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NIOS Class 10 Science and Technology Chapter 5 Atomic Structure
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Atomic Structure
Chapter: 5
INTEXT QUESTIONS 5.1
1. Name two charged particles which constitute all matter.
Ans: Electrons and protons.
2. Describe a cathode ray tube.
Ans: 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?
Ans: Electron.
4. Why do the canal rays obtained by using different gases have different e/m values?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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.
Ans: 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.
Ans: 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.
Ans: The 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?
Ans: 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?
Ans: It is a neutral subatomic particle present in the nucleus of the atom.
2. How many neutrons are present in the α-particle?
Ans: An α-particle contains two neutrons.
3. How will you distinguish between an electron and a proton?
Ans: The electron and proton can be distinguished in terms of their charge and mass. While the electron is negatively charged, the proton is positively charged. Secondly, the proton is much heavier than the electron; it is about 1840 times heavier.
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.
Ans: No of protons = 11
No. of electrons = 11
No. of neutrons = 12.
2. What is the mass number of an atom which has 7 protons and 8 neutrons?
Ans: Mass number = number of protons + number of neutrons Therefore, mass number = 7 + 8 = 15.
3. Calculate the number of electrons, protons and neutrons in 40 Ar/ 18 and 49 /K 19.
Ans: 40 Ar 18: Number of protons = atomic number = 18
Number of electrons = number of protons = 18
Number of neutrons = mass number – number of protons = 40-18 = 22
40/ K 19 Number of protons = atomic number = 19 Number of electrons = number of protons = 19
Number of neutrons = mass number – number of protons = 40 – 19 = 21.
INTEXT QUESTIONS 5.6
1. How many shells are occupied in the nitrogen (atomic number =7) atom?
Ans: The electronic configuration of nitrogen is 2, 5. Thus, two shells are occupied. The first shell (capacity = 2) is completely filled while the second shell (capacity = 8) is partially filled.
2. Name the element which has completely filled first shell.
Ans: Helium.
3. Write the electronic configuration of an element having atomic number equal to 11.
Ans: The electronic configuration of an element having atomic number 11 is 2, 8, 1.
| TERMINAL EXERCISE |
1. How did J.J.Thomson discover the electron? Explain his “plum pudding” model of the atom.
Ans: J.J. Thomson discovered the electron through his experiments with cathode rays. In 1897, he conducted experiments using a cathode ray tube, a sealed glass tube containing low-pressure gas. When a high voltage was applied across the tube, a beam of particles was produced, which he called cathode rays.
Thomson observed that these cathode rays were attracted to a positively charged plate, suggesting that they were negatively charged particles. By measuring the deflection of the cathode rays in magnetic and electric fields, Thomson was able to determine the charge-to-mass ratio of these particles. He found that it was much larger than that of any known atom, leading him to conclude that they were fundamental particles, which he named “corpuscles” but later became known as electrons.
Based on his discovery of the electron, Thomson proposed the “plum pudding” model of the atom. In this model, he envisioned the atom as a positively charged sphere with embedded negatively charged electrons scattered throughout, much like plums in a pudding. This model suggested that the atom was electrically neutral overall, with the positive and negative charges balancing each other out.
Thomson’s plum pudding model was a significant step forward in understanding atomic structure because it provided a way to reconcile the observed properties of cathode rays with the existing understanding of atoms at the time. However, later experiments, such as Ernest Rutherford’s gold foil experiment, led to the development of the nuclear model of the atom, which replaced Thomson’s model.
2. What made Thomson conclude that all atoms must contain electrons?
Ans: Thomson’s model states that a large sphere of uniform positive charge with a number of small negatively charged electrons scattered around this outer surface. This model was called the plum pudding model.
3. Identify the following subatomic particles:
(a) The number of these in the nucleus is equal to the atomic number.
Ans: Proton has the same number as the atomic number.
(b) The particle that is not found in the nucleus.
Ans: Electrons are not found in nucleus of an atom
(c) The particle that has no electrical charge.
Ans: – Neutron has no electrical charges on it.
(d) The particle that has a much lower mass than the others subatomic particles.
Ans: Electron have a much lower mass than the other subatomic particle.
4. Which of the following are usually found in the nucleus of an atom?
(a) Protons and neutrons of.
(b) Protons, neutrons and electrons.
(c) Neutrons only.
(d) Electrons and neutrons only.
Ans: (a) Protons and neutrons of.
5. Describe Ernest Rutherford’s experiment with alpha particles and gold foil. How did this lead to the discovery of the nucleus?
Ans: Ernest Rutherford and his co-workers were working in the area of radioactivity. They were studying the effect of alpha (α) particles on matter. The alpha particles are helium nuclei, which can be obtained by the removal of two electrons from the helium atom. In 1910, Hans Geiger (Rutherford’s technician) and Ernest Marsden (Rutherford’s student) performed the famous α-ray scattering experiment. This led to the failure of Thomson’s model of atom. Let us learn about this experiment.
(i) Ray scattering experiment: In this experiment a stream of α particles from a radioactive source was directed on a thin (about 0.00004 cm thick) piece of gold foil. On the basis of Thomson’s model it was expected that the alpha particles would just pass straight through the gold foil and could be detected by a photographic plate placed behind the foil. However, the actual results of the experiment, Fig. 5.5, were quite surprising.
It was observed that:
(i) Most of the α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by small angles.
(iii) A few particles were deflected by large angles.
(iv) About 1 in every 12000 particles experienced a rebound.
6. What does the atomic number tell us about an atom?
Ans: The atomic number tell us about an atomic are:
Number of Electrons in a Neutral Atom: In an electrically neutral atom, the number of electrons surrounding the nucleus is equal to the number of protons. This means that atoms typically have the same number of positive protons as negative electrons, resulting in a balanced overall charge of zero.
Chemical Properties: The number of protons, and hence the atomic number, also determines the chemical properties of an element. Elements with similar atomic numbers often exhibit similar chemical behaviour because they have the same number of electrons in their outer shells, which determines how they interact with other atoms.
Identity of the Element: The atomic number uniquely identifies a specific element. Every atom of a particular element has the same number of protons in its nucleus, and this number defines the element. For example, all atoms of Hydrogen have 1 proton, all Oxygen atoms have 8 protons, and so on. Therefore, the atomic number essentially tells you what element you’re dealing with.
7. What is the relationship between the numbers of electrons and protons in an atom?
Ans: Relationship between the atomic mass and atomic number (the number of protons). This problem was solved later by Chadwick by discovering neutron, the third particle constituting the atom. The problem of the stability of the atom and the distribution of electrons in the atom was solved by Neils Bohr by proposing yet another model of the atom. This is discussed in the next section.
8. How did Neils Bohr revise Rutherford’s atomic model?
Ans: The Neil’s Bhor states that in an atomic model there are different energy levels in that and an electron releases some energy when it travels from the upper to lower levels and gains or needs some energy when it travels lower to upper energy levels.
9. What is understood by a stationary state?
Ans: A stationary state is called stationary because the system remains in the same state as time elapses, in every observable way. For a single-particle Hamiltonian, this means that the particle has a constant probability distribution for its position, its velocity, its spin, etc.
10. What is a shell? How many electrons can be accomodate in L-shell?
Ans: According to Bohr’s Atomic model electrons revolve around the nucleus in a specific circular path known as orbit or called a shell. Shells have stationary energy levels, the energy of each shell is constant. Each stationary orbit or shell is associated with a definite amount of energy.
The L-shell is the second principal energy level (n = 2) in an atom.
According to the Aufbau principle, which governs how electrons fill atomic orbitals, the L-shell can hold a maximum of 8 electrons.
11. State the rules for writing the electronic configuration of elements.
Ans: The electronic configuration of an element is states that about the different energy levels of that element. And are rules that the electron first get in the K shell and then passes to the other shell in order of K>L>M.
