EDULABZ INTERNATIONAL 3 STRUCTURE OF ATOMS

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1 3 STRUCTURE OF ATOMS I. Multiple choice questions (Tick the correct option). 1. Which one of the following proposed the atomic theory of matter? (a) John Dalton (b) J.J. Thomson (c) Rutherford (d) Niels Bohr 2. Nucleons are (a) protons only (b) neutrons only (c) protons and neutrons (d) protons and electrons 3. The maximum number of electrons in the L shell are (a) 2 (b) 18 (c) 32 (d) 8 4. Atoms of the same element having different physical properties, but same chemical properties, are called (a) isobars (b) allotropes (c) isotopes (d) none of the above 5. The outermost shell of an atom cannot have more than (a) 2 electrons (b) 8 electrons (c) 18 electrons (d) 32 electrons 6. Alpha particles consist of (a) 2 protons and 3 neutrons (b) 2 protons and 1 neutron (c) 2 protons and 2 neutrons (d) 2 protons and 2 electrons 7. The nuclear fuel in stars is (a) helium (b) hydrogen (c) carbon (d) oxygen 8. The radioactive isotope used to cure thyroid cancer is (a) P 32 (b) Co 60 (c) Na 24 (d) I Number of the types of particles in an atom is (a) one (b) two (c) three (d) four 10. Which one of the following scientists put forward the theory regarding the structure of an atom? (a) James Chadwick (b) Rutherford (c) Niels Bohr (d) John Dalton 11. The maximum number of electrons that can be accommodated in any energy level of an atom is given by the formula (a) n 2 (b) 2n 2 (c) n (d) 2n 2 + x 12. In order to exist independently by itself an atom must have... electrons in its outermost shell or two electrons if it has only one shell. (a) six (b) twelve (c) eight (d) ten Chemistry Class VIII 1 Question Bank

2 13. Electrons present in the outermost shell determine the... of an atom. (a) valency (b) capacity (c) energy (d) place 14. An ionic bond is a bond between the ions with (a) opposite charges (b) similar charges (c) no charges (d) neutral ions 15. Sharing of electrons takes place in.... (a) an ionic bond (b) a covalent bond (c) both (a) and (b) (d) does not take place at all 16. An alpha particle is (a) an ionised helium atom (b) an ionised hydrogen atom (c) an isotope of hydrogen (d) an isotope of carbon Ans. 1. (a) 2. (c) 3. (d) 4. (c) 5. (b) 6. (c) 7. (b) 8. (d) 9. (c) 10. (b) 11. (b) 12. (c) 13. (a) 14. (a) 15. (b) 16. (a). II. Match the statements/words in Column A with those in Column B. Column A Column B 1. Proton (a) number of protons in the nucleus 2. Atom (b) negatively charged 3. Electron (c) carries no charge 4. Atomic number (d) positively charged 5. Neutron (e) electrically neutral 6. Valence electrons (f) electricity 7. Argon (g) force of attraction that holds atoms together in a molecule 8. Chemical bond (h) hydrogen 9. Fusion (i) determine the chemical properties 10. Fission (j) noble gas Ans. 1. (d) 2. (e) 3. (b) 4. (a) 5. (c) 6. (i) 7. (j) 8. (g) 9. (h) 10. (f) III. A. Fill in the blanks with appropriate words: 1. Dalton said that... could not be divided are emitted by radioactive elements. 3. An ion which has a positive charge is called a The outermost shell of an atom is known as The... of an atom is very hard and dense. Chemistry Class VIII 2 Question Bank 2

3 6. Neutrons are... particles having mass equal to that of protons. 7. Atomic number is the number of... present in the nucleus. 8. Ionic compounds are... of electricity. 9. MgCl 2 is an example of... compound. 10. The number of electrons donated or accepted by an atom so as to have the stable electronic configuration of the nearest noble gas is called A pair of electrons is shared in a/an The atom providing a pair of electrons is called the... atom. Ans. 1. atoms 2. High energy radiations 3. proton 4. valence shell 5. nucleus 6. neutral 7. protons 8. conductors 9. an ionic 10. valency 11. covalent bond 12. donor B. Fill in the blanks with appropriate words from the given list. [List: Chemical reaction, hydrogen, two, fusion, nuclear, anu, J.J. Thomson, electrically neutral]. 1. According to Maharishi Kanad, the tiniest of the tiny particle of a pure substance, is called An atom is the smallest unit of an element which takes part in a proposed the apple pie pudding model of an atom. 4. Mass of an electron is 1/1837 times less than the mass of one atom of Neutrons are... particles having mass equal to one atom of hydrogen. 6. During nuclear... the hydrogen nuclei fuse to form helium nuclei. 7. The attractive forces between the nucleons in the nucleus of an atom are called... forces. 8. The K-shell of any atom cannot have more than... electrons. Ans. 1. anu 2. chemical reaction 3. J.J. Thomson 4. hydrogen 5. electrically neutral 6. fusion 7. nuclear 8. two C. Choose the correct words from the brackets. 1. Valency is the number of (electrons/protons) donated or accepted by an atom of an element so as to have an electronic configuration of the nearest noble gas. Chemistry Class VIII 3 Question Bank

4 Ans. IV. 2. The nuclear reaction in which two lighter nuclei are fused together to form a heavier nucleus is called nuclear (fission/fusion). 3. If an element X has six electrons in its outer or valence shell, its valency is (2 + /2 /1 ). 4. An application of nuclear fission is the (atomic bomb/hydrogen bomb). 5. The element (magnesium/silver) exhibits variable valency. 1. electrons 2. fusion atomic bomb 5. silver A. State whether the following statements are true or false. If false, write the correct statement. 1. According to the modern atomic theory, atoms of the same element are alike in all respects. Ans. False: According to the modern atomic theory, atoms of the same element may not be alike in all respects as seen in the case of isotopes. 2. J.J. Thomson saw that the cathode rays were deflected by a negatively charged plate in the cathode tube. Ans. False: J.J. Thomson saw that the cathode rays were deflected by a positively charged plate in the cathode tube. 3. An atom consists of a central nucleus containing protons and neutrons and the electrons revolve around the nucleus in imaginary paths called shells. Ans. True. 4. Atomic number is equal to the number of protons and also equal to the number of neutrons. Ans. False: Atomic number is equal to the number of protons and also equal to the number of electrons in a neutral atom. 5. An element X has atomic number 19. Its electronic configuration is 2,8,9. Ans. False: An element X has atomic number 19. Its electronic configuration is 2, 8, 8, The core of an atom is called the nucleus. Ans. True 7. An atom always acquires stable configuration of the nearest noble gas by losing electrons from the valence shell. Ans. False: An atom always acquires stable electronic configuration of the nearest noble gas by losing electrons from or gaining electrons in their valence shell. Chemistry Class VIII 4 Question Bank 4

5 8. Helium is the only gas which has 2 electrons in its valence shell. Ans. True C denotes that the element carbon has mass number 6 and atomic number 12. Ans. False: 12 C 6 denotes that the element carbon has mass number 12 and atomic number Non-metals form cations by accepting electrons in their valence shell. Ans. False: Non-metals form anions by accepting electrons in their valence shell. 11. During nuclear fission, only slow neutrons can break uranium atom. Ans. True 12. Nuclear radiations under medical supervision can cure cancer. Ans. True 13. The α-particles are ejected from the nucleus of a radioactive element. Ans. True 14. γ-radiations are given out by the collapse of electrons of an atom. Ans. False: γ-radiations are given out as radiations from the nucleus of radioactive materials. 15. Chadwick discovered the electron. Ans. False: Chadwick discovered the neutron. 16. A proton is 1/1837 times heavier than an electron. Ans. False: An electron is 1/1837 times heavier than a proton. 17. During nuclear fission nuclei of lighter elements fuse to form heavy nuclei. Ans. False: During nuclear fusion, nuclei of lighter elements fuse to form heavy nuclei. 18. Sodium is a radioactive metal. Ans. False: Sodium is not a radioactive element. 19. An atom on the whole has a positive charge. Ans. False: An atom on the whole is electrically neutral. 20. The maximum number of electrons in the first shell can be 8. Ans. False: The maximum number of electrons in the first shell can be Deuterium is an isotope of hydrogen. Ans. True 22. α-particles are nuclei of hydrogen. Ans. False: α-particles are nuclei of helium atoms. Chemistry Class VIII 5 Question Bank

6 23. Cations are negatively charged particles. Ans. False : Cations are positively charged particles. 24. Metals always form negatively charged ions. Ans. False: Metals always form positively charged ions. 25. L-shell of an atom can have a maximum of 18 electrons. Ans. False: L-shell of an atom can have a maximum of 8 electrons. 26. γ-radiation have the least penetrating power among the three kinds of radiation. Ans. False: γ-radiations have the highest penetrating power among the three kinds of radiations. 27. During nuclear fusion, nuclei of lighter elements fuse to form heavy nuclei. Ans. True 28. K-shell of any atom can accommodate a maximum of eight electrons. Ans. False: K-shell of any atom can accommodate a maximum of 2 electrons. V. Name the following. 1. The sub-atomic particle with negative charge and negligible mass. 2. Protons and neutrons present in the nucleus. 3. The electrons present in the outermost shell. 4. A phenomenon due to which heavy nucleus splits into lighter nuclei. 5. Arrangement of electrons in the shells of an atom. 6. The binding force between atoms in a molecule of a compound. 7. The number of protons present in the nucleus of an atom. 8. The sum of the number of protons and neutrons of an atom. 9. Atoms of the same element with same atomic number but different mass number. 10. The smallest unit of an element which takes part in a chemical reaction. 11. The philosopher of India who proposed anu. 12. The scientist who discovered the electron. 13. The scientist who discovered the proton. 14. The scientist who discovered the neutron. 15. The scientist whose contribution lead to the discovery of the atomic nucleus. 16. The sub-atomic particle having no charge. 17. The shell of an atom which can accommodate a maximum of two electrons. Chemistry Class VIII 6 Question Bank 6

7 18. The type of element [i.e. metal, non-metal or noble gas] to which an element X having electronic configuration 2,8,8 belongs. 19. A radioactive element other than uranium and radium. 20. A slow moving particle used for splitting nucleus of an atom of uranium. 21. A charged particle formed by the donation of electrons from the valence shell. 22. A phenomenon due to which hydrogen atoms fuse in the sun. 23. A charged particle having +2 electric charge and 4 amu mass. 24. A highly penetrating radiation given by radioactive elements. 25. The outermost shell of an element. 26. The phenomenon due to which an atom spontaneously gives radiations. Ans. 1. Electron 2. Nucleons 3. Valence electrons 4. Fission 5. Electronic configuration 6. Chemical bond 7. Atomic number 8. Atomic mass 9. Isotopes 10. Atom 11. Maharishi Kanad 12. J.J. Thomson. 13. E. Goldstein 14. James Chadwick 15. Rutherford 16. Neutron 17. K-shell 18. Noble gas 19. Thorium 20. Neutron 21. Cations 22. Nuclear fusion 23. Alpha particle 24. Gamma radiation 25. Valence shell 26. Radioactivity. VI. Give reasons for the following. 1. As per the modern atomic theory, atoms are divisible. Ans. Atoms consist of the sub-atomic particles, like neutrons, protons and electrons. These have been isolated and studied. 2. J.J. Thomson s experiment further led to the discovery of protons. Ans. J.J. Thomson found out that the cathode rays consist of negatively charged particles in an atom the electrons. Since an atom is electrically neutral, scientists were of the opinion that there must be some oppositely charged particles present in the atom. This led to the discovery of protons. 3. The maximum number of electrons in the L-shell is 8. Ans. The maximum number of electrons that can be present in any shell of an atom is given by the formula 2n 2, where n denotes the number of the shell. Therefore, the second shell or L shell can accomodate a maximum of = 8 electrons. 4. Electronic configuration of calcium whose atomic number is 20 is 2,8,8,2 and not 2,8,10. Chemistry Class VIII 7 Question Bank

8 Ans. The Octet rule states that the maximum number of electrons that the outermost shell of an electrically neutral and chemically stable atom can have is 8, except the atom which has only one shell and can have only 2 electrons (duplet). Therefore, Ca (at. no. 20) will have the electronic configuration 2,8,8,2. 5. Argon atomic number = 18, is considered a noble gas. Ans. All inert gases, except helium, have eight electrons in their valence shells. Therefore, argon (at. no. 18; electronic configuration 2,8,8) has 8 electrons in its valence shell, which is thus complete. Hence it is considered a noble gas. 6. Metals lose electrons and form positive ions while non-metals gain electrons and form negative ions. Ans. Metals have 1, 2 or 3 electrons in their valence shells. Thus, they can lose these electrons to form positive ions. Non-metals have 5, 6 or 7 electrons in their valence shell. They gain 3, 2 or 1 electron respectively to become negative ions. 7. Isotopes of elements have same number of protons and electrons but different number of neutrons. Ans. Isotopes are atoms of the same element having same atomic number but different mass number. Thus, isotopes have the same number of protons and electrons but different number of neutrons. 8. Gamma radiations are used in medical applications to destroy cancerous tumors. Ans. Gamma radiations are highly penetrating radiations and in general affect abnormal cells more than healthy cells. Hence, they are used in controlled measures by doctors to destroy abnormal cells in cancerous tumors. 9. Uranium whose atomic number is 92 and mass number 235 is considered radioactive, but sodium whose atomic number is 11 and mass number 23 is not considered radioactive. Ans. Uranium contains a heavy atomic nucleus. The nucleus of uranium is unstable since the electrostatic repulsion between the positively charged protons and the attractive force between the neutrons and protons are not balanced. Thus, uranium is radioactive while sodium whose nucleus is stable, is not radioactive. 10. Nuclear reactors are used as power plants. Ans. Nuclear energy obtained during nuclear fission is converted into Chemistry Class VIII 8 Question Bank 8

9 mechanical and electrical energy by a turbine and a generator respectively. Since nuclear fission takes place in nuclear reactors, they are used as power plants. 11. Addition of a neutron to the nucleus of an atom would increase its mass number. Ans. Mass number of an atom is equal to the sum of the number of protons and neutrons in the atom of an element. Therefore, addition of a neutron to the nucleus of an atom increases its mass number. 12. The atomic number of an element accounts for the number of positive charges in the nucleus. Ans. The atomic number of an element is equal to the number of protons present in the atom of an element. Since protons are the only positively charged particles present in the nucleus of an atom, therefore atomic number accounts for the number of positive charges in the nucleus. 13. Covalent compounds have low melting and boiling points. Ans. Covalent compounds are formed by the sharing of electrons between the atoms of elements. These compounds are usually neutral and their intermolecular forces are weak. It is easy to break these forces and therefore, these compounds have low melting and boiling points. 14. An ionic bond is formed between sodium and chlorine. Ans. Electronic configuration of sodium Na is 2,8,1 while that of chlorine C1 is 2,8,7. Therefore, sodium can donate its valence electron to chlorine to form sodium chloride. Thus, an ionic bond is formed between an atom of sodium and chlorine. 15. A nuclear reactor is surrounded with thick steel and concrete. Ans. To prevent the leakage of radioactive radiations, thick steel and concrete walls are built around nuclear reactors as these radiations are very harmful. 16. There is no charge on the atom of an element, although every atom contains negatively charged particles. Ans. An atom of an element has equal number of negatively and positively charged particles. Thus, it is electrically neutral and there is no charge on the atom. 17. Nuclear fusion cannot be used to generate electricity. Ans. Nuclear fusion takes place at extremely high temperature and Chemistry Class VIII 9 Question Bank

10 pressure. There is no way to produce such a high temperature or obtain materials that can withstand such high temperatures. So, nuclear fusion cannot be used to generate electricity. VII. Short answer-type questions. 1. Choose from the table letters (A to E) that represent (a) a pair of isotopes. (b) an element with mass number 19. (c) an element having atomic number 7. Element Neutron Proton Electron A B C D E Ans. (a) B and E. Atoms of elements having the same atomic number (no. of protons) but different mass number or different number of neutrons are isotopes. (b) C: Mass number = no. of protons + no. of neutrons. (c) D: Atomic number = no. of protons. 2. State whether the following elements having the given electronic configuration are metals, non-metals or noble gases. (a) Electronic configuration 2, 8, 8, 1. (b) Electronic configuration 2, 8, 2. (c) Electronic configuration 2, 1. (d) Electronic configuration 2, 8, 5. (e) Electronic configuration 2, 8. Ans. (a) Metal (b) Metal (c) Metal } Atoms of elements having 1, 2 or 3 electrons in their valence shell are metals. (d) Non-metal: Atoms of elements having 4,5,6 or 7 electrons in their valence shell are non-metals. (e) Noble gas: Atoms of elements having an octet (of electrons in the valence shell) are noble gases. 3. State the following (a) The valency of an element X whose electronic configuration is 2, 8, 7. Chemistry Class VIII 10 Question Bank 10

11 (b) The number of electrons in the L-shell of an atom whose atomic number is 14. (c) The number of neutrons in an element whose mass number is 19 and atomic number 9. Ans. (a) 1: Valency is the number of electrons gained or lost by an atom during a chemical combination (to attain a noble gas configuration). (b) 8: Atomic number = 14 Electronic configuration = 2 (K), 8 (L), 4 (M)... No. of electrons in the L shell is 8. (c) 10: Mass number = No. of protons + no. of neutrons Atomic number = No. of protons No. of neutrons = Mass number atomic number 4. State the mass number, atomic number, number of protons and electronic configuration of the following atoms: (a) P, (b) Q, (c) 14 7 R. Ans. (a) Mass no. 40[... in A Z X, element X has at. mass A, and at. no. Z] Atomic no. = 20 No. of protons = 20 [At. no. = no. of protons] Electronic configuration = 2, 8, 8, 2 (b) Mass no. = 27 At. no. = 13 No. of protons = 13 Electronic configuration = 2, 8, 3 (c) Mass no. = 14 At. no. = 7 No. of protons = 7 Electronic configuration = 2, 5 5. Show the geometric configuration of magnesium atom Mg and chlorine atom C1. Ans. Geometric configuration of magnesium atom. Mass number; A = 24; Atomic; number; Z = 12; Number of; protons = Z = 12; Number of; neutrons = A Z = 12; Number of; electrons = Z = 12; Electronic; configuration = 2, 8, 2. Chemistry Class VIII 11 Question Bank

12 Geometric configuration of Chlorine Mass number; A = 35; Atomic number; Z = 17; Number of protons = Z = 17; Number of neutrons = A Z = = 18; Number of electrons = Z = 17; Electronic configuration = 2, 8, An atom has one electron more than the noble gas configuration. What type of ion would it tend to form? Ans. An atom having one more electron than the noble gas configuration will tend to lose that electron to attain a noble gas configuration. Thus, it will form a cation with + 1 charge. VIII. Question Answers. 1. What are the main properties of cathode rays? Ans. (i) Cathode rays consists of negatively charged particles called electrons. (ii) They travel in straight lines from the cathode to the anode. (iii) They are deflected towards a positively charged plate. (iv) The cathode rays have kinetic energy and cast the shadow of an object in their path. 2. What are isotopes? Explain with the help of a suitable example. Ans. The atoms of an element having the same atomic number but different mass numbers are called isotopes. Chlorine has two isotopes: chlorine Cl and chlorine Cl Cl Cl p + = 17 p + = 17 e = 17 e = 17 n 0 = 18 n 0 = 20 Chemistry Class VIII 12 Question Bank 12

13 3. Compare and contrast nuclear fission and nuclear fusion. Ans. Nuclear fission Nuclear fusion (i) The process of (i) The process in which two splitting of the light nuclei combine at an nucleus of heavy extremely high temperature atoms is called and pressure to form a stable nuclear fission. nucleus, with the release of an enormous amount of energy, is called nuclear fusion. (ii) It is applied for (ii) It is used in the production making atom bombs of hydrogen bomb which and in nuclear reac- produces 1000 times the tors for the produc- energy produced by an atom tion of electricity. bomb. 4. What are the harmful effects of radioactivity and safety requirements against radioactive radiations? Ans. The harmful effects of radioactivity are: (i) harmful gene mutation in living organisms. (ii) skin and other types of cancer. (iii) large-scale destruction when uncontrolled. (iv) birth of deformed babies. The following safety requirements should be taken against radioactive radiations. (i) There should be a special concrete wall or a thick shield of lead around nuclear reactors so as to prevent leakage of radioactive radiations. (ii) Workers in nuclear power plants should wear lead lined aprons and gloves and special lead glasses to protect their bodies and eyes from radiation. (iii) The nuclear materials should be kept in either aluminium or stainless steel cans and away from each other to avoid any explosion. 5. List the properties of α-rays, β-rays and γ-rays. Ans. Properties of α-rays, β-rays and γ-rays. Chemistry Class VIII 13 Question Bank

14 α-rays β-rays γ-rays Nature Positively charged Negatively charged Neutral particles particles. particles. similar to X-rays. They are the nuclei They are electrons. They have no mass of helium atoms. or charge. Penetrating Small penetrating More penetrating Penetrating power power. power than α-rays times more than α-rays. Action on a Affect photographic Affect photographic Have little effect Photographic plate. plate. on photographic Plate plate. 6. (a) Define: (i) Valency (ii) Variable valency. (b) Name two elements having variable valency and state their valencies. Ans. (a) (i) Valency: The number of electrons donated or accepted by an atom of an element so as to achieve a stable electronic configuration of the nearest noble gas. (ii) Variable valency: When elements exhibit more than one valency due to loss of electrons from the valence shell as well as from the shell before the valence shell, it shows variable valency. (b) Copper and silver show variable valency. Cu 1+, Cu 2+ and Ag 1+, Ag Define: (i) Radioactivity (ii) Nuclear fusion (iii) Nuclear fission (iv) Mass number Ans. (i) Radioactivity: The phenomenon due to which certain elements emit highly penetrating radiation due to spontaneous disintegration of their nuclei is known as radioactivity. (ii) Nuclear fusion: A nuclear reaction in which two lighter nuclei are fused together to form a heavier nucleus is called nuclear fusion. (iii) Nuclear fission: A nuclear reaction in which a heavy atomic nucleus breaks up into two smaller nuclei, with the release of a very large amount of energy is called nuclear fission. (iv) Mass number: Mass number of an atom is the sum of the number of protons and number of neutrons in the atom of the element. Chemistry Class VIII 14 Question Bank 14

15 8. How does the modern atomic theory contradict Dalton s atomic theory? Ans. According to the modern atomic theory: (i) Atoms can be created or destroyed by nuclear fusion and fission reactions. (ii) Atoms are divisible and consists of protons, electrons and neutrons. (iii) Atoms of the same element may not be alike as seen in case of isotopes. 9. Explain the rule according to which electrons are filled in various energy levels. Ans. The Bohr-Bury scheme of electronic configuration explains the rule according to which electrons are filled in various energy levels. They are: (i) The maximum number of electrons that can be present in any shell or orbit of an atom is given by the formula 2n 2, where n is the serial number of the shell. (ii) The Octet rule determines the actual number of electrons present in a shell. It states that the maximum number of electrons that the outermost shell of an electrically neutral and chemically stable atom can have is 8, except if the atom has only one shell (as in case of hydrogen and helium), which can have only 2 electrons (duplet). 10. How do the three isotopes of hydrogen differ? Ans. The three isotopes of hydrogen are (i) Protium ( 1 H 1 ) or ordinary hydrogen: It has one proton and one electron. (ii) Deuterium ( 2 H 1 ) or heavy hydrogen: It has one proton, one neutron and one electron. (iii) Tritium ( 3 H 1 ) or very heavy hydrogen: It has one proton, two neutrons and one electron. Thus, the number of neutrons differ in the three isotopes of hydrogen. 11. Name the three fundamental particles of an atom. Give the symbol with charge on each particle. Chemistry Class VIII 15 Question Bank

16 Ans. The three fundamental particles of an atom are: (i) Proton: 1 p + The positive charge on the proton is equal to coulomb. (ii) Electron: 1 e 0 The negative charge on the proton is equal to coulombs. (iii) Neutron: 0 n 1 The neutron is neutral with no charge. 12. Differentiate between: (a) proton and electron (b) stable and unstable elements. Ans. (a) Proton Electron Mass g g Charge coulombs coulombs (positive) (negative) Symbol 1 p+ or 1 H 1 1 e0 or e (b) Stable element Unstable element (i) The electrostatic force The electrostatic force of of repulsion between repulsion between the the positively charged positively charged protons protons and the attrac- and the attractive force tive force between between the neutrons and neutrons and protons protons do not balance (nuclear force) balance each other in an unstable each other in a stable atom. element. (ii) Contain light atomic Contain heavy atomic nucleus. nucleus. **** Chemistry Class VIII 16 Question Bank 16