CBSE - IX SCIENCE CHEMISTRY (Structure of atom) Time Allowed: 3 Hrs. Maximum Marks: 54 INSTRUCTIONS

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1 CBSE - IX SCIENCE CHEMISTRY (Structure of atom) Time Allowed: 3 Hrs. Maximum Marks: 54 General: INSTRUCTIONS (i) (ii) (iii) (iv) (v) (vi) (vii) The question paper comprises of four sections, A, B, C and D, You are to attempt all the sections All questions are compulsory. All questions of sections A, B, C and D are to be attempted separately. Question number 1 to 10 in section A are one marks. Question numbers 11 to 16 in section B are two marks. Question numbers 17 to 20 in section C are three marks. Question number 21 to 24 in section D is five marks. SECTION A 1. What is the relation between mass number and atomic number? Ans. Mass number = Atomic number + No. of neutrons 2. Which shell determines the nature of an element? Ans. Valence shell electrons determine the nature of an element 3. Write the electronic configuration of chlorine. Ans. Atomic number of chlorine is 17. K L M 2 8 7

2 4. What is the general name of the elements having 2 or 8 electrons in the valence shell of their atoms? Ans. Noble gases or inert gases (due to completely filled outermost shell, they are inactive and are very stable). 5. Which elements have tendency to gain electrons? Ans. Elements which consist of 4 or more electrons in the valence shell, have the tendency to gain electrons. 6. Define valence electrons? Ans. The electrons present in the outermost shell of an atom are known as the valence electrons. 7. Name the scientists who discovered (1/2 +1/2 M) (a) Nucleus of atom (b) Proton Ans. (a) Rutherford (1/2 M) (b) E. Goldstein (1/2 M) 8. Name the particles which are present in the nucleus and what type of charge is there on them? Ans. Protons and neutrons are present in the nucleus. Protons are positively charged particles and neutrons have no charge on them. 9. Give the maximum number of electron that can be accommodated in the outermost orbit. Ans. The maximum number of electrons that can be accommodated in the outermost orbit is Define atomic number. Ans. The atomic number is defined as the total number of protons present in the nucleus of an atom.

3 SECTION B 11. What are isotopes? Give example? (1+1 M) Ans. If we have a set of atoms of an element which are having same atomic number but different mass numbers then the atoms are said to be isotope of each other. Example of isotopes: Isotopes of Hydrogen are, Protium (1H 1 ), Deuterium (1H 2 ) and Tritium (1H 3 ) 12. What were the characteristics of the electron that were defined by J.J Thomson? (1+1 M) Ans. According to J.J Thomson electron has following characteristics:- 1). Electrons are present inside a positively charged sphere. 2). Atom as a whole is electrically neutral, since the number of electrons (- ve charge) in an atom is equal to number of protons (+ve charge). 13. Explain the two types of valency. (1+1 M) Ans. (i) Electrovalency: The number of electrons lost or gained by one atom of an element to achieve the inert gas electron configuration is called electrovalency. (ii) Covalency: The number of electrons shared by one atom of an element to achieve the nearest inert gas electron configuration is called covalency. 14. Draw a sketch of Bohr s model of an atom with four shells? (2 M) Ans. The sketch for Bohr s model of an atom with four shells is given as follows

4 15. Give the isotopes of carbon and oxygen. (1+1 M) Ans. Isotopes of carbon= C, 6C and 6C Isotopes of oxygen= O, 8Oand 8O 16 How many neutrons are present in 24 12Mg (2 M) Ans. Mass number=24 Atomic number =12 Thus,No.of protons=12 No. of neutrons=mass number-no. of protons =24-12 =12 SECTION C 17. An element X has 11 protons. What is the valency of the element? What will be the name of element? (3 M)

5 Ans. Element has 11 protons. So it will have 11 electrons Electronic configuration of X = 2, 8, 1 The valency of the element X = 1 (As it has one electron in its outermost shell) Since, Number of protons = Number of electrons = atomic number The atomic number of X = 11 Therefore, the name of X is sodium (Na) 18. Why do atoms combine? Ans. Atoms combine with one another to attain inert gas electron arrangement in order to become more stable. An atom can achieve inert gas electron arrangement in three ways-(1.5 M) (i) By losing one or more electrons to another atom. (1/2 M) (ii) By gaining one or more electrons from another atom. (1/2 M) (iii) By sharing one or more electrons with another atom. (1/2 M) 19. (a) Who was the first to present the model of the structure of an atom? (b) What he proposed in this model? (1+2 M) Ans. (a) J.J. Thomson was the first to present the model of the structure of an atom. (b)thomson proposed that: (i) An atom consists of a positively charged sphere and the electrons are embedded in it.

6 (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. 20. What are isobars? Give example. (3 M) Ans. Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. For example: calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40. The total number of nucleons is the same in the atoms of this pair of elements. (2 M) SECTION D 21. What was the concept of atom, which was proposed by Neil Bohr? (5 M) Ans. According to Neil Bohr: (i) An atom is made up of two particles- electrons and protons. Electrons have negative charge and protons have positive charge. An atom has equal number of electrons and protons, thus they are neutral. (ii)the protons are located in a small nucleus at the centre of an atom. Thus the nucleus is positively charged. (iii)the electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells represented either by letters K, L, M, N, O and P or 1, 2, 3, 4, 5 and 6. (iv)each energy level is associated with a fixed amount of energy. The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has the maximum energy. (v)the change in the energy level of an electron takes place only when it jumps from a lower energy level to a higher energy level or when it comes down from a higher energy level to a lower energy level.

7 22.(a) Who discovered presence of Neutrons in the nucleus of an atom? (b)an atom of phosphorous can be written as P, (i) What does the figure 31 and 15 indicates? (ii) What is the number of protons in an atom of phosphorous? (iii) What is the number of electrons in an atom of phosphorous? (iv) What is the number of neutrons in an atom of phosphorous? (1+4 M) Ans.(a) J. Chadwick discovered presence of neutrons in the nucleus of an atom. (b) (i) The figure 31 indicates the mass number of phosphorous and figure 15 indicates the atomic number of phosphorous (ii) The number of proton is equal to the atomic number, thus phosphorous has 15 (iii) (iv) protons. In an atom, the number of proton is equal to the number of electrons, thus phosphorous has 15 electrons. Mass number= number of protons + number of neutrons 31=15+ number of neutrons Number of neutrons = 31-15= (a)what observations were noticed by Rutherford during alpha particle scattering experiment? (b) What information about the nucleus was provided by the alpha particle scattering experiment of Rutherford? (3+2 M) Ans. (a) When fast moving alpha particles were allowed to strike a very thin gold foil in vacuum, Rutherford noticed that: (i) Most of the alpha particles passed straight through the gold foil without any deflection from their original path. (ii)few alpha particles were deflected through small angles and very few were deflected through large angles.

8 (iii)very few (about 1 in 20,000) alpha particles completely rebounded on hitting the gold foil and turned back on their path. (b) (i) Nucleus of an atom is positively charged. (1/2 M) (ii) Nucleus of an atom is very dense and hard. (1/2 M) (iii) Nucleus of an atom is very small as compared to the size of the atom as a whole. 24. State the applications of radioactive isotopes. (5 M) Ans. Radioactive isotopes are widely used in power plants, medicines and industries. (i) Radioactive isotope such as Uranium-235 is used as a fuel in nuclear reactors of nuclear power plants for generating electricity. (ii) Radioactive isotopes are used as tracers in medicines to detect the presence of tumors, blood clots, etc, in a human body. (iii) Iodine-131 radioisotope is used to determine the activity of thyroid gland, which helps in the treatment of diseases like goiter. (iv)cobalt-60 radioisotope is used in the treatment of cancer. (v) Radioactive isotopes are used in industry to detect leakage in underground oil pipelines, gas pipelines and water pipes.