Rajasthan Board RBSE Class 9 Science Important Questions Chapter 4 Structure of the Atom Important Questions and Answers.
RBSE Class 9 Science Chapter 4 Important Questions Structure of the Atom
Objective Type Questions
Question 1.
Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2
Answer:
(b) 2, 8, 2
Question 2.
Rutherford’s ‘alpha (α) particles scattering experiment’ resulted into discovery of:
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Answer:
(c) Nucleus in the atom
Question 3.
The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
(a) 16X31
(b) 16X31
(c) 16X16
(d) 16X15
Answer:
(a) 16X31
Question 4.
Dalton’s atomic theory successfully explained :
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportions
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Answer:
(d) (i), (ii) and (iv)
Question 5.
Which of the following statements about Rutherford’s model of atom are correct?
(i) Considered the nucleus as positively charged.
(ii) Established that the α-particles are four times heavy as a hydrogen atom.
(iii) Can be compared to solar system.
(iv) Was in agreement with Thomson’s model.
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) Only (i)
Answer:
(a) (i) and (iii)
Question 6.
Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer:
(d) (ii) and (iv)
Question 7.
In the Thomson’s model of atom, which of the following statements are correct?
(i) The mass of the atom is assumed to be uniformly distributed over the atom.
(ii) The positive charge is assumed to be uniformly distributed over the atom.
(iii) The electrons are uniformly distributed in the positively charged sphere.
(iv) The electrons attract each other to stabilise the atom.
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)
Answer:
(a) (i), (ii) and (iii)
Question 8.
Rutherford’s α-particle scattering experiment showed that:
(i) Electrons have negative charge.
(ii) The mass and positive charge of the atom is concentrated in the nucleus.
(iii) Neutron exists in the nucleus.
(iv) Most of the space in atom is empty.
Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv)
Answer:
(b) (ii) and (iv)
Question 9.
The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16
Answer:
(b) 10
Question 10.
Identify the Mg2+ ion from the fig. where, n and p represent the number of neutrons and protons respectively :
Answer:
Question 11.
In a sample of ethyl ethanoate (CH3COOC2H5), the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons.
(b) One of the oxygen atoms has gained two neutrons.
(c) The two oxygen atoms are isotopic.
(d) The two oxygen atoms are isobars.
Answer:
(c) The two oxygen atoms are isotopic.
Question 12.
Elements with valency 1 are :
(a) Always metals
(b) Always metalloids
(c) Either metals or non-metals
(d) Always non-metals
Answer:
(c) Either metals or non-metals
Question 13.
The first model of an atom was given by :
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Answer:
(d) J.J. Thomson
Question 14.
An atom with 3 protons and 4 neutrons will have a valency of :
(a) 3
(b) 7
(c) 1
(d) 4
Answer:
(c) 1
Question 15.
The electron distribution in an aluminium atom is :
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8
Answer:
(a) 2, 8, 3
Question 16.
Which of the following fig. do not represent Bohr’s model of an atom correctly?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Answer:
(c)
Question 17.
Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Answer:
(a) An atom has equal number of electrons and protons.
Question 18.
Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order : (i) Rutherford’s atomic model, (ii) Thomson’s atomic model, (iii) Bohr’s atomic model.
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Answer:
(c) (ii), (i) and (iii)
Question 19.
Fill in the blanks in the following statements :
(a) Rutherford’s α-particle scattering experiment led to the discovery of the ……………….
(b) Isotopes have same ………………. but different ……………….
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be ………………. and ………………. respectively.
(d) The electronic configuration of silicon is ………………. and that of sulphur is ……………….
Answer:
(a) nucleus
(b) same atomic numbers, mass numbers
(c) 0 and -1
(d) Si = 2, 8, 4 and S = 2, 8, 6
Question 20.
Match the items in column A with column B :
Column A | Column B |
(i) 18Ar40 and 20Ca40 | (a) Isotopic |
(ii) 17Cl35 and 17Cl37 | (b) Electrons in L shell of Ne |
(iii) 3, 5 | (c) Isomeric |
(iv) 8 | (d) Valency of phosphorus |
Answer:
Column A | Column B |
(i) 18Ar40 and 20Ca40 | (c) Isomeric |
(ii) 17Cl35 and 17Cl37 | (a) Isotopic |
(iii) 3, 5 | (d) Valency of phosphorus |
(iv) 8 | (b) Electrons in L shell of Ne |
Question 21.
Match the items in column A with column B :
Column A | Column B |
(i) Maximum electrons in a shell | (a) ρ + n |
(ii) Mass number | (b) Number of protons |
(iii) Atomic number | (c) Isotope of hydrogen |
(iv) Deuterium | (d) 2n2 |
Answer:
Column A | Column B |
(i) Maximum electrons in a shell | (d) 2n2 |
(ii) Mass number | (a) ρ + n |
(iii) Atomic number | (b) Number of protons |
(iv) Deuterium | (c) Isotope of hydrogen |
Question 22.
Read the statements carefully and identify whether they are True or False-
1. All atoms, except hydrogen, have neutrons in the nucleus.
2. Outermost, orbit of an atom consisting eight atoms is called octet.
3. Neutron is charged and has mass equal to proton.
4. The maximum number of electrons in M shell of an atom is 32.
Answer:
1. True
2. True
3. False
4. False
Very Short Answer Type Questions
Question 1.
Why did Rutherford select a gold foil in his a-ray scattering experiment?
Answer:
Rutherford selected a gold foil in his α-ray scattering experiment because gold has high malleability and can be hammered into thin sheets.
Question 2.
Is Cl-35 and Cl-37 have different valencies?
Answer:
No, it is because these are isotopes of chlorine that have same atomic number but different mass number.
Question 3.
Name the isotope used for treatment of cancer.
Answer:
Isotope of cobalt: Co-60.
Question 4.
What is the charge and mass of α-particles?
Answer:
Charge is 2 and mass is 4 amu.
Question 5.
What is the mass of a neutron?
Answer:
The mass of a neutron is 1.675 × 10-24 g.
Question 6.
Which is much closer to the nucleus of an atom out of K and L shells?
Answer:
K shell is much closer to the nucleus of an atom.
Question 7.
Which shell can accommodate a maximum of 32 electrons?
Answer:
Fourth shell can accommodate a maximum of 32 electrons.
Question 8.
Name the radioisotope used for examining the circulation of blood in the body.
Answer:
Na-24 is the radioisotope used for examining the circulation of blood in the body.
Question 9.
What are valence electrons?
Answer:
The electrons present in the outermost shell of an atom are known as valence electrons.
Question 10.
What is an anion?
Answer:
When an atom gains one or more electrons, it becomes negatively charged and is known as anion.
Question 11.
How does an atom become a cation?
Answer:
When an atom acquires positive charge by losing one or more electrons, it is called cation.
Question 12.
What are nucleons?
Answer:
Protons and neutrons reside in the nucleus of an atom and are thus called nucleons.
Question 13.
Why isotopes of an element are chemically similar?
Answer:
Isotopes of an element are chemically similar because these have same electronic configuration.
Question 14.
Why an atom is electrically neutral?
Answer:
An atom is electrically neutral because it contains equal number of positively charged particles and negatively charged particles, i.e. protons and electrons.
Question 15.
Why noble gases show least reactivity?
Answer:
Noble gases have their outermost octet completely filled up, so these gases show least reactivity.
Question 16.
Define atomic number. How is it denoted?
Answer:
Number of protons of an atom determines its atomic number. It is denoted by ‘Z’.
Question 17.
Which study led to the conclusion that atoms are not indivisible?
Answer:
Study of static electricity and the condition under which electricity is conducted by different substances led to the conclusion that atoms are not indivisible.
Question 18.
What is the charge and mass of alpha particle?
Answer:
Charge = + 2 units
Mass = 4 units
Question 19.
An uncharged particle is found in the nucleus of an atom. Identify this uncharged particle.
Answer:
Neutron.
Question 20.
Name the scientist who concluded that the size of nucleus is very small as compared to the size of atom.
Answer:
Rutherford.
Question 21.
Helium atom has two electrons in its valence shell but its valency is not two. Why?
Answer:
Helium atom has two electrons in its valence shell but its valency is not two as its outermost shell is completely filled, its combining capacity or valency is zero.
Question 22.
Write the symbols of two isotopes of uranium.
Answer:
Uranium 238 (92U238) and Uranium 235 (92U235).
Question 23.
If an atom contains one electron and one proton, will it carry any charge or not?
Answer:
It will not carry any charge because the positive charge on the proton neutralises the negative charge on the electron.
Question 24.
Name the scientist who discovered neutrons.
Answer:
J. Chadwick.
Question 25.
Give two uses of isotopes in the field of medicines.
Answer:
- An isotope of cobalt is used in the treatment of cancer.
- An isotope of iodine is used in the treatment of goitre.
Question 26.
Why do noble gases show low reactivity?
Answer:
The outermost shell of the atoms of noble gases are complete. So, they show little chemical reactivity.
Question 27.
Is an atom electrically neutral?
Answer:
Yes, because number of protons (+ve charge) inside the nucleus are equal to the number of electrons (-ve charge) outside the nucleus.
Question 28.
Are noble gases inert?
Answer:
The outermost shell of the atoms of noble gases are completely filled. So, they do not show affinity towards chemical reactions.
Question 29.
What is meant by electronic configuration of elements?
Answer:
The systematic distribution of electrons in various orbits of an atom is called electronic configuration of elements.
Question 30.
What kind of elements have a tendency to lose electrons? What are they commonly called?
Answer:
The elements having 1, 2 or 3 valence electrons have the tendency to loose electrons. They are commonly called metals.
Question 31.
What kind of elements have a tendency to gain electrons? What are they commonly called?
Answer:
The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.
Question 32.
How are the canal rays different from electrons in terms of charge and mass?
Answer:
Electrons are negatively charged particles, mass of which is approximately 1/2000 that of canal rays.
Short Answer Type Questions
Question 1.
In what way the Rutherford proposed atomic model? What were the drawbacks of Rutherford’s model?
Answer:
Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus.
Drawbacks: The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.
Question 2.
Give sketch of Thomson’s proposed atomic model. What are the limitations of J. J. Thomson’s model?
Answer:
Thomson proposed the model of an atom to be similar to a Christmas pudding. The electrons are studded like currents in a positively charged sphere like Christinas pudding and the mass of the atom was supposed to be uniformly distributed.
Limitations : Thomson’s atomic model failed to explain how the positively charge holds the negative charge inside the atom. It also failed to explain an atom’s stability. It could not mention anything about the nucleus of an atom.
Question 3.
Is there any relationship between atomic number, mass number, isotopes, isobars and valency of an atom? Explain.
Answer:
Atomic number: It tells the number of protons (Z).
Atomic Mass: Total number of proton and neutron (A) is called atomic mass.
Isotopes: When atoms of same element have same number of protons but . different number of neutrons is called isotopes.
Isobars: When atoms of different element have saihe atomic mass but different atomic number, such atoms are called isobars.
Valency: It is the combining capacity of an atom or it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.
Question 4.
Give important properties of cathode rays.
Answer:
- Cathode rays travel in a straight line.
- Cathode rays can rotate a light wheel placed in their path.
- They ionise gas through which they pass.
- They are deflected by magnetic field.
- They can penetrate through thin metallic sheet.
- Mass of cathode ray particle is too small.
Question 5.
How has atomic number improved the definition of an element?
Answer:
- Atomic number of an element = number of proton = number of electron
- Atomic number gives the position of the element in a periodic table. An element can now be defined as a substance comprising of atoms all of which have same atomic number.
Question 6.
State the properties of isotopes.
Answer:
Isotopes have the following uses :
- Treatment of disease : Radioactive isotopes are used for the treatment of dreadful diseases like cancer. Cobalt-60 is used to kill malignant cells in patients suffering from cancer.
- It is used in chemical analysis.
- It is used to detect disease in plants.
- Dating of plants/animals being obtained by using carbon-14 after excavation.
- Thyroid disorders can be treated by I-131.
- Leukaemia can be treated by using P-32.
- Uranium-235 can produce electricity.
Question 7.
Give reasons :
(i) Mass number of an atom excludes the mass of an electron.
(ii) Nucleus of an atom is charged.
(iii) Alpha-particle scattering experiment was possible by using gold foil only and not by foil of any other metal.
Answer:
(i) Mass number of an atom excludes the mass of an electron because electrons have negligible mass in comparison to protons and neutrons.
(ii) Nucleus of an atom consists of protons and neutrons. Protons are positively charged particles. So, the nucleus of an atom is charged.
(iii) Because an extremely thin film was required for the experiment and it was only possible by using gold, as gold is a highly malleable metal.
Question 8.
(i) An ion X2+ contains 10 electrons and 12 neutrons. What is the atomic number and mass number of the element X?
(ii) Is it possible in an atom to have 12 protons and 13 electrons?
(iii) Why helium gas is inert?
Answer:
(i) Atomic number = 12,
Mass number = 24.
(ii) No, it is not possible. An atom is electrically neutral. The number of positively charged particles (protons) is always equal to the number of negatively charged particles (electrons).
(iii) Helium atom has completely filled outermost shell. Thus, it is inert.
Question 9.
Define the following terms :
(i) Electronic configuration
(ii) Valence shell
(iii) Valency
Answer:
(i) The distribution of electrons amongst different orbits of an atom is known as electronic configuration.
(ii) The outermost shell of an atom is called its valence shell.
(iii) The combining capacity of an atom is called its valency or the number of electrons lost or gained by an atom to acquire noble gas configuration.
Question 10.
Describe briefly Thomson’s model of an atom.
Answer:
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 as a whole is electrically neutral.
Question 11.
Give four characteristics of isotopes.
Answer:
All isotopes of an element consist of the same number of protons inside their nuclei. Hence, they have the same atomic number.
- All isotopes of an element consist of different number of neutrons in their nuclei. Hence, they have different mass number.
- All isotopes of an element give identical chemical reactions.
- Isotopes of an element have same electronic configuration.
Question 12.
What are canal rays? Who discovered them? What is the charge and mass of canal ray?
Answer:
New radiations in a gas discharge tube which are positively charged are known as -canal rays. They were discovered by E. Goldstein. Charge on canal rays is positive and its mass is one unit.
Question 13.
If Z = 3, what would he the valency of the element? Also, name the element.
Answer:
Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
Valence shell has 1 electron in the outermost shell, so valency of element is 1.
The element is lithium.
Question 14.
Write three -main features of Rutherford’s nucleus model of an atom.
Answer:
On the basis of α-particle scattering experiment, Rutherford proposed a model of atom. According to him :
- There is positively charged centre in an atom called the nucleus which contains the whole mass of the atom.
- The electrons revolve around the nucleus.
- The size of the nucleus is very small as compared to the size of the atom.
Question 15.
Write the electronic configuration of any one pair of isotopes and isobars.
Answer:
Question 16.
State the major drawback in Rutherford’s model of an atom. Mention two features of Bohr’s model which helped compensate this drawback.
Answer:
The major drawback of Rutherford’s model of an atom is that it does not explain the stability of an atom. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. So, revolving electron would lose energy and finally fall into the nucleus.
Two features of Bohr’s model which helped to resolve this drawback :
Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
While revolving in these discrete orbits, the electrons do not radiate energy.
Question 17.
Define isotopes. Why do isotopes have same atomic number but different mass number? Explain with the help of an example.
Answer:
Atoms of the same element, having the same atomic number but different mass numbers are called isotopes.
Isotopes have same atomic number but different mass number because they contain different number of neutrons.
For example: In nature chlorine occurs in two isotopic forms; 17Cl35 and 17Cl37. Here atomic number of both the atoms is same but due to the difference in the number of neutrons, their mass number is different.
Question 18.
If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element? Name the element.
Answer:
The maximum numbers of electrons that can occupy K and L shells of an atom are 2 and 8 respectively. Therefore, if K and L shells of an atom are full then the total number of electrons in the atom would be 2 + 8 = 10 electrons. So, the valency of this element is zero. The element is neon (Ne).
Question 19.
Define the term “Half life”.
Answer:
Radioactive elements are unstable and disintegrate with time emitting alpha and beta particles along with gamma rays. The rate of disintegration depends on the amount of substance. As the amount of the substance decreases, disintegration becomes slower. But it takes a long time for whole of the substance to decay. Thus, the rate of radioactive decay is measured in terms of half time. The time taken by half of the atoms of radioactive element to disintegrate is called its half-time.
Question 20.
Give difference between isotopes and isobars.
Answer:
Isotopes | Isobars |
1. They are atoms of same element. | 1. They are atoms of different elements. |
2. Have same atomic number. | 2. Have different atomic number. |
3. Have different mass number. | 3. Have same mass number. |
4. Have similar chemical properties. | 4. Have different chemical properties. |
Question 21.
Explain Rutherford’s atomic model.
Answer:
Rutherford proposed a model of an atom on the basis of α-particles scattering experiment. This is known as Rutherford’s nuclear model of atom.
- An atom consists a heavy positively charged core called nucleus.
- Nucleus is surrounded by electrons.
- Electrons and nucleus are held together by electrostatic force of attraction.
- Size of nucleus is very small as compared to the size of atom.
- Almost the entire mass of the atom is concentrated in the nucleus.
Question 22.
What was Chadwick’s experiment?
Answer:
In Chadwick’s experiment, he bombarded beryllium atoms with high speed particles.
- The rays emitted from beryllium during bombardment had speeded about one-tenth the speed of light.
- Later Chadwick was able to show that these rays consisted of neutral particles called neutron.
Question 23.
Give the number of electron, proton and neutron in 27CO59 and 47Ag108.
Answer:
In 27CO59
Number of protons = 27
Number of electrons = 27
Number of neutrons = 59 – 27 = 32
In 47Ag108
Number of protons = 47
Number of electrons = 47
Number of neutrons = 108 – 47 = 61
Long Answer Type Questions
Question 1.
Explain Rutherford’s alpha particle scattering experiment. Write the observations and describe Rutherford’s atomic model on the basis of observations.
Answer:
Ernest Rutherford wanted to know how the electrons are arranged within an atom. In this experiment Rutherford passed beams of alpha (α) particles through a thin gold foil to observe the atom. He chose gold foil because he wanted as thin layer as possible and gold foil was about 1000 atoms thick.
α-particles are doubly-charged helium ions. Since they have a mass of 4μ, the fast moving α-particles have a considerable amount of energy. It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since α-particles were much heavier than protons, he did not expect to see large deflections. But the experiment gave unexpected results. According to the experiment following observations were made-
- Most of the fast moving α-particles passed straight through the gold foil.
- Some of the α-particles were deflected by the foil by small angles.
- Surprisingly one out of every 12000 particles appeared to rebound.
Rutherford concluded from the α-particle scattering experiment that-
- Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
- Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
- A very small fraction of α-particles were deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
On the basis of experiment, Rutherford put forward the nuclear model of an atom, which had the following features-
- There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
- The electron revolve around the nucleus in circular paths.
- The size of the nucleus is very small as compared to the size of the atom.
Question 2.
Give electronic configuration of the first 18 elements.
Answer:
From the Bohr-Bury scheme, electronic configuration of first 18 elements can be written as follows:
Question 3.
Chlorine occurs in nature in two isotopic forms with masses 35 μ and 37 μ. What should be the mass of chloride for the chemical calculations? Explain.
Answer:
The mass of an atom of any natural element is taken as the average mass of all the naturally occuring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, we have to know the percentage of each isotopic form and then the average mass is calculated.
Similarly the average mass of chlorine has to be calculated :
Chlorine occurs in nature in two isotopic forms with masses 35 μ and 37 μ in the ratio of 3 : 1.
So, the average mass of chlorine will be
So the mass of chlorine for the chemical calculations should be taken 35.5 μ.
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