In this post, question answer of structure of atom are provided which includes NCERT questions as well.
1. What are canal rays?
Answer: Canal rays are positively charged radiations which led to the discovery of positively charged sub-atomic particle called proton.
2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: The atom will be electrically neutral as one negative charge balances one positive charge.
3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson’s model of an atom, “an atom consists of a positively charged sphere and the electrons are embedded in it”. The negative and positive charges are equal in magnitude. So the atom is electrically neutral.
4. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: As per Rutherford’s model of an atom, the protons which are positively charged are present in the nucleus of an atom.
5. Draw a sketch of Bohr’s model of an atom with three shells.
Answer: The sketch of Bohr’s model of an atom is:
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?
Answer: On using any metal foil, the observations of the alpha-particle scattering experiment would remain the same as all atoms would have the same structure.
7. Name the three sub-atomic particles of an atom.
Answer: The sub-atomic particles of an atom are:
Positively charged particles: Protons
Negatively charged particles: Electrons
Neutral particles: Neutron
8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer: Atomic Mass of He = 4 u
Atomic number = Number of protons = 2
Atomic mass = Number of protons + Number of neutrons
4 = 2 + Number of neutrons
Number of neutrons = 4-2 = 2
So, helium has 2 neutrons.
9. Write the distribution of electrons in carbon and sodium atoms.
Answer: (a) Carbon:
Atomic number = 6
Number of protons = Number of electrons = 6
Distribution of electrons = 2, 4
(b) Sodium:
Atomic number = 11
Number of protons = Number of electrons = 11
Distribution of electrons = 2, 8, 1
10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: K shell can hold maximum 2 electrons and L shell can hold maximum 8 electrons. When both the shells are full, there will be (8 + 2) electrons, i.e., 10 electrons in the atom.
11. How will you find the valency of chlorine, sulphur and magnesium?
Answer: Valency is the combining capacity of an atom of an element.
(a) Chlorine: Atomic number = 17
Number of protons = Number of electrons = 17
Distribution of electrons = 2,8,7
Chlorine needs 1 electron to complete its outermost orbit. So, its valency is -1.
(b) Sulphur: Atomic Number = 16
Number of protons = Number of electrons = 16
Distribution of electrons = 2,8,6
Sulphur needs 2 electrons to complete its outermost orbit. So, its valency is -2.
(c) Magnesium: Atomic Number = 12
Number of protons = Number of electrons = 12
Distribution of electrons = 2,8,2
Magnesium loses 2 electrons to acquire noble gas electronic configuration. So its valency is +2
12. If number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the atom?
(ii) What is the charge on the atom?
Answer: Given that number of electrons = number of protons = 8
(i) Atomic Number = Number of protons = 8
(ii) Since the number of electrons are equal to the number of protons. So, the atom is electrically neutral.
13. With the help of given table, find out the mass number of oxygen and sulphur atom.
| Element | Symbol | Atomic Number | No. of proton | No. of neutron | No, of electron | Distribution of electron (K, L, M, N) | Valency |
|---|---|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 0 | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 2 | 2 | 2 | 0 |
| Lithium | Li | 3 | 3 | 4 | 3 | 2, 1 | 1 |
| Beryllium | Be | 4 | 4 | 5 | 4 | 2, 2 | 2 |
| Boron | B | 5 | 5 | 6 | 5 | 2, 3 | 3 |
| Carbon | C | 6 | 6 | 6 | 6 | 2, 4 | 4 |
| Nitrogen | N | 7 | 7 | 7 | 7 | 2, 5 | 3 |
| Oxygen | O | 8 | 8 | 8 | 8 | 2, 6 | 2 |
| Fluorine | F | 9 | 9 | 10 | 9 | 2, 7 | 1 |
| Neon | Ne | 10 | 10 | 10 | 10 | 2, 8 | 0 |
| Sodium | Na | 11 | 11 | 12 | 11 | 2, 8, 1 | 1 |
| Magnesium | Mg | 12 | 12 | 12 | 12 | 2, 8, 2 | 2 |
| Aluminium | Al | 13 | 13 | 14 | 13 | 2, 8, 3 | 3 |
| Silicon | Si | 14 | 14 | 14 | 14 | 2, 8, 4 | 4 |
| Phosphorous | P | 15 | 15 | 16 | 15 | 2, 8, 5 | 3 |
| Sulphur | S | 16 | 16 | 16 | 16 | 2, 8, 6 | 2 |
| Chlorine | Cl | 17 | 17 | 18 | 17 | 2, 8, 7 | 1 |
| Argon | Ar | 18 | 18 | 22 | 18 | 2, 8, 8 | 0 |
| Potassium | K | 19 | 19 | 21 | 19 | 2, 8, 8, 1 | 1 |
| Calcium | Ca | 20 | 20 | 20 | 20 | 2, 8, 8, 2 | 2 |
Answer: For oxygen, Number of protons = 8
Number of neutrons = 8
Mass number = Number of (protons + neutrons) = 8+8 = 16
For sulphur, Number of protons = 16
Number of neutrons = 16
Mass number = Number of (protons + neutrons) = 16+16 = 32
14. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Answer: The three sub-atomic particles are as follows:
| Isotopes of Hydrogen | Atomic Number | Mass Number | No. of proton | No. of electron | No. of neutron |
|---|---|---|---|---|---|
| Protium (1H1) | 1 | 1 | 1 | 1 | 0 |
| Deuterium (1D2) | 1 | 2 | 1 | 1 | 1 |
| Tritium (1T3) | 1 | 3 | 1 | 1 | 2 |
15. Write the electronic configuration of any one pair of isotopes and isobar.
Answer: Isotopes: Atoms of same element having same atomic number but different mass number, e.g.,
| Isotope | Atomic Number | Electronic configuration |
|---|---|---|
| 6C12 | 6 | 2, 4 |
| 6C14 | 6 | 2, 4 |
Isobars: Atoms of different elements with different atomic number but same mass number, e.g.,
| Isobar | Atomic Number | Electronic configuration |
|---|---|---|
| 18Ar40 | 18 | 2, 8, 8 |
| 19K40 | 19 | 2, 8, 8, 1 |
| 20Ca40 | 20 | 2, 8, 8, 2 |
16. Compare the properties of electrons, protons and neutrons.
Answer: The difference between the properties of electrons, protons and neutrons is as follows:
| S No. | Electrons | Protons | Neutrons |
|---|---|---|---|
| 1. | Negatively charged | Positively charged | No charge |
| 2. | Mass is negligible, i.e., 1/1800 times of proton | Mass is 1 amu | Mass is 1 amu |
| 3. | Get attracted towards positive charge | Get attracted towards negative charge | Do not get attracted as they are neutral |
| 4. | Present outside the nucleus | Present inside the nucleus | Present inside the nucleus |
17. What are the limitations of J.J. Thomson’s model of the atom?
Answer: 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 centre of the atom and electrons are distributed around it.
18. What are the limitations of Rutherford’s model of the atom?
Answer: 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.
19. Describe Bohr’s model of the atom.
Answer: Bohr’s model of the atom is described as:
(1) Atom has nucleus in the centre.
(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
20. Compare all the proposed models of an atom given in this chapter.
Answer: Comparison between Thomson, Rutherford and Bohr’s model of atom is given as:
21. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer: 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: 2n2, where 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).
22. Define valency by taking examples of silicon and oxygen.
Answer: Valency is defined as the combining capacity of an atom.
Atomic number of oxygen = 8
Atomic number of silicon = 14
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 that of silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4
23. Explain with examples: (i) Atomic number (ii) Mass number (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Answer: (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 number is 6.
(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus, e.g., there are 6 protons and 6 neutrons inside nucleus of carbon atom, so mass Number = number of protons + number of neutrons = 6 + 6 = 12.
(iii) Isotopes: Isotopes are atoms of the same element which have different mass number but same atomic number, e.g., 1H1, 1H2, 1H3.
(iv) Isobars: Isobars are atoms having the same mass number but different atomic numbers, e.g., 18Ar40, 19K40, 20Ca40.
Two uses of isotopes are:
(i) An isotope of iodine is used in the treatment of goitre.
(ii) An isotope of uranium is used as a fuel in nuclear reactors.
24. Na+ has completely filled K and L shells. Explain.
Answer: Sodium atom (Na), has atomic number =11
Number of protons = Number of electrons = 11
Electronic configuration of Na = 2, 8, 1
Sodium atom (Na) looses 1 electron to become stable and form Na+ ion. So the electronic configuration of Na+ ion is 2,8. Hence it has completely filled K and L shells.
25. If bromine atom is available in the form of say, two isotopes 7935Br (49.7%) and 8135Br (50.3%), calculate the average atomic mass of bromine atom.
Answer: The average atomic mass of bromine is calculated as:
26. 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?
Answer: Let the percentage of 168X be x and the percentage of 168X be (100 – x), then
27. If Z = 3, what would be the valency of the element? Also, name the element.
Answer: Given that, Z = 3
So, the electronic configuration is 2, 1
It has 1 valence electron, that the elements lose easily to acquire noble gas stable electronic configuration.
So, its valency is 1.
Name of the element is Lithium.
28. Composition of the nuclei of two atomic species X and Y are given as under:
| Element X | Element Y | |
| Protons | 6 | 6 |
| Neutrons | 6 | 8 |
Give the mass number of X and Y. What is the relation between the two species?
Answer: Mass number of X = Protons + Neutrons = 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
Number of protons = 6 (For both X and Y). So, the atomic number is same, i.e., 6.
As the atomic number is same, so both X and Y are isotopes of same element.
29. 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 proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer: (a) False (b) False (c) True (d) False
30. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:
(a) Atomic nucleus (b) Proton (c)Electron (d) Neutron
Answer: (a) Atomic nucleus
31. Isotopes of an element have: (a) the same physical properties (b) different number of neutrons (c) different number of neutrons (d) different atomic numbers.
Answer: (b) different number of neutrons
32. Number of valence electrons in C4- ion are: (a) 16 (b) 8 (c) 17 (d) 18
Answer: (b) 8
33. 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
Answer: (d) 2, 8, 1
34. Complete the following table.
| Atomic Number | Mass Number | No. of neutrons | No. of protons | No. of electrons | Name of the element |
|---|---|---|---|---|---|
| 9 | – | 10 | – | – | – |
| 16 | 32 | – | – | – | Sulphur |
| – | 24 | – | 12 | – | – |
| – | 2 | – | 1 | – | – |
| – | 1 | 0 | 1 | 0 | – |
Answer: The complete table is as:
| Atomic Number | Mass Number | No. of neutrons | No. of protons | No. of electrons | Name of the element |
|---|---|---|---|---|---|
| 9 | 19 | 10 | 9 | 9 | Fluorine |
| 16 | 32 | 16 | 16 | 16 | Sulphur |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 1 | 2 | 1 | 1 | 1 | Deuterium |
| 1 | 1 | 0 | 1 | 0 | Hydrogen |
35. The nucleons are: (a) protons and electrons (b) neutrons and electrons (c) protons and neutrons (d) none of these
Answer: (c) protons and neutrons
36. The isotope deuterium of hydrogen has: (a) no neutrons and one proton (b) one neutron and two protons (c) one electron and two neutron (d) one proton and one neutron
Answer: (d) one proton and one neutron.
37. The electrons present in the outermost shell are called: (a) valency electrons (b) octet electrons (c) duplet electrons (d) valence electron
Answer: (d) valence electrons
38. An alpha-particle contains: (a) 4 positive charge and 2 mass unit (b) 2 positive charge and 4 mass unit (c) 2 positive charge and 2 mass unit (d) 4 positive charge and 4 mass unit.
Answer: (b) 2 positive charge and 4 mass unit
39. The atomic number of sodium is 11 and its mass number is 23. It has: (a) 11 neutrons and 12 protons (b) 12 protons and 11 electrons (c) 11 electrons and 12 neutrons (d) 12 electrons and 11 neutrons
Answer: (c) 11 electrons and 12 neutrons
40. The electronic configuration of chlorine is: (a) 2, 7 (b) 2, 8, 8, 7 (c) 2, 8, 7 (d) 2, 7, 8
Answer: (c) 2, 8, 7
41. The isotope used to remove the brain tumours and treatment of cancer is: (a) U-235 (b) Na-24 (c) Iodine (d) Co-60
Answer: (d) Co-60
42. In the α-scattering experiment, few α-particles rebounded because: (a) Most of the space in the atom is occupied (b) Positive charge of the atom occupies very little space (c) The mass of the atom is concentrated in the centre (d) all positive charge and mass of the atom is concentrated in small volume.
Answer: (d) all positive charge and mass of the atom is concentrated in small volume.
43. Draw the atomic structure of hydrogen atom.
Answer: The atomic structure of hydrogen atom is:
44. Why are some elements chemically inert?
Answer: Some elements are chemically inert because their outermost shell is completely filled.
45. Why is atom electrically neutral?
Answer: It has same number of protons and electrons, i.e., positive charge equals negative charge.
46. What is the charge and mass of alpha-particles?
Answer: Charge is +2 and Mass is 4 u.
47. What are valence electrons?
Answer: Electrons present in the outermost shell of an atom are called valence electrons.
48. An atom has atomic number 12, what is its valency and name the element?
Answer: Atomic number = 12
So, Protons = Electrons = 12
Electrons Configuration = 2, 8, 2
So, its valency is 2
Element is Magnesium (Mg).
49. Find the number of neutrons in 2713X.
Answer: Mass number = 27
So, p + n = 27
⇒ 13 + n = 27
⇒ n = 14
Hence, number of neutron = 14.
50. Where is the mass of an atom concentrated?
Answer: Mass of an atom is concentrated in nucleus.
51. Name two elements with same number of protons and neutrons?
Answer: Carbon (Protons = Neutrons = 6) and Oxygen (Protons = Neutrons = 8).
52. Draw the atomic structure of sodium atom.
Answer: The atomic structure of 11Na23 atom is:
53. Name the isotope used for treatment of cancer.
Answer: Isotope of cobalt, i.e., Co-60.
54. ZXA or AZX. What does this symbol represent?
Answer: X represents Symbol of element.
A represents Mass number.
Z represents Atomic number.
55. Can the value of ‘Z’ be same for two different atoms?
Answer: No, two different atoms cannot have same atomic number.
56. Can the value of A’ be same for two different atom?
Answer: Yes, it can be same, e.g. Ar, K and Ca have mass number, 40, while there atomic number are different.
57. Name the scientist who discovered protons and neutrons in an atom.
Answer: Protons were discovered by E. Goldstein in 1866 and neutrons were discovered by J, Chadwick in 1932.
58. What is the contribution of Bohr and Bury together in the structure of atom’s explanation?
Answer: Both Bohr and Bury gave the distribution of electrons into different atoms by giving the formula 2n2, where n = shell number.
59. Draw the atomic structure of (i) an atom with same number of sub-atomic particles, (ii) an atom with same number of electrons in L and M shell.
Answer: (i) An atom with same number of sub-atomic particles is Helium (2He4).
No. of protons = 2
No. of electrons = 2
No. of neutrons = 2
(ii) An atom with L and M shell filled, i.e., Argon (18Ar40) whose electronic configuration is 2, 8, 8.
60. What is an octet? Why would atoms want to complete their octet?
Answer: When the outermost shell of an atom i.e., L, M or N are completely filled with 8 electrons in the shell, it is said an octet. Atoms would want to complete their octet because they want to become stable.
61. Find the valency of 147N and 3517Cl.
Answer: Atomic number of nitrogen = 7, p = 7, e = 7
Electronic configuration = 2, 5
Valency = 3
Because either it will gain 3 electrons to complete its octet.
Atomic number of chlorine = 17, p = 17, e = 17
Electronic configuration = 2, 8, 7
Valency = 1
Because it will gain 1 electron to complete its octet.
62. Pick up the isotopes among the following and state reason. 7X14, 17X35, 14X24, 17X37
Answer: The isotopes are 17X35 and 17X37 as both the atoms show same atomic number but different mass number.
63. Pick up atoms which have same number of neutrons from the following: 11Y23, 14Y28, 12Y24, 13Y27
Answer: 11Y23 and 12Y24: Number of neutrons are 12
14Y28 and 13Y27: Number of neutrons are 14.
64. What are nucleons? What is the name given to those atoms which have same number of nucleons in it?
Answer: Protons and neutrons present in the nucleus are called nucleons. Isobaric elements have same number of nucleons in it and called as isobars.
65. Give the difference between three sub-atomic particles.
Answer: Three sub-atomic particles are electron, proton and neutron. The difference among them is as:
| Particle | Electron | Proton | Neutron |
|---|---|---|---|
| Discovered by | J.J. Thomson | E. Goldstein | J. Chadwick |
| Charge | -1 | +1 | 0 |
| Symbol | e | p | n |
| Mass | 1/1800 | 1 | 1 |
66. Give the names of three isotopes of hydrogen.
Answer: Three atomic species of hydrogen are: Protium (1H1), Deuterium (1H2) and Tritium (1H3).
67. Atomic mass exists as whole number, why do we write the atomic mass of chlorine as 35.5 u.
Answer: Chlorine has two isotopes and the mass of an atom is taken as the average mass of all the naturally occurring atoms of that element. This is obtained by knowing the percentage of each isotopic form and then the average mass is calculated.
The abundance of Cl-35 is 75% while of Cl-37 is 25%.
So, the average atomic mass of Cl is 35.5 u.
Calculation: [35*(75/100)] + [37*(25/100)] = 35.5 u
68. Give difference between isotopes and isobars.
Answer: The difference between isotopes and isobars is:
| Isotopes | Isobars |
|---|---|
| Atoms of same element | Atoms of different element |
| Have same atomic number | Have different atomic number |
| Have different mass number | Have same mass number |
| Number of protons and electrons are same in these atoms | Number of protons and electrons are not same in these atoms |
| e.g., 1H1, 1H2 and 1H3 | e.g., 18Ar40, 19K40 and 20Ca40 |
69. Number of protons and electrons are same in an atom. Then why is it wrong to say that atomic number of an atom is equal to its number of electrons.
Answer: Atomic number ≠ Number of electrons, although number of protons = number of electrons because the electron’s number can change in an atom by loss, or gain of it. But the proton’s number remain constant (as it does not take part in loss or gain).
70. An atom is electrically neutral, but on loss or gain of electrons, why does it become charged?
Answer: An atom is electrically neutral because of same number of protons and electrons. But it becomes charged, to become stable atom by lose or gain of electrons. Hence, in that case of losing or gaining of electron,
Number of protons ≠ Number of electrons
If it loses electrons p > e; hence positive charge is obtained.
If it gains electrons e > p; hence negative charge is obtained.
71. What is valency? Explain different types of valencies.
Answer: The combining capacity of an atom is called its valency. There are 2 types of valencies.
Covalency: When an atom share electrons.
Electrovalency: When an atom lose or gain electrons.
Some atoms also show zero valency when there outermost shell is completely filled.
72. In the structure of an atom why are protons present in the centre and are not pulled outside by the electrons as both are oppositely charged with same unit of charge?
Answer: Protons are heavy with mass 1 unit and hence are concentrated in the centre of the atom. The mass of electrons is negligible i.e., 1/1800 times less than that of protons. Hence, electrons are not able to attract the protons and pull them out of the nucleus, although their charge is of same value.
73. According to you, among the structure of atom studied which model is correct and why?
Answer: Bohr’s model of an atom is the best model and is correct because it gives the explanation of nucleons (protons and neutrons) in the centre and how electrons revolve around the nucleons in their discrete, special orbits, so electrons don’t loose/radiate energy and remain bonded in their shell.
74. Give an activity to understand the implications of Rutherford’s a scattering experiment by a gold foil.
Answer: To understand the implications of Rutherford’s a-particle scattering experiment:
Activity: Let a child stand in front of a wall with his eyes closed. Let him throw stones at the wall from a distance. He will hear sound for each strike of stone on the wall. This is like a nucleus of the atom. But if a blind-folded child has to throw stones at a barbed-wire fence, most of the stones would not hit the fencing and no sound would be heard.
This is because there are lots of gap in the fence which allows the stone to pass through them. This is like empty space in an atom through which a-particles will pass through. Based on the above activity and similar reasoning Rutherford concluded the a-particle scattering experiment as:
(1) Most of the space inside the atom is empty as a-particles passed through the foil.
(2) Very few particles deflected from their path, this show that positive charge occupies less space.
(3) A very small fraction of a-particles are deflected by 180°, this shows that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
75. What are isotopes? State its characteristics and give uses of isotopes.
Answer: Atoms of same element with same atomic number but different mass number are called as isotopes.
Characteristics:
(1) Physical properties of the isotopes are different e.g. mass, density.
(2) Chemical properties of the isotopes are same due to same number of electrons.
Uses:
(1) Uranium isotope is used as a fuel in nuclear reactor (U-235).
(2) Cobalt isotope is used for treatment of cancer (Co-60).
(3) Iodine isotope is used in the treatment of goitre.
76. Explain Rutherford’s α-particle scattering experiment and give its observation and conclusion drawn.
Answer: Fast moving α-particles were made to fall on a thin gold foil. Particles have +2 charge and 4 u mass, and considerable amount of energy.
Observations:
(1) Most of the α-particles passed straight through the foil.
(2) Some of the α-particles were deflected by small angles by the foil.
(3) One out of every 12000 particles rebounded.
Conclusion from observation:
(1) Most of the space inside the foil is empty.
(2) Positive charge of atom occupies very less space.
(3) Mass of the atom is concentrated in the centre with all positive charge concentrated in small volume within the atom.
77. Establish the relationship between atomic number, mass number, isotopes, isobars and valency of an atom.
Answer: Atomic number: Gives the number of protons (Z)
Mass number: Gives the number of protons and neutrons (A)
Isotopes: When atoms of same element have same number of protons (Z) but different number of a neutrons (n), such atoms are called isotopes.
Isobars: When atom of different element have same mass number (A) but different atomic number (Z) such atoms are called isobars.
Valency: It is the combining capacity of an atom.
78. Aryan could not solve the following question in the group; his group mate explained him and solved his difficulty. What information do you get from the given figure about the atomic number, mass number and valency of the given atom X’. The question was as follows:
(a) What is the answer for the above question?
(b) Name the element X’.
(c) What value of Aryan’s friend is reflected in this behaviour?
Answer: (a) The atomic number is 5.
The mass number is 11.
The valency is 3.
(b) The element X’ is boron.
(c) Aryan’s friend showed the value of helping and caring nature.
Read More: 52 Q&A of Is Matter Around Us Pure?
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