A abampere (aA) The unit of electric current in the CGSeniu system, defined as that current that, if flowing through two parallel conductors of negligible cross section and infinite length, placed 1 cm apart in vacuo, would produce on each conductor a force of 1 dyne per centimeter of length. 1 abampere = 1 abcoulomb/s = r statampere (where c = speed of light in cm/s) = 10 ampere. aberration Imperfect image formation due to geometric imperfections in the optical elements of a system ablation 1 . The wasting of glacier ice by any process (calving, melting, evaporation, etc.). 2. The shedding of molten material from the outer sur- face of a meteorite or tektite during its flight through the atmosphere. absolute age The age of a natural substance, of a fossil or living organism, or of an artifact, obtained by means of an absolute dating method. See absolute dating method. absolute density Density in kg/m' or, more commonly, in g/cm\ both at STP. Cf. density, relative density abso
Structure of Atom class9 CHEMISTRY MCQ & SAQ
Structure of Atom class9 CHEMISTRY NCERT SOLUTION:
1. What are canal rays?
Answer
Canal rays are positively charged radiations that can pass through a perforated cathode plate. These rays consist of positively charged particles known as protons.
2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer
An electron is a negatively charged particle, whereas a proton is a positively charged particle. The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.
Page No: 49
1. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
Answer
As per Thomson’s model of the atom, an atom consists of both negative and positive charges which are equal in number and magnitude. So, they balance each other as a result of which the atom as a whole is electrically neutral.
2. On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
Answer
On the basis of Rutherford's model of an atom, protons are present in the nucleus of an atom.
3. Draw a sketch of Bohr's model of an atom with three shells.
Answer
4. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer
If an α-particle scattering experiment is carried out using a foil of any metal as thin as gold foil used by Rutherford, there would be no change in observations. But since other metals are not so malleable, such a thin foil is difficult to obtain. If we use a thick foil, then more α-particles would bounce back and no idea about the location of positive mass in the atom would be available with such certainty.
1. Name the three subatomic particles of an atom.
Answer
The three subatomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons
2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer
Number of neutrons = Atomic mass - Number of protons
Therefore, the number of neutrons in the atom = 4 - 2 = 2
Page No: 50
1. Write the distribution of electrons in carbon and sodium atoms
Answer
? The total number of electrons in a carbon atom is 6. The distribution of electrons in a carbon atom is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 4 electrons
Or, we can write the distribution of electrons in a carbon atom as 2, 4.
? The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atoms is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 8 electrons
Third orbit or M-shell = 1 electron
Or, we can write the distribution of electrons in a sodium atom as 2, 8, 1.
2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer
The maximum capacity of the K shell is 2 electrons and the L shell can accommodate a maximum of 8 electrons in it. Therefore, there will be ten electrons in the atom.
Page No: 52
1. How will you find the valency of chlorine, sulfur and magnesium?
Answer
If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than
4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.
The distribution of electrons in chlorine, sulfur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively.
Therefore, the number of electrons in the outermost shell of chlorine, sulfur, and magnesium atoms are 7, 6, and 2 respectively.
? Thus, the valency of chlorine = 8 -7 = 1
? The valency of sulfur = 8 - 6 = 2
? The valency of magnesium = 2
1. 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?
Answer
(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.
(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.
2. With the help of Table 4.1, find out the mass number of oxygen and sulfur atoms.
Answer
Mass number of oxygen = Number of protons Number of neutrons
= 8 8
= 16
Mass number of sulfur = Number of protons Number of neutrons
= 16 16
= 32
Page No: 53
1. For the symbol, H, D and T tabulate three subatomic particles found in each of them.
Answer
2. Write the electronic configuration of any one pair of isotopes and isobars.
Answer
12C6 and 14C6 are isotopes, have the same electronic configuration as (2, 4)22Ne10and 22Ne11 are isobars. They have different electronic configurations as given below:
22Ne10 – 2, 8
22Ne11 – 2, 8, 1
Page No: 54
Exercise
1. Compare the properties of electrons, protons and neutrons.
Answer
2. What are the limitations of J.J. Thomson's model of the atom?
Answer
The limitations of J.J. Thomson's model of the atom are:
→ It could not explain the result of the scattering experiment performed by rutherford.
→ It did not have any experimental support.
3. What are the limitations of Rutherford's model of the atom?
Answer
The limitations of Rutherford's model of the atom are
→ It failed to explain the stability of an atom.
→ It doesn't explain the spectrum of hydrogen and other atoms.
4. Describe Bohr's model of the atom.
Answer
→ The atom consists of a small positively charged nucleus at its center.
→ The whole mass of the atom is concentrated at the nucleus and the volume of the nucleus is much smaller than the volume of the atom.
→ All the protons and neutrons of the atom are contained in the nucleus.
→ Only certain orbits known as discrete orbits of electrons are allowed inside the atom.
→ While revolving in these discrete orbits electrons do not radiate energy. These orbits or cells are represented by the letters K, L, M, N etc. or the numbers, n = 1, 2, 3, 4, . . as shown in the figure below.
5. Compare all the proposed models of an atom given in this chapter.
Answer
6. Summarize the rules for writing the distribution of electrons in various shells for the first eighteen elements.
Answer
The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.
→ If n gives the number of orbit or energy level, then 2n2 gives the maximum number of electrons possible in a given orbit or energy level. Thus,
First orbit or K-shell will have 2 electrons,
Second orbit or L-shell will have 8 electrons,
Third orbit or M-shell will have 18 electrons.
→ If it is the outermost orbit, then it should have not more than 8 electrons.
→ There should be stepwise filling of electrons in different orbits, i.e., electrons are not accompanied in a given orbit if the earlier orbits or shells are incompletely filled.
7. Define valency by taking examples of silicon and oxygen.
Answer
The valency of an element is the combined capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element.→ Valency of Silicon: It has electronic configuration: 2,8,4
Thus, the valency of silicon is 4 as these electrons can be shared with others to complete the octet.
→ Valency of Oxygen: It has electronic configuration: 2,6
Thus, the valency of oxygen is 2 as it will gain 2 electrons to complete its octet.
Page No: 55
8. 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 the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
(ii) Mass number: The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of a boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 6 = 11.
(iii) Isotopes: These are atoms of the same element having the same atomic number, but different mass numbers. For example, chlorine has two isotopes with atomic number 17 but mass numbers 35 and 37 represented by
(iv) Isobars: These are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example, Ne has atomic number 10 and sodium has atomic number 11 but both of them have mass numbers as 22 represented by -
Two uses of isotopes:
→ One isotope of uranium is used as a fuel in nuclear reactors.
→ One isotope of cobalt is used in the treatment of cancer.
9. Na has completely filled K and L shells. Explain.
Answer
The atomic number of sodium is 11. So, a neutral sodium atom has 11 electrons and its electronic configuration is 2, 8, 1. But Na has 10 electrons. Out of 10, K-shell contains 2 and L-shell 8 electrons respectively. Thus, Na has completely filled K and L shells.
10. If a bromine atom is available in the form of, say, two isotopes 79 / 35Br (49.7%) and 81 / 35Br (50.3%), calculate the average atomic mass of the bromine atom.
Answer
It is given that two isotopes of bromine are 79 / 35Br (49.7%) and 81 / 35Br (50.3%). Then, the average atomic mass of a bromine atom is given by:
11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 16 / 8 X and 18 / 8 X in the sample?
Answer
It is given that the average atomic mass of the sample of element X is 16.2 u.
Let the percentage of isotope 18 / 8 X be y%. Thus, the percentage of isotope 16 / 8 X will be (100 - y) %.
Therefore,
18y 1600 - 16y = 1620
2y 1600 = 1620
2y = 1620 - 1600
y= 10
Therefore, the percentage of isotope 18 / 8 X is 10%.
And, the percentage of isotope 16 / 8 X is (100 - 10) % = 90%.
12. If Z = 3, what would be the valency of the element? Also, name the element.
Answer
By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).
Therefore, the element with Z = 3 is lithium.
13.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 numbers of X and Y. What is the relation between the two species?
Answer
Mass number of X = Number of protons Number of neutrons
= 6 6
= 12
Mass number of Y = Number of protons Number of neutrons
= 6 8
= 14
These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.
14. 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.
? False
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
? False
(c) The mass of an electron is about 1 / 2000times that of a proton.
? True
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
? False
15. Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
? (a) Atomic nucleus
16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
? (c) different number of neutrons
17. Number of valence electrons in Cl -ion are:
(a) 16
(b) 8
(c) 17
(d) 18
? (b) 8
Page No: 56
18. 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
19. Complete the following table.
Answer
Structure of Atom class9 CHEMISTRY MCQ
1. When alpha-particles are sent through a thin metal foil, most of them go straight through the foil because
(a) alpha-particles are positively charged
(b) alpha-particles are much smaller than electrons
(c) alpha-particles move with low velocity
(d) most part of the atom is empty
► (d) most part of the atom is empty
2. The number of electrons in an atom of Fluorine is 9. Its electronic configuration is _______.
(a) 8, 1
(b) 7, 2
(c) 2, 7
(d) 1, 8
► (c) 2, 7
3. N shell can have a maximum of _______ electrons.
(a) 32
(b) 2
(c) 18
(d) 8
► (a) 32
4. Rutherford’s scattering experiment falls for very small scattering angles because
(a) the impact parameter between the a-particle source and the nucleus of the target is very large as compared to the size of the nucleus.
(b) whole nuclear charge of the target atom is not screened by its electrons at all
(c) the velocity of a-particles is large
(d) the gold foil is thin
► (b) whole nuclear charge of the target atom is not screened by its electrons at all
5. Nucleus consist of:
(a) Proton and electron
(b) Proton and neutron
(c) Neutron and electron
(d) Only neutron
► (b) Proton and neutron
6. Which of the following is the correct electronic configuration for magnesium?
(a) 2, 8, 1
(b) 2, 8, 3
(c) 2, 8, 4
(d) 2, 8, 2
► (d) 2, 8, 2
7. The electronic configuration of an atom with atomic number 19 is:
(a) 2, 8, 8, 1
(b) 1, 2, 2, 8
(c) 2, 8, 9
(d) 2, 10, 7
► (a) 2, 8, 8, 1
8. Which of the following pairs have identical values of charge/mass?
(a) A proton and a deuterium
(b) A proton and a neutron
(c) A deuterium and an a-particle
(d) An electron and y-rays
► (c) A deuterium and an a-particle
9. If Z = 6, what would be the valency of the element?
(a) 6
(b) 2
(c) 4
(d) 8
► (c) 4
10. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4
► (c) 1
11. Bohr proposed that while revolving in discrete orbits, the electrons:
(a) Gain energy
(b) Lose energy
(c) Do not radiate energy
(d) First lose energy and then gain energy.
► (c) Do not radiate energy
12. A neutral element A has 3 electrons, the atomic number of an element is _______.
(a) 1
(b) 2
(c) 5
(d) 3
► (d) 3
13. Lightest particle is –
(a) Neutron
(b) Electron
(c) Proton
(d) None of these
► (b) Electron
14. Argon has an atomic number of 18 and a mass number of 40. Calcium has atomic 20 and mass number 40. Calcium and argon are examples of _______.
(a) isotopes
(b) isobars
(c) isotones
(d) isochores
► (b) isobars
15. The number of valence electrons in Na is –
(a) 1
(b) 2
(c) 3
(d) 4
► (a) 1
16. Why do isotopes have different physical properties?
(a) Isotopes of an element have different electronic configurations.
(b) Isotopes have the same atomic number.
(c) Isotopes of an element have different mass numbers.
(d) Isotopes of an element have different atomic numbers.
► (c) Isotopes of an element have different mass numbers.
17. The electronic configuration of Rb(37) will be
(a) 2 , 8 , 1 8 , 9
(b) 2 , 8 , 1 8 , 8 , 1
(c) 2, 8,8 ,1 8,1
(d) 2 ,8 ,1 8 ,1 ,8
► (b) 2 , 8 , 1 8 , 8 , 1
18. Mass number of element is –
(a) Number of protons in its nucleus
(b) Number of electrons and protons in the atom
(c) Number of neutrons and protons in the nucleus
(d) Number of neutrons in the nucleus
► (c) Number of neutrons and protons in the nucleus
19. When a gas at reduced pressure is subjected to an electric discharge, the rays originating from the negative electrode are–
(a) Cathode rays
(b) Anode rays
(c) x-rays
(d) Radiation ray
► (a) Cathode rays
20. The increasing order of the energy levels in an atom is _______ .
(a) K > L > M > N
(b) L > M > N > K
(c) L < M < N < K
(d) K < L < M < N
► (d) K < L < M < N
21. Atomic number is equal to:
(a) Number of protons + Number of electrons
(b) Number of protons
(c) Number of protons + Number of neutrons
(d) Number of neutrons
► (b) Number of protons
22. Neutron is a fundamental particle which have–
(a) +1 unit charge and 1 unit mass
(b) No charge and 1 unit mass
(c) Have no charge and mass
(d) Have –1 unit charge and 1 unit mass
► (b) No charge and 1 unit mass
23. The atomic number of an element having mass number 23 and number of neutrons 12 is ________.
(a) 23
(b) 11
(c) 12
(d) 35
► (b) 11
24. Which form of iodine is used for the treatment of goiter?
(a) An isotone
(b) An isobar
(c) An isochore
(d) An isotope
► (d) An isotope
25. According to Dalton's atomic theory –
(a) Electrons, protons and neutrons are found in the atom
(b) Atoms are indivisible
(c) Atoms of various elements have the same properties
(d) Atoms are destroyed in the chemical reaction
► (b) Atoms are indivisible
26. The maximum number of electrons that can be accommodated in an orbit is given by the formula _______ where n is the number of electrons.
(a) 2n + 2
(b) 2(n + 2)
(c) 2/n2
(d) 2n2
►(d) 2n2
27. An atom which has a mass number of 14 or 8 neutrons is–
(a) Isotope of oxygen
(b) Isobar of oxygen
(c) Isotope of carbon
(d) Isobar of carbon
► (c) Isotope of carbon
28. What is a valence shell?
(a) Third shell
(b) First shell
(c) Outermost shell
(d) Second shell
► (c) Outermost shell
29. The electron should be filled in the orbital in accordance with the increasing order of their energy. This statement is related with
(a) Aufbau principle
(b) Pauli principle
(c) Hund's principle
(d) Plank's rules
► (a) Aufbau principle
30. Which of the following statements is always correct?
(a) An atom has an equal number of electrons and protons.
(b) An atom has an equal number of electrons and neutrons.
(c) An atom has an equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutron
► (a) An atom has an equal number of electrons and protons.
Question. Which of the following does not have 8 valence electrons:
(a) He
(b) Ne
(c) Ar
(d) Cl
Answer : A
Question. Which of the following particles was discovered first?
(a) Neutron
(b) Electron
(c) Proton
(d) Meson
Answer : B
Question. Isotopes of an element have:
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
Answer : C
Question. The electron should be filled in the orbital in accordance with the increasing order of their energy. This statement is related with
(a) Aufbau principle
(b) Pauli principle
(c) Hund's principle
(d) Plank's rules
Answer : A
Question. Which of the following elements has the electron configuration 2?
(a) Magnesium
(b) Helium
(c) Sodium
(d) Hydrogen
Answer : B
Question. Light energy is treated as a form of matter because it consists of:
(a) Photons or bundles of energy
(b) Electrons or wave like matter
(c) Neutrons
(d) None of the Above
Answer : A
Question : Though the three fundamental particles are present in almost all elements, one element does not possess:
(a) Proton
(b) Neutron
(c) Electron
(d) Nucleons
Answer : B
Question. According to J.J Thomson, an atom is a positively charged sphere. Which subatomic particles are embedded in it?
(a) Electrons
(b) Canal Rays
(c) Protons
(d) Neutrons
Answer : A
Question. The smallest fundamental particle, of an atom with a unit negative charge is called:
(a) Electron
(b) Proton
(c) Neutron
(d) Alpha-particle
Answer : A
Question. Neutron Possesses
(a) Positive charge
(b) No charge
(c) Negative charge
(d) None of the above.
Answer : B
Question. Each of the stationary orbits are associated with:
(a) A certain amount of velocity.
(b) A certain amount of mass.
(c) A certain amount of energy.
(d) A certain amount of force.
Answer : C
Question. Which of the following is the correct electronic configuration for magnesium?
(a) 2, 8, 1
(b) 2, 8, 3
(c) 2, 8, 4
(d) 2, 8, 2
Answer : D
Question. What are the essential conditions for the production of anode rays?
(a) High voltage and low pressure
(b) High voltage and high pressure
(c) Low voltage and high pressure
(d) Low voltage and low pressure
Answer : A
Structure of Atom class9 CHEMISTRY SAQ
Why are some elements chemically inert?
Answer: Because their outermost shell is completely filled.
Question 3. Why are atoms electrically neutral?
Answer: It has the same number of protons and electrons, (positive charge = negative charge).
Question 4. What is the charge and mass of a-particles?
Answer: Charge is + 2
Mass is 4 a.m.u.
Question 5. What are valence electrons?
Answer: Electrons present in the outermost shell of an atom are called valence electrons.
Question 6. An atom has atomic number 12, what is its valency and name of the element?
Answer: Atomic number = 12
∴ Protons = Electrons = 12 Electrons Configuration = K L M -2 8 2
∴ Valency = 2
Element is magnesium.
Question 7. Find the number of neutrons in 2713X.
Answer: Mass number = 27
∴ p + n = 27 p = 13, (Atomic No. = Number of protons)
∴ 13 + n = 27
∴ n = 14
∴ Neutron =14
Question 8. Where is the mass of atom concentrated?
Answer: Mass of an atom is concentrated in the nucleus.
Question 9. Name two elements with the same number of protons and neutrons?
Answer: Carbon (Protons = Neutrons = 6)
Oxygen (Protons = Neutrons = 8)
Question 10. Draw the atomic structure of sodium atoms.
Answer:
Question 11. Name the isotope used for treatment of cancer.
Answer: Isotope of cobalt.
Question 12. AZX What does this symbol represent?
Answer: X —> Symbol of element
A —> Mass number
Z —> Atomic number
Question 13. Can the value of ‘Z’ be the same for two different atoms?
Answer: No, (Z = atomic number), two different atoms cannot have the same atomic number.
Question 14. Can the value of A’ be the same for two different atoms?
Answer: Yes, it can be e.g. Ca and Ar has A-40 (i.e., mass number).
Question 1. Name the scientist who discovered protons and neutrons in atoms.
Answer: Protons were discovered by E. Goldstein in 1866 and neutrons were discovered by J. Chadwick in 1932.
Question 2. What is the contribution of Bohr and Bury together in the structure of an 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.
Question 3. Draw the atomic structure of (i) an atom with the same number of subatomic particles, (ii) an atom with the same number of electrons in the L and M shell.
Answer: (i) An atom with same number of subatomic particles is 24He
No. of protons = 2
No. of electrons = 2
(ii) An atom with L and M shell filled —->K L M- 2 8 8
Question 4. 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 called an octet. Atoms would want to complete their octet because they want to become stable.
Question 5. Find the valency of 147N and 3517Cl.
Answer: The atomic number of nitrogen = 7, No. of protons = 7, No. of electrons = 7
Electronic configuration = K L M =2 5 –
Valency = 3
Because either it will gain three electrons or share 3 electrons to complete its octet.
The atomic number of chlorine = 17, p = 17, e=17
Electronic configuration = K L M= 2 8 7
Valency = 1
Because it will gain 1 electron to complete its octet.
Question 6. Pick up the isotopes among the following and state the reason.
Answer: The isotopes are 3517X and 3717X as both the atoms show the same atomic number but different mass number.
Question 7. Pick up atoms which have the same number of neutrons from the following:
Answer:
Question 8. What are nucleons? What is the name given to those atoms which have the same number of nucleons in it?
Answer: Protons and neutrons present in the nucleus are called nucleons Isobaric elements have the same number of nucleons in it.
Question 9. Give the difference between three subatomic particles.
Answer: Three subatomic particles are electron, proton and neutron
Question 10. Give the names of three atomic species of hydrogen.
Answer: Three atomic species of hydrogen are:
Question 11. Atomic Mass exists as a whole number, so why do we write that the atomic mass of chlorine is 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.
Question 12. Give difference between isotopes, and isobars.
Answer:
Question 13. Number of protons and electrons are the same in an atom. Then why is it wrong to say that the 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 remains constant (as it does not take part in loss or gain).
Question 14. An atom is electrically neutral, on loss or gain of electrons why does it become charged?
Answer: An atom is electrically neutral because it has the same number of protons and electrons. But it becomes charged, becomes a stable atom, loses or gains electrons. Hence,
Number of protons ≠ Number of electrons
If it loses electrons p > e; hence +ve charge is obtained.
If it gains electrons e > p; hence -ve charge is obtained.
Question 15. 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.
Some atoms also show zero valency when their outermost shell is completely filled.
Question 16. With the help of an activity in daily life, how can you prove that atoms are divisible.
Answer: Activity
Take a scale, rub it on your hair, and try to attract a small bit of paper.
Now divide the bit of paper further into smaller pieces.
Again bring the charged scale near to these pieces of paper.
You will observe that the bits of paper still get attracted.
Conclusion: This activity shows that an atom contains charges and these charges are opposite in nature which shows the attraction. Hence here scale and paper both are oppositely charged and hence attract each other. Also, every atom has at least one subatomic particle.
Question 17. In the structure of an atom why are protons present in the center and are not pulled outside by the electrons as both are oppositely charged with the same unit of charge?
Answer: Protons are heavy with mass 1 unit and hence are concentrated in the center
of the atom. The mass of electrons is negligible, i.e.1/1800 times less than that of protons. Hence they are not able to attract the protons and pull them out of the nucleus, although their charge is of the same value.
Question 18. According to you, among the structures of atoms 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 center and how electrons revolve around the nucleons in their discrete, special orbits, so electrons don’t lose/radiate energy and remain bonded in their shell.
Structure of Atom class9 CHEMISTRY MCQ & SAQ
Question 1. Give an activity to understand the implications of Rutherford's scattering experiment with 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 a 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 gaps 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 shows 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.
Question 2. What are isotopes? State its characteristics, give uses of isotopes?
Answer: Atoms of same element with same atomic number but different mass number are isotopes.
Characteristics:
(1) Physical properties of the isotopes are different e.g. mass, density.
(2) Chemical properties of the isotopes are the same due to the same number of electrons.
Uses:
(1) Uranium isotope is used as a fuel in a nuclear reactor (U-235).
(2) Cobalt isotope is used for treatment of cancer (Co-60).
(3) Iodine isotope is used in the treatment of goiter.
Question 3. Explain Rutherford’s α-particle scattering experiment and draw its observations and conclusions.
Answer: Rutherford’s α-particle scattering experiment:
Fast moving α-particles were made to fall on a thin gold foil. Particles have + 2 charge and 4u mass, and a 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 an atom occupies very less space.
(3) Mass of the atom is concentrated in the center with all positive charges concentrated in small volume within the atom.
Question 4. 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 the same element have the same number of protons (Z) but different numbers of neutrons (s) such atoms are called isotopes.
Isobars — When atoms of different elements have the same mass number (A) but different atomic number (Z) such atoms are called isobars.
Valency — It is the combining capacity of an atom.
Question 1. Aryan could not solve the following question in the group; his group mate explained to him and solved his difficulty. The question was as follows:
What information do you get from the given figure about the atomic number, mass number and valency of the given atom X’.
(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 behavior?
Answer: (a) The atomic number is 5.
The mass number is 11.
The vacancy is 3.
(b) The element X’ is boron.
(c) Aryan’s friend showed the value of helping and caring nature.
Comments
Post a Comment