class 9th, science chapter 3, important questions

  1. What are isotopes? Give two examples.
  2. How is the valency of an atom related to its electronic configuration?
  3. Calculate the atomic number of the element whose atomic nucleus has mass number of 23 and neutron number 12.What is the symbol for this element?
  4. Nucleus of an atom has 5 protons and 6 neutrons.What would be the atomic number,the mass number,the number of electrons and the number of valency electrons per atom of this element?
  5. What valency will be shown by an element having atomic number 15?
  6. An element has atomic number 16 .How many electrons will be present in K,L and M energy shells of its atom? What will its electro-valency be?
  7. What are valence electrons? Write the number of Valency electrons present in the elements: carbon,sodium and phosphorus.
  8. Draw the electronic structure of sodium and calcium with atomic number 11 and 20 respectively.
  9. Give four differences between cathode rays and canal rays.
  10. Write the main characteristic properties of cathode rays.
  11. Define electron, proton and neutrons.
  12. What is meant by atomic number of an element? Does the atomic number of an element change when its atoms from ions? Give one example each of diatomic and triatomic molecules.
  13. An element M has atomic mass 24 and atomic number 12. How many neutrons does its atom contain? How many electrons will be present in K,L and M energy shells of its atoms ?
  14. An element has atomic number 17. How many electrons will be present in the K,L and M energy shells of the atom?What will its electro-valency be?
  15. Elements A, B, C and D have atomic numbers 11,8,1 and 17 respectively.Give the chemical formulae of the compounds formed between (i) A and D (ii) B and C.
  16. Which fundamental particle is equal in number in Mg2+ and AL3+ ions ?An atom has electronic configuration of 2,8,7