Question 1. What are canal rays?
Canal rays are positively charged radiations which led to the discovery of positively charged sub-atomic particles called protons.
Question 2. If an atom contains one electron and one proton, will it carry any charge or not?
The atom will be electrically neutral as one – ve charge balances one + ve charge.
Question 3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
According to Thomson’s model of an atom
(i) An atom consists of a positively charged sphere and the electrons are embedded in it,
(ii) The negative and positive charges are equal in magnitude. So the atom is electrically neutral.
Question 4. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
s per Rutherford’s model of an atom, the protons which are positively charged are present in the nucleus of an atom.
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Question 5. Draw a sketch of Bohr’s model of an atom with three shells.
Question 6. What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
On using any metal foil, the observations of the a-particle scattering experiment would remain the same as all atoms would have the same structure.
Question 7. Name the three sub-atomic particles of an atom.
The sub-atomic particles of an atom are
Question 8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Question 9. Write the distribution of electrons in carbon and sodium atoms.
Question 10. If the K and L shells of an atom are full, then what would be the total number of electrons in the atom?
K shell can hold 2 electrons and L shell can hold 8 electrons. When both the shells are full, there will be (8 + 2) 10 electrons in the atom.
Question 11. How will you find the valency of chlorine, sulfur, and magnesium?
Question 12. If the K and L shells of an atom are full, then what would be the total number of electrons in the atom?
K shell can hold 2 electrons and L shell can hold 8 electrons. When both the shells are full, there will be (8 + 2) 10 electrons in the atom.
Question 13. How will you find the valency of chlorine, sulfur, and magnesium?
Question 14. If the number of electrons in an atom is 8 and the number of protons is also 8, then
(i) What is the atomic number of the atom? and
(ii) What is the charge on the atom?
Question 15. With the help of given Table 4.1, find out the mass number of oxygen and sulfur atom.
Table: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells
Question 16. For the symbols, H, D, and T tabulate three sub-atomic particles found in each of them
Question 17. Write the electronic configuration of any one pair of isotopes and isobar.
Isotopes: Atoms of the same element having the same atomic number but a different mass number.
Question 18. Compare the properties of electrons, protons, and neutrons.
Question 19. What are the limitations of J.J. Thomson’s model of the atom?
According to J.J. Thomson’s model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the center of the atom and electrons are distributed around it.
Question 20. What are the limitations of Rutherford’s model of the atom?
According to Rutherford’s model of an atom the electrons are revolving in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, the atom would be highly unstable. But we know that atoms are quite stable.
Question 21. Describe Bohr’s model of the atom.
Bohr’s model of the atom
(1) Atom has a nucleus in the center.
(2) Electrons revolve around the nucleus.
(3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(4) While revolving in discrete orbits the electrons do not radiate energy.
(5) These orbits or shells are called energy levels.
(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4
Question 23. Compare all the proposed Bohr’s models of an atom given in this chapter.
Answer:
Question 23. Summarise the rules for the writing of the distribution of electrons in various shells for the first eighteen elements.
The rules for writing of distribution of electrons in various shells for the first eighteen elements are:
(i) The maximum number of electrons present in a shell is given by the formula-2 n2
∵ n = orbit number i.e., 1, 2, 3
∵ Maximum number of electrons in different shells are:
K shell n = 1 2n2 => 2(1)2 = 2
L shell n = 2 2n2 => 2(2)2 = 8
M shell n = 3 2n2 => 2(3)2 = 18
N shell n = 4 2n2 => 2(4)2 = 32
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. (Shells are filled step-wise).
Question 24. Define valency by taking examples of silicon and oxygen.
Valency is the combining capacity of an atom.
Atomic number of oxygen = 8 Atomic number of silicon = 14 K L M
Electronic configuration of oxygen = 2 6 –
Electronic configuration of silicon =2 8 4
In the atoms of oxygen, the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.
Hence, the combining capacity of oxygen is 2 and silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4
Question 25. Explain with examples:
(i) Atomic number (ii) Mass number,
(iii) Isotopes and (iv) Isobars.
Give any two uses of isotopes
(i) Atomic number: The atomic number of an element is equal to the number of protons in the nucleus of its atom. e.g., Oxygen has 6 protons hence atomic no. = 6.
(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.
Nucleons = number of protons + number of neutrons Example: Protons + Neutrons = Nucleus = Mass number 6 + 6 = 12
(iii) Isotopes: Isotopes are atoms of the same element which have different mass numbers but the same atomic numbers.
(iv) Isobars: Isobars are atoms having the same mass number but different atomic numbers.
Both calcium and argon have the same mass number but a different atomic number.
Two uses of isotopes are:
(i) An isotope of iodine is used in the treatment of goiter.
(ii) An isotope of uranium is used as a fuel in nuclear reactors.
Question 26. Na+ has completely filled K and L shells. Explain.
Sodium atom (Na), has atomic number =11
Number of protons =11
Number of electrons = 11
Electronic configuration of Na = K L M – 2 8 1
Sodium atom (Na) loses 1 electron to become stable and form Na+ ion. Hence it has completely filled K and L shells.
Question 27. If a bromine atom is available in the form of say, two isotopes 7935Br (49.7%) and 8135Br (50.3%), calculate the average atomic mass of the bromine atom.
Question 28. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 168X and 188X in the sample?
Let the percentage of 168X be x and the percentage of 168X be 100 – x.
Question 29. If Z = 3, what would be the valency of the element? Also, name the element.
Z = 3, (i.e, atomic number —> z)
∴ Electronic configuration = 2, 1
Valency = 1
The name of the element is lithium.
Question 30. Composition of the nuclei of two atomic species X and Y are given as under
X – Y
Protons =6 6
Neutrons = 6 8
Give the mass number of X and Y. What is the relation between the two species?
Mass number of X = Protons + Neutrons
= 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
As the atomic number is same i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of the same element.
Question 31. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of the proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
(a) False (b) False
(c) True (d) False
Question 32. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus (c) Proton
(b)Electron (d)neutron
(a) Atomic nucleus
Question 33. Isotopes of an element have
(a) the same physical properties (c) different numbers of neutrons
(b) different numbers of neutrons (d) different atomic numbers.
(c) different number of neutrons
Question 34. Number of valence electrons in Ct ion are :
(a) 16
(b) 8
(c) 17
(d) 18
(b) 8
Question 35. Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
(d) 2, 8, 1
Question 36. Draw the atomic structure of hydrogen atom.
Question 37. Why are some elements chemically inert?
Because their outermost shell is completely filled.
Question 38. Why is an atom electrically neutral?
It has the same number of protons and electrons, (positive charge = negative charge).
Question 39. What is the charge and mass of a-particles?
Charge is + 2
Mass is 4 a.m.u.
Question 40. What are valence electrons?
Electrons present in the outermost shell of an atom are called valence electrons.
Question 41. An atom has atomic number 12, what is its valency and name the element?
Atomic number = 12
∴ Protons = Electrons = 12 Electrons Configuration = K L M -2 8 2
∴ Valency = 2
Element is magnesium.
Question 42. Find the number of neutrons in 2713X.
Mass number = 27
∴ p + n = 27 p = 13, (Atomic No. = Number of protons)
∴ 13 + n = 27
∴ n = 14
∴ Neutron =14
Question 43. Where is the mass of taiji concentrated?
Mass of an atom is concentrated in the nucleus.
Question 44. Name two elements with the same number of protons and neutrons?
Carbon (Protons = Neutrons = 6)
Oxygen (Protons = Neutrons = 8)
Question 45. Draw the atomic structure of the sodium atom.
Question 45. Name the isotope used for the treatment of cancer.
Isotope of cobalt.
Question 46. AZX What does this symbol represent?
X —> Symbol of element
A —> Mass number
Z —> Atomic number
Question 47. Can the value of ‘Z’ be the same for two different atoms?
No, (Z = atomic number), two different atoms cannot have the same atomic number.
Question 48. Can the value of A’ be the same for two different atoms?
Yes, it can be e.g. Ca and Ar has A-40 (i.e., mass number).
Question 49. Name the scientist who discovered protons and neutrons in an atom.
Protons were discovered by E. Goldstein in 1866 and neutrons were discovered by J, Chadwick in 1932.
Question 50. What is the contribution of Bohr and Bury together in the structure of the atom’s explanation?
Both Bohr and Bury gave the distribution of electrons into different atoms by giving the formula 2n2, where n = shell number.