Chapter 5. ATOMIC STRUCTURE
Chapter 5. Atomic Structure Class 10 Science and Technology (212) NIOS Textbook Solutions
Chapter 5. ATOMIC STRUCTURE
INTEXT QUESTIONS 5.1
1. Name two charged particles which constitute all matter.
Answer: The two charged particles that constitute all matter are electrons and protons .
2. Describe a cathode ray tube.
Answer: A cathode ray tube consists of two metal electrodes in a partially evacuated glass tube. The negatively charged electrode is called cathode while the positively charged electrode is called anode. These electrodes are connected to a high voltage source.
3. Name the negatively charged particles emitted from the cathode in the cathode ray tube?
Answer: The negatively charged particles emitted from the cathode in a cathode ray tube are electrons.
4. Why do the canal rays obtained by using different gases have different e/m values?
Answer: When the electrons emitted from the cathode collide with the neutral atoms of the gas present in the tube, these remove one or more electrons present in them. This leaves behind positive charged particles which travel towards the cathode. As the atoms of different gases have different number of protons present in them, these give positively charged ions with different e/m values.
INTEXT QUESTIONS 5.2
1. Describe Thomson’s model of atom. What is it called?
Answer: According to Thomson’s model, atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it. This model was called as plum pudding model.
2. What would have been observed in the α-ray scattering experiment if the Thomson’s model was correct?
Answer: If the Thomson’s model was correct, then most of the α-particles in the α-ray scattering experiment would have passed straight through the atom.
3. Who performed the α-ray scattering experiment and what were the observations?
Answer: The α-ray scattering experiment was performed by Geiger and Marsden. When a stream of α-particles from a radioactive source was directed on a thin piece of gold foil, most of the α-particles passed straight through the gold foil, some α-particles were deflected by small angles, a few particles by large angles and very few experienced a rebound.
4. Describe the model of atom proposed by Rutherford.
Answer: According to Rutherford’s model, the atom contains a dense and positively charged region called nucleus at its centre and the negatively charged electrons move around it. All the positive charge and most of the mass of atom is contained in the nucleus.
INTEXT QUESTIONS 5.3
1. Give any two drawbacks of Rutherford’s model of atom.
Answer: The Rutherford’s model could not explain the stability of the atom, the distribution of electrons and the relationship between the atomic mass and atomic number (the number of protons).
2. State the postulates of Bohr’s model.
Answer: The two postulates of Bohr’s model are :
(i) The electrons move in definite circular paths of fixed energy around a central nucleus.
(ii) The electron can change its orbit or energy level by absorbing or releasing energy.
3. How does Bohr model of an atom explain the stability of the atom?
Answer: The Bohr’s model explains the stability of atom by proposing that the electron
does not lose energy when present in a given energy level.
INTEXT QUESTIONS 5.4
1. What is a neutron and where is it located in the atom?
Answer: It is a neutral subatomic particle present in the nucleus of the atom.
2. How many neutrons are present in the α-particle?
Answer: An α-particle (alpha particle) contains 2 neutrons.
3. How will you distinguish between an electron and a proton?
Answer: The difference between an electron and a proton :
|
Electron |
Proton |
|
It has a negative charge (−1). |
It has a positive charge (+1). |
|
It has a very small mass. |
It is about 1840 times heavier than an electron. |
INTEXT QUESTIONS 5.5
1. A sodium atom has an atomic number of 11 and a mass number of 23. Calculate the number of protons, electrons and neutrons in a sodium atom.
Answer: Here, Atomic number = Number of protons = 11 and Mass number = 23
Electrons = 11
We know that, Number of neutrons = Mass number − Atomic number
Neutrons = 23 − 11 = 12
2. What is the mass number of an atom which has 7 protons and 8 neutrons?
Answer: Here, Number of protons = 7 and Number of neutrons = 8 .
We know that, Mass number = Number of protons + Number of neutrons
= 7 + 8 = 15
The mass number of the atom is 15.
3. Calculate the number of electrons, protons and neutrons in 
and 
.
Answer: We know that, Number of neutrons = Mass number − Atomic number
|
Atom |
Electrons |
Protons |
Neutrons |
|
|
18 |
18 |
40 − 18 = 22 |
|
|
19 |
19 |
49 − 19 = 30 |
INTEXT QUESTIONS 5.6
1. How many shells are occupied in the nitrogen (atomic number =7) atom?
Answer: Nitrogen has atomic number = 7, so it has 7 electrons.
Electronic configuration of nitrogen = 2, 5 .
First shell (K shell) = 2 electrons
Second shell (L shell) = 5 electrons
So, 2 shells are occupied in the nitrogen atom.
2. Name the element which has completely filled first shell.
Answer: The element which has a completely filled first shell is Helium (He).
[ The first shell can hold a maximum of 2 electrons, and helium has 2 electrons, so its first shell is completely filled.]
3. Write the electronic configuration of an element having atomic number equal to 11.
Answer: An element with atomic number = 11 has 11 electrons.
The electronic configuration = 2. 8. 1
TERMINAL EXERCISE
1. How did J.J.Thomson discover the electron? Explain his “plum pudding” model of the atom.
Answer: J. J. Thomson discovered the electron through cathode ray experiments, where he observed negatively charged particles moving in straight lines from the cathode.
His “plum pudding” model suggests that an atom is a positively charged sphere with negatively charged electrons embedded in it like raisins in a pudding. The positive charge is spread uniformly, and electrons are scattered inside to balance it, making the atom electrically neutral.
2. What made Thomson conclude that all atoms must contain electrons?
Answer: Thomson concluded that all atoms must contain electrons because cathode rays were produced irrespective of the type of gas used and the material of the electrodes. This showed that electrons are present in all atoms.
3. Identify the following subatomic particles:
(a) The number of these in the nucleus is equal to the atomic number
(b) The particle that is not found in the nucleus
(c) The particle that has no electrical charge
(d) The particle that has a much lower mass than the others subatomic particles
Answer: (a) The number of these in the nucleus is equal to the atomic number → Protons
(b) The particle that is not found in the nucleus → Electron
(c) The particle that has no electrical charge → Neutron
(d) The particle that has a much lower mass than the other subatomic particles → Electron
4. Which of the following are usually found in the nucleus of an atom?
(a) Protons and neutrons only
(b) Protons, neutrons and electrons
(c) Neutrons only
(d) Electrons and neutrons only
Answer: (a) Protons and neutrons only.
[ The nucleus of an atom contains protons and neutrons, while electrons move around the nucleus in shells. ]
5. Describe Ernest Rutherford’s experiment with alpha particles and gold foil. How did this lead to the discovery of the nucleus?
Answer: Ernest Rutherford performed the gold foil experiment by directing a beam of alpha particles at a very thin sheet of gold foil and observing their scattering on a fluorescent screen.
Most alpha particles passed straight through, some were deflected, and a few bounced back. This showed that most of the atom is empty space, but a small, dense, positively charged region exists at the centre. This led to the discovery of the nucleus, where nearly all the mass and positive charge of the atom is concentrated.
6. What does the atomic number tell us about an atom?
Answer: The atomic number tells us the number of protons present in the nucleus of an atom. In a neutral atom, it is also equal to the number of electrons.
7. What is the relationship between the numbers of electrons and protons in an atom?
Answer: The relationship between the number of electrons and protons in an atom is that the number of electrons is equal to the number of protons.
8. How did Neils Bohr revise Rutherford’s atomic model?
Answer: Niels Bohr revised Rutherford’s atomic model by proposing that:
(i) Electrons revolve around the nucleus only in certain fixed circular paths called shells or orbits.
(ii) Each shell has a fixed amount of energy and is called a stationary state.
(iii) Electrons do not lose energy while moving in these fixed orbits.
(iv) Energy is absorbed or emitted only when an electron moves from one shell to another.
9. What is understood by a stationary state?
Answer: A stationary state is a fixed energy state of an atom in which an electron moves around the nucleus without gaining or losing energy. In this state, the electron remains in a particular shell or orbit.
10. What is a shell? How many electrons can be accomodate in L-shell?
Answer: A shell is a fixed energy level or orbit around the nucleus in which electrons are present.
The L-shell is the second shell of an atom and can accommodate a maximum of 8 electrons.
11. State the rules for writing the electronic configuration of elements.
Answer: The rules for writing the electronic configuration of elements are:
(i) Electrons are filled in shells in order starting from the innermost shell (K, L, M, N …).
(ii) The maximum number of electrons in a shell is given by the formula 2n² , where n = shell number.
[ If n =1 , then K shell = 2 × 1² = 2
If n = 2 , then L shell = 2 × 2² = 2 × 4 = 8
If n = 3 , then M shell = 2 × 3² = 2 × 9 = 18
If n = 4 , then N shell = 2 × 4² = 2 × 16 = 32 ]
(iii) The outermost shell cannot contain more than 8 electrons.
(iv) A shell is not occupied until the inner shells are filled first.
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