Structure of Atom
Class: 9 | Chapter: 4
Extra Questions & Answer
1. What is the maximum number of electrons which can be accommodated in ‘M’ shell ?Answer: The maximum number of electrons that can be accommodated in 'M' shell is 2 + 8 + 8 = 18 .
2. Why atom is nuetral in nature ?
Answer: Atom has equal number of Protons (positive charge) and Electrons ( negative charge) , these equal and opposite charges neutralise each other. Therefore, an atom as a whole is nuetral in nature.
3. Write the correct representation of an element ‘X’ which contains 3 electrons and 4 neutrons.
Answer: Here X is lithium and representation is 3Li7.
4. What will be the valency of an atom if it contains 15 protons and 16 neutrons ?
Answer: This atom has 15 protons So, it has same electrons i.e. 15 and it's electronic configuration will be 2,8,5 . Hence, valency will be 8 - 5 = 3.
5. One electron is present in the outermost shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outermost shell ?
Answer: The elements which have 1 or 2 or 3 electrons in outermost shell, generally they are of metallic nature and if last electron is removed from the outermost shell then it will aquire one positive charge.
6. Which isotope of hydrogen is present in heavy water ?
Answer: Deuterium is present in heavy water.
7. Chemical formula of a metal sulphate is MSO4. What will be the formula of its oxide ?
Answer: Chemical formula of a metal sulphate is MSO4, it is indicating that Metal M has valency 2 and when it will form it's oxide, then formula will be MO becouse oxide ion also has valency 2.
8. Will 35Cl and 37Cl have different valencies ? Justify your answer.
Answer: No, they have same valencies becouse both are isotopes of Chlorine and isotopes have same atomic number ( same no. of electrons). Therefore, their electronic configuration is 2,8,7 and valency is 1.
9. Write the electronic configuration of a positively charged Magnesium ion (Mg2+). Atomic number of magnesium is 12.
Answer: Here Magnesium has lost 2 electrons from last shell So, electronic configuration is 2,8.
10. Why do inert gases have a zero valency ?
Answer: Valency is combining capacity of an element. Inert gases have fully filled electronic configuration therefore they don't have tendency to combine with other elements. Hence they are called to have zero valency or zero combining capacity.
11. Why do isotopes show similar chemical properties ?
Answer: Isotopes have same atomic no.( No. Of protons / No. of electrons) And different mass number. Chemical properties of an element depends upon electronic configuration and that is same for isotopes. So, they show similar chemical properties.
12. An atom of an element has electronic configuration 2,8,6.
(I) what will be it's atomic number ?
(II) write valency.
Answer: (i) 16
(ii) 8 - 6 = 2
13. What is an octate? Why would atoms want to complete their octate ?
Answer: Octate indicates the presence of 8 electrons in outermost shell of an atom and this configuration provides the stability. Therefore, atoms want to complete their octate Or want to have this configuration to gain stability.
14. Why did Rutherford select a gold foil for his experiment ?
Answer: Rutherford needed a very thin sheet for his experiment and gold is most malleable metal so, it was easy to get thin shaped foil by using gold.
15. Which subatomic particle determines the mass of an atom ?
Answer: Protons and nuetrons determine the mass of an atom.
16. Which subatomic particle is absent in an ordinary hydrogen atom ?
Answer: Nuetron.
17. Electron attributes negative charge, protons attribute positive charge. An atom has both but why there is no charge ?
Answer: There is no net charge becouse charges are in equal amount so, they nuetralise each other.
18. Write the electronic configuration of an element whose atomic number is 12.
Answer: K L M
2 8 2
19. What do you understand by ground state of an atom ?
Answer: Ground state of an atom indicates the lowest energy state . Excited states can also be achieved by absorbing or emition of energy.
20. Give two important applications of radioactive isotopes.
Answer: 1. Co - 60 is used in cancer treatment.
2. Fe - 59 is used in anemia diagnosis.
21. Which isotope of hydrogen is radioactive ?
Answer: Tretium
22. An element ‘A’ has valency +3, while another element ‘B’ has valency -2. Give the formula of their compound formed when ‘A’ reacts with ‘B’.
Answer: By cross multiplication of valencies, the formula of the compound is A2B3.
23. The electronic configuration of phosphorus atom is 2, 8, 5. Give the electronic configuration of P3- ion.
Answer: By gaining 3 electrons in last shell, the electronic configuration will be 2,8,8.
24. Where is the mass of atom concentrated ?
Answer: The whole mass of atom is concentrared at the centre of atom, which is known as nucleus.
25. In Rutherford’s model of an atom, fast moving alpha (a)-particles were made to fall on a thin gold foil. State two properties of a-particles.
Answer: 1. Alpha particles are heavy and positively charged particles.
2. They are energetic particles.
26. What will be the charge on an atom with mass number one and atomic number one ?
Answer: No charge becouse it is having one proton and one electron.
27. What characteristic feature is seen in the configurations of chemically inactive elements ?
Answer: Chemically inactive elements have completely filled electronic configuration. They are very stable and don't have any tendency to react with other elements.
28. Atomic number is defined in terms of protons and not in terms of electrons. Why ?
Answer: Electrons are a part of chemical reactions. During chemical reactions, electrons can be gained, lost or exchanged So, their number can be changed during reactions but protons don't participate. Therefore, atomic number is defined in terms of protons.
29. Why are anode rays also called canal rays ?
Answer: Anode rays are also called canal rays becouse these are passed through hole or canal
of cathode.
30. What would you conclude from the observation that cathode rays rotate a light paddle wheel placed in their path ?
Answer: This observation concludes that cathode rays have particles nature and have mass and velocity.
31. Find valencies of the elements having atomic numbers 10 and 15.
Answer: The element having atomic number 10 would have electronic configuration 2,8.
The element having atomic number 15 would have electronic configuration 2,8,5.
32. The atom of an element has 9 protons, 9 electrons and 10 neutrons.
(a) What is the atomic number of the element ?
(b) What is the mass number of the element ?
(c) Name the element and give its electronic configuration.
(d) Predict the valency of the element.
Answer: (a) Atomic number = No. Of protons / No. Of electrons
Therefore, atomic number = 9
(b) Mass number = No. Of protons + No. Of nuetrons
Mass number = 9 + 10 = 19
(c) Element is Fluorine and it's electronic configuration is 2,7.
(d) valency = 8 - 7 = 1
33. Give difference between isotopes and isobars.
Answer: Isotopes are the atoms of an element which have same atomic number but different mass number means they have same no. of protons and different no. of nuetrons.
Isobars are the atoms of different elements which have same mass number but different atomic number means they have same no. of nucleons.
34. According to you, among the structure of atom studies which model is correct and why ?
Answer: Bohr's model is correct becouse it tells about position of sub - atomic particles in atom and fixed energy of shells.
35. An atom has atomic number 12, what is its valency and name the element ?
Answer: This atom has electronic configuration 2,8,2 . So, it's valency will be 2 and it is Magnesium.
36. Why did Rutherford select a gold foil in his a-ray scattering experiment ?
Answer: Rutherford needed a very thin sheet for his experiment and gold is most malleable metal so, it was easy to get thin shaped foil by using gold.
37. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain.
Answer: Helium atom has electrons in only first shell and this shell can have 2 electrons maximum so, it can't gain or loose any electron. Therefore it's valency is zero.
38. Why are Bohr’s orbits called stationary states ?
Answer: Bohr's orbits are called stationary states becouse they have fixed energy value.
39. The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it ?
Answer: No. of protons will be equal to no. of electrons . So, the no. of protons will be 7.
Mass Number is the sum of no. of protons and nuetrons. Here mass number is 18 . So, no. of nuetrons will be 18 - 7 = 11.
40. If K and L shells of an atom are completely filled what will be its name ?
Answer: K shell can contain maximum of 2 electrons and L shell can contain 8 electrons. Therefore in total, that atom can have 2 + 8 = 10 electrons.