula unit mass of NaCI (Atomic mass of Na = 23 u,
    Cl = 35.5 u)
  9. (a) Give an example in each of the following cases:
    (i) a divalent anion, (ii) a trivalent cation. (iii) a monovalent anion.
    (b) Calculate the mass of the following:
    (i) 2 moles of carbon dioxide, (ii) 6.022 × 1023 molecules of carbon dioxide.
  10. (a) Write the chemical formulae of the following:
    (i) Calcium oxide, (ii) Magnesium chloride, (iii) Aluminium hydroxide
    (b) Calculate the number of molecules of sulphur (•%) present in 128 g of solid sulphur.
    (Atomic mass S = 32 u)


  1. Give the postulates of Dalton’s atomic theory.
  2. (a)  Give one point of difference between an atom and an ion.
    (b)  Give one example each of a polyatomic cation and an anion.
    (c)  Identify the correct chemical name of FeSO3: Ferrous sulphate. Ferrous sulphide, Ferrous sulphite.
    (d)  Write the chemical formula for the chloride of magnesium.
  3. (a) Define: (i) Molecular mass, (ii) Avogadro constant.
    (b) Calculate the number of molecules in 50 g of CaCO3.
    (Atomic mass of Ca = 40 u, C = 12 u and 0 u)
    (c) If one mole of sodium atom weighs 23 g, what is the mass (in g) of one atom of sodium ?
  4. Calculate the molecular masses of H2 , O2, Cl2, CO2, CH4, C2H6, C2H4,NH3, CH3OH,H2S.
  5. Calculate the molar mass of the following substances:
    (a) Ethyne, C2H2
    (b) Sulphur molecule,S8
    (c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
    (d) Hydrochloric acid, HCI (e) Nitric acid, HNO3


  1. Calcium chloride when dissolved in water dissociates into its ions according to the following equation:
    CaCl2 (aq)  Ca2+(aq) + 2Cl (aq)

    Calculate the number of ions obtained from CaCl2 when 222 g of it is dissolved in water.

  2. The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.
  3. Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury is present in 225 g of pure

    HgS ? Molar mass of Hg and S are 200.6 g


    and 32 g



  4. The mass of one steel screw is 4.11 g. Find the mass of one mole of these steel screws. Compare this value with the mass of the earth (5.98 × 1024kg). Which one of the two is heavier and by how many times ?
  5. Compute the difference in masses of 10:* moles each of magnesium atoms and magnesium ions.
    (Mass of an electron = 9.1 ×



  6. Which has more number of atoms ? (i) 100 g of W2 or (ii) 100 g of NH3
  7. Compute the number of ions present in 5.85 g of sodium chloride.
  8. A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold ?
  9. Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1 ×

     g). Which one is heavier ?

  10. A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1 % of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.
  11. A sample of ethane (C2H6) gas has the same mass as 1.5 × 1020 molecules of methane (CH4). How many C2H6 molecules does the sample of gas contain ?
    (a) Calculate the number of molecules of SO2 present in 44 g of it.
    (b) If one mole of oxygen atoms weighs 16 grams, find the mass of one atom of oxygen in (grams).
  12. Sodium is represented as
    (a) What is its atomic mass ?
    (b) Write its gram atomic mass.
    (c) How many atoms of Na will be there in 11.5 g of the sample ?
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