Class 9 Chemistry: Chapter 01 Structure of Atom - Questions and Answers
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Class 9 Chemistry Questions and Answers
Chapter 01 Structure of Atom
* Law of conservation of mass: Mass is neither created nor destroyed in a chemical reaction. In a chemical reaction, the total mass of reactants is equal to the total mass of products. This law was proposed by Antoine Lavoisier.
* Law of constant proportion: A simple whole-number ratio exists between the masses of combining elements in a compound.
Eg:-
i.In carbon monoxide, the ratio of the masses of carbon and oxygen is 3:4
ii. In water, the mass ratio between hydrogen and oxygen is 1:8
* Dalton's Atomic Theory: Major Ideas.
• Matter is made up of minute particles called atoms.
• Atoms can neither be created nor destroyed.
• Atoms of the same elements are identical in properties, size and mass.
• Atoms of different elements differ in their properties and mass.
• Atom is the smallest particle that can take part in a chemical reaction.
• Compounds are formed when atoms of two or more elements combine in a simple ratio.
* Sub Atomic particles: According to Dalton's theory, it is impossible to have subatomic particles in an atom.
But the electrification of substances by rubbing different objects, the results of experiments conducted by Faraday and Davy by passing electricity through solutions gave indications of the presence of subatomic particles in atoms. Later the presence of subatomic particles electron (having negative charge), proton (having positive charge) and neutron (neutral particle) were confirmed.
* Relative atomic mass.
It is the method of expressing the mass of an atom by comparing its mass with the mass of a standard atom. 1/12 mass of carbon- 12 atom is accepted as one atomic mass unit (u).
* Thomson's model of the atom: Through discharge tube experiment, Goldstein predicted the presence of positively charged particles and J J Thomson found out the presence of negatively charged particle (electron) in the atom. Based on this, J J Thomson put forward Thomson's atom model. This is also called the plum pudding model.
According to this model, the atom is a sphere of positive charge and electrons are embedded somewhere in it. The total positive charge and total negative charge in an atom are equal and hence atom is electrically
neutral.
* Rutherford's model of Atom: It was confirmed the presence of nucleus and positive charge in an atom through a Gold foil experiment by Rutherford. Rutherford's atom model was proposed based on the inferences of the Gold foil experiment.
* Rutherford's Planetary Model of Atom: Major Ideas.
• Atoms have a centre called the nucleus.
• Size of the nucleus is too small compared to the size of the atom.
• Almost the complete mass of an atom is concentrated in its nucleus.
• Electrons revolve around the nucleus in shells.
Limitations of Rutherford's atom model.
According to electromagnetic theory, while electrons revolving around the nucleus they should emit electromagnetic radiation, lose energy and come closer to the nucleus and finally, electrons should collapse into the nucleus. But this does not happen in an atom. Rutherford failed to explain this.
* Bohr model of the atom: This model was proposed by Niels Bohr by resolving the limitations of Rutherford's atom model.
* Main Ideas of Bohr Model:
• Electrons revolve around the nucleus in fixed paths called orbits or shells.
• Electrons in each shell have definite energy and hence the shells are called energy levels.
• As long as an electron revolves in a particular orbit, its energy remains constant.
• The energy of the shells increases as the distance from the nucleus increases. These shells are named
K, L, M, N …. Or 1,2,3,4 … respectively.
* Chargeless particle in atoms: If the particles of an atom were only protons and electrons, the total mass of an atom should be the total mass of the protons and electrons in it. But it was realised that the mass of an atom was not equal to the masses of total masses of electrons and protons in it. This indicated the presence of another particle in the atom. In 1932, James Chadwick confirmed the presence of a neutral particle within the nucleus of an atom called a neutron. Its mass is slightly greater than that of the proton.
* Fundamental particles within an atom: Electron, proton and neutron are the fundamental particles of an atom. Of them electron is a negatively charged particle, the proton is positive and the neutron is charge less.
Protons and neutrons are confined inside the nucleus and electrons revolve around the nucleus in the orbit. The mass of proton and neutron is approximately equal and is 1u each. As the mass of an electron is too small compared to proton and neutron, an electron's mass is considered zero for practical purpose.
* the Mass number of atom (A): The total number of protons and neutrons in an atom is called the mass number and is represented by the letter 'A'.
* Atomic number: The total number of protons in an atom is called Atomic number(Z).
• The following relation can be used to find the fundamental particles in an atom.
Atomic number Z = Number of protons = Number of electrons
Mass number A = Number of protons + Number of neutrons
Number of neutrons = Mass number - Atomic number = A – Z
• Normally the number of protons and electrons in an atom are equal. Hence atomic number is also equal to the total number of electrons
in an atom.
The mass number and atomic number is expressed as superscript and subscript respectively to the left of the symbol as shown below.
Sodium: 11²³Na: Atomic number = 11,
mass number = 23,
Number of protons = 11,
number of electrons = 11,
number of neutrons = 23 – 11 = 12.
* Electronic configuration in an atom: The electrons in an atom are occupied in various shells. There is a certain limit for each shell to accommodate electrons in them. The electron accommodating capacity of the shells 1,2,3 & 4 (Or K,L.M&N) are 2,8,18,32 respectively. If the shell number is 'n', the maximum electron that can be accommodated in the shell is calculated using the formula 2n².
The filling of electrons will be according to the following principle.
• Maximum electrons in a shell will be 2n²
• Electrons will be filled in the lower energy shells first and then filled in the next higher shells and so on.
• The maximum electrons that can be accommodated in the outermost shell shall be 8.
* Electronic configuration and Bohr Model of a few elements are given.
* Isotopes: The number of protons in an atom determines the element. That is, an element will be the same even if there is a change in the number of neutrons or electrons in its atom.
Atoms of the same element having the same atomic number and different mass number are called Isotopes.
Example: Atoms of ordinary hydrogen contain one proton only and no neutron. So its atomic number and mass number are one. This hydrogen is known as protium. Hydrogen atoms that contain one proton and one neutron (mass number 2) are called deuterium. Hydrogen atoms of mass number 3 are called Tritium. That is protium, deuterium and tritium are the isotopes of hydrogen. Most of the elements have isotopes. To identify isotopes, their mass number is written along with the name of the
element.
Example:- Carbon -12, Carbon -13 and Carbon -14 are the isotopes of carbon.
Isotopes are of great importance in diverse fields.
• Deuterium, an isotope of hydrogen, is used in nuclear reactors.
• Carbon -14 is used to determine the age of fossils.
• Phosphorous-31 is used as a tracer in plants.
• Iodine -141 & Cobalt -60 are used for diagnosis and treatment of cancer and tumour. Uranium-235 is used as fuel for nuclear reactors.
PRACTICE QUESTIONS AND ANSWERS
1. The presence of which fundamental particle was discovered by J J Thomson? What is the charge of this article?
Answer: Electron. It is a negatively charged particle.
2. Which was the experiment that led to the Rutherford Atom Model.
Answer: Bypassing alpha particle through the thin gold foil.
3. Name the scientist who confirmed the presence of protons in an atom.
Answer: Rutherford.
4. According to the Rutherford model, where is the location of electrons in an atom?
Answer: In shells.
5. What was the major limitation of the Rutherford Atom Model?
Answer: According to electromagnetic theory, electrons should radiate electromagnetic radiation while it is revolving around the nucleus and eventually collapse into the nucleus when their energy is lost completely. But this does not happen in atoms. Rutherford could not explain the reason for this.
6. What are the major ideas proposed by Bohr Atom Model?
Answer: Electrons revolve around the nucleus of an atom in fixed paths called orbits or shells.
• Electrons in each shell have a definite energy. Hence the shells are also called Energy levels.
• As long as an electron revolves in a particular orbit, its energy remains constant.
• The energy of the shells increases as the distance from the nucleus increases.
7. The shells of atoms are also called energy levels. How are the shells represented?
Answer: They are represented as 1,2,3,4,5 Or K,L,M,N,O.
8. Which is the neutral particle in an atom? Who did confirm the presence of this article?
Answer: The neutron is the neutral fundamental particle. James Chadwick.
9. What are the fundamental particles? Where are they located in an atom?
Answer: Electron, proton and Neutron.
Protons and neutrons are confined in the nucleus and electrons revolve around the nucleus in shells.
10. Which are the fundamental particles having more less similar mass?
Answer: Proton and neutron.
11. Explain mass number and atomic number.
Answer: Total number of protons and neutrons is called the mass number. The number of protons is called the atomic number.
12. An atom consists of a subatomic charged particle. But an atom is electrically neutral. Why?
Answer: The number of protons and electrons present in an atom is equal. Since the charge of proton and electron are equal and opposite, the total charge of an atom will be zero.
13. Which is the fundamental particle that displaced during chemical reactions?
Answer: Electrons.
14. An atom consists of three types of fundamental particles. Name the particle that determines the identity of an element?
Answer: Protons.
15. The atomic number and mass number of an atom X are 11 and 23 respectively. How does it express along with its symbol? Find the number of protons, electrons and neutrons in this atom.
Answer: 11²³X
Number of protons = 11
Number of Electrons = 11 (number of protons = number of electrons)
Number of neutrons = A – Z = 23 – 11 = 12
16. Write down the equation for finding out the number of maximum electrons that can be accommodated in a shell. And find out the number of electrons in the first four shells.
Answer:
The maximum number of electrons in 'n' shell = 2n²
Number of electrons in the first shell = 2n² = 2x1² = 2
Number of electrons in the second shell = 2n² = 2x2² = 8
Number of electrons in the third shell = 2n² = 2x3² = 18
Number of electrons in the fourth shell = 2n² = 2x4² = 32
17. It is given a few elements and their atomic numbers. Write down their electronic configuration and draw Bohr Atom model. a. Boron -5 b. Sodium - 11, c. Argon -18 d.Beryllium -4
18. What are Isotopes? Which are the isotopes of hydrogen? Which isotope of hydrogen is used in nuclear rectors?
Answer: Atoms having the same atomic number but a different mass number are called isotopes. Protium (1¹H), Deuterium (1²H), Tritium (1³H) are the isotopes of hydrogen.
Deuterium is used in Nuclear rectors.
19. Carbon -14, Phosphorous -31, Cobalt – 60 and Uranium -235 are a few isotopes having great importance. Write down one use of each.
Answer: Carbon -14: Used for finding out the age of fossils.
Phosphorous - 31: Used as tracers for identifying the nutrient exchange in plants.
Cobalt -60: Used for diagnosis and treatment.
Uranium - 235: Used as fuel in Nuclear reactor.
20. Who were the two scientists who conducted the experiment by passing a current through solutions and gave the indication of the presence of charged particles in atoms?
Answer: Michael Faraday and Humphry Davy.
21. Fill the second pair according to the first pair.
a. K shell: 2 Electrons; M shell: …….. b. Deuterium: Mass number - 2 ; Protium: ……..
c. Carbon -12: 6 protons; Carbon -14: ….. protons
Answer:
a. 18 b. 1 c. 6 protons
22. Find the relation and match suitably.
23. Certain ideas of Dalton’s atomic theory are given below. Out of these find the wrong ones and rewrite them correctly.
(a) Molecule is the smallest particle that can take part in chemical reactions.
(b) Atoms of the same elements are not identical in properties, size and mass.
(c) Matter is made up of minute particles called atoms.
(d) Compounds are formed when atoms of two or more elements combine in a simple ratio.
Answer:
*Atom is the smallest particle that can take part in chemical reactions.
*Atoms of the same element are identical in properties, size and mass.
24. a)Name the path along which electrons revolve around the nucleus.
b)What happens to the energy of electrons as the distance from the nucleus increases?
Answer:
a) Shell or Orbit
b) The energy of electrons increases as the distance from the nucleus increases.
25. According to the Electromagnetic Theory a charged body in circular motion continuously emits energy as radiations.
What is the limitation of Rutherford’s atom model based on this theory?
Answer:
While electrons revolve around the nucleus, they should emit radiations, lose energy and come closure to the nucleus. Finally, the negatively charged electrons should collapse into the nucleus.
26. The maximum number of electrons that can be accommodated in the outermost shell of an atom(except the first shell) is ____________
Answer: 8
27. Name the scientist who suggested the planetary model of the atom.
(J.J Thomson, Rutherford, Neils Bohr )
Answer:
Rutherford
28. Negatively charged particles in an atom are known as _____
(Electrons, protons , Neutrons)
Answer:
Electrons
29. Name the Scientist who discovered negatively charged particles in an atom.
J.J Thomson, Rutherford, Goldstein, Chadwick)
Answer:
J.J Thomson
30. Name the part of an atom that contains protons and neutrons
(Shell, Nucleus, Neutron, Orbit)
Answer:
Nucleus
31. Atoms of elements are made up mainly of protons, electrons and neutrons.
a) Which are the particles present in the nucleus of an atom?
b) Which one of them has a positive charge?
c) Which one of these particles has a very negligible mass?
Answer:
a)Neutron, Proton
b) Proton
c) Electron
32. Give a reason for the following.
a) There are positively charged particles and negatively charged particles in the atom of an element. But atoms are electrically neutral.
b) Most of the mass of an atom is concentrated in its nucleus.
Answer:
a) The positively charged particles and negatively charged particles are equal in number.
b)The protons and neutrons are the particles that possess mass. They are present in the nucleus.
33. Certain findings in connection with the structure of the atom are given below. Select the scientist related to each finding from the names given in the bracket.
(Niels Bohr, Michael Faraday, Sir Humphrey Davy, Rutherford, J.J.Thomson)
a) Most of the mass of an atom is concentrated in its nucleus.
b) The rays originating from the cathode are made up of negatively charged particles.
c)Two types of electric charges are present in substances.
d) Electrons revolve around the nucleus in fixed paths called orbits or shells.
Answer:
a) Rutherford
b) J.J.Thomson
c) Sir Humphrey Davy
d) Niels Bohr
34. In the atom of an element, electrons are present in three shells. The outermost shell has 7 electrons. There are 18 neutrons in the nucleus of an atom.
a) Write down the electronic configuration of the given atom.
b) What is its mass number?
c) How many electrons are present in it?
Answer:
a) 2,8,7
b) 35
c) 17
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