Exploration - Science solution
Class 9 - Chapter 8: Journey Inside the Atom
Exercise Solution
Question 1
Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford’s Gold Foil Experiment.
| Statement | Correct / Incorrect | Reason |
|---|---|---|
| (i) The experiment clearly showed the existence of neutrons in the nucleus. | Incorrect | Rutherford's experiment did not discover neutrons. The neutron was discovered later by James Chadwick (1932). |
| (ii) The results disproved the Plum Pudding Model and led to the idea of a nucleus at the centre of the atom. | Correct | The large deflections of a few alpha particles proved that the positive charge is concentrated in a very small central nucleus, contradicting Thomson's model. |
| (iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre. | Correct | Only a dense, positively charged nucleus could produce such large deflections and occasional backward scattering. |
| (iv) The way alpha particles were deflected showed that electrons move around the nucleus. | Incorrect | The Gold Foil Experiment provided evidence for the nucleus only. It did not explain the arrangement of electrons around the nucleus. |
Final Answer: The correct statements are (ii) and (iii).
Question 2
Which of the following statements are correct or incorrect according to Bohr's Atomic Model? Give a reason for each statement.
| Statement | Correct / Incorrect | Reason |
|---|---|---|
| (i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus. | Incorrect | According to Bohr, electrons moving in fixed orbits do not lose energy. Therefore, atoms remain stable. |
| (ii) Electrons can exist anywhere around the nucleus with no fixed energy. | Incorrect | Electrons occupy only specific shells having definite energy levels. |
| (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. | Correct | This is the basic postulate of Bohr's atomic model. |
| (iv) Electrons can be found between energy levels as they move around the nucleus. | Incorrect | Electrons cannot exist between two energy levels. They jump directly from one shell to another by absorbing or releasing energy. |
Final Answer: Only statement (iii) is correct.
Question 3
The composition of the nuclei of three atomic species X, Y and Z is given below.
| Species | Protons | Neutrons | Mass Number |
|---|---|---|---|
| X | 18 | 19 | 37 |
| Y | 17 | 18 | 35 |
| Z | 17 | 20 | 37 |
(i) Relation between Y and Z
Y and Z have the same number of protons (17) but different numbers of neutrons.
Therefore, Y and Z are isotopes of the same element.
(ii) Relation between Z and X
Z and X have the same mass number (37) but different atomic numbers.
Therefore, Z and X are isobars.
Question 4
What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part based on the few alpha particles that bounced back or were deflected at large angles?
From the observations of the Gold Foil Experiment, Rutherford concluded that:
- The positive charge of the atom is concentrated in a very small region called the nucleus.
- The nucleus is located at the centre of the atom.
- The nucleus is extremely small, dense and contains almost all the mass of the atom.
- Most of the atom is empty space, which is why most alpha particles passed straight through the foil.
| Observation | Conclusion |
|---|---|
| Most alpha particles passed straight through. | Most of the atom is empty space. |
| A few particles were deflected. | The positive charge is concentrated. |
| Very few particles bounced back. | The nucleus is very small, dense and massive. |
Question 5
Arrange the following statements in the correct chronological order to show how atomic models evolved over time.
Correct Order:
- (iv) Dalton's model described atoms as indivisible particles.
- (ii) Thomson's Plum Pudding Model proposed electrons embedded in a positively charged sphere.
- (iii) Rutherford's Nuclear Model proposed a dense central nucleus.
- (i) Bohr's Model proposed fixed energy orbits around the nucleus.
| Scientist | Atomic Model | Main Contribution |
|---|---|---|
| John Dalton | Solid Sphere Model | Atoms are indivisible particles. |
| J. J. Thomson | Plum Pudding Model | Electrons are embedded in a positively charged sphere. |
| Ernest Rutherford | Nuclear Model | Discovered the dense central nucleus. |
| Niels Bohr | Bohr's Atomic Model | Electrons move in fixed energy shells without losing energy. |
Chronological Order: (iv) → (ii) → (iii) → (i)
Question 6
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
Electrons are negatively charged particles, whereas the nucleus contains positively charged protons. According to the law of electrostatic attraction, opposite charges attract each other. Therefore, the positively charged nucleus continuously attracts the electrons and keeps them bound to the atom.
According to Bohr's Atomic Model, electrons revolve only in certain fixed energy levels (shells). While moving in these allowed orbits, they do not lose energy. As a result, they neither fall into the nucleus nor fly away from the atom. They remain stable unless they absorb sufficient energy to move to a higher energy level or leave the atom.
| Reason | Explanation |
|---|---|
| Electrostatic Attraction | The positively charged nucleus attracts negatively charged electrons. |
| Fixed Energy Levels | Electrons revolve only in permitted shells having fixed energy. |
| No Energy Loss | Electrons do not lose energy while moving in these stationary orbits. |
| Atomic Stability | The atom remains stable because electrons neither collapse into the nucleus nor escape from the atom. |
Question 7
Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure.
Reason (R): The number of electrons is equal to the number of protons in an atom.
Answer:
Option (ii): Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
| Statement | Explanation |
|---|---|
| Assertion | Correct. The discoveries of electrons, protons and neutrons helped scientists understand the internal structure of atoms. |
| Reason | Correct. In a neutral atom, the number of electrons equals the number of protons. |
| Why Reason is not the explanation? | Equal numbers of electrons and protons explain electrical neutrality, not how the discovery of subatomic particles led to the development of atomic structure. |
Question 8
Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
Given:
- Atomic Number (Z) = 12
- Mass Number (A) = 24
Calculations:
- Number of Protons = Atomic Number = 12
- Number of Electrons = Number of Protons = 12
- Number of Neutrons = Mass Number − Atomic Number
- = 24 − 12 = 12
| Particle | Number |
|---|---|
| Protons | 12 |
| Neutrons | 12 |
| Electrons | 12 |
Electronic Configuration
The 12 electrons are distributed according to the Bohr-Bury rules as follows:
| Shell | Electrons |
|---|---|
| K | 2 |
| L | 8 |
| M | 2 |
Electronic Configuration = 2, 8, 2
Question 9
Find the following information for the elements shown in Fig. 8.17.
| Figure | Element | Symbol | Total Electrons | Valence Electrons | Valency | Protons | Atomic Number |
|---|---|---|---|---|---|---|---|
| (a) | Helium | He | 2 | 2 | 0 | 2 | 2 |
| (b) | Lithium | Li | 3 | 1 | 1 | 3 | 3 |
| (c) | Sodium | Na | 11 | 1 | 1 | 11 | 11 |
| (d) | Carbon | C | 6 | 4 | 4 | 6 | 6 |
Note: The identification is based on the electronic arrangements shown in Fig. 8.17 of the chapter
Question 10
Both Rutherford's and Bohr's models have electrons orbiting the nucleus. Why did Rutherford's model fail to explain atomic stability, while Bohr's model succeeded?
Answer:
Rutherford proposed that electrons revolve around the positively charged nucleus like planets revolve around the Sun. However, according to the laws of classical physics, a charged particle moving in a circular path continuously loses energy. Therefore, the electrons should gradually lose energy, spiral inward, and finally fall into the nucleus. If this happened, atoms would collapse and matter would not exist in a stable form. Hence, Rutherford's model failed to explain the stability of atoms.
Bohr solved this problem by proposing that electrons move only in certain fixed energy levels (shells) called stationary orbits. While revolving in these permitted orbits, electrons do not lose energy. They can move from one shell to another only by absorbing or emitting a fixed amount of energy. Thus, Bohr's model successfully explained why atoms remain stable.
| Rutherford's Model | Bohr's Model |
|---|---|
| Electrons revolve around the nucleus. | Electrons revolve around the nucleus in fixed energy levels. |
| Could not explain why electrons do not lose energy. | Explained that electrons do not lose energy while moving in allowed orbits. |
| Predicted that electrons should fall into the nucleus. | Electrons remain stable in stationary orbits. |
| Failed to explain atomic stability. | Successfully explained atomic stability. |
Question 11
An atom 70X has 31 electrons. How many neutrons are there in its nucleus?
Given:
- Mass Number (A) = 70
- Atomic Number (Z) = 31
- Electrons = 31
Since the atom is neutral,
Number of Protons = Number of Electrons = 31
Number of Neutrons = Mass Number − Atomic Number
= 70 − 31 = 39
Answer: The nucleus contains 39 neutrons.
Question 12
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
Given:
- Number of Protons = 79
- Mass Number = 197
(i) Number of Neutrons
= Mass Number − Number of Protons
= 197 − 79
= 118
(ii) Number of Electrons
Since the atom is electrically neutral,
Number of Electrons = Number of Protons = 79
| Particle | Number |
|---|---|
| Protons | 79 |
| Neutrons | 118 |
| Electrons | 79 |
Question 13
Complete Table 8.5.
| Atomic Number | Mass Number | Number of Neutrons | Number of Protons | Number of Electrons | Name of the Element |
|---|---|---|---|---|---|
| 5 | 11 | 6 | 5 | 5 | Boron |
| 7 | 14 | 7 | 7 | 7 | Nitrogen |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 15 | 31 | 16 | 15 | 15 | Phosphorus |
| 1 | 1 | 0 | 1 | 1 | Hydrogen |
Explanation
- Atomic Number (Z) = Number of Protons = Number of Electrons (for a neutral atom).
- Mass Number (A) = Number of Protons + Number of Neutrons.
- Number of Neutrons = Mass Number − Atomic Number.
Question 14
Aman was discussing the structure of an atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions:
Given:
- Mass Number (A) = 35
- Number of Neutrons (N) = 18
Formula Used:
Atomic Number (Z) = Number of Protons = Mass Number − Number of Neutrons
Z = 35 − 18 = 17
Since the atom is electrically neutral,
Number of Electrons = Number of Protons = 17
(i) How many electrons and protons does element X have?
Answer:
- Number of Protons = 17
- Number of Electrons = 17
(ii) What is its atomic number?
Answer:
The atomic number of element X is 17.
(iii) Identify the element X.
Answer:
The element having atomic number 17 is Chlorine (Cl).
(iv) Write its electronic configuration.
Answer:
Electronic configuration of chlorine is:
2, 8, 7
| Shell | Number of Electrons |
|---|---|
| K | 2 |
| L | 8 |
| M | 7 |
(v) How many valence electrons does it have?
Answer:
The outermost shell (M-shell) contains 7 electrons.
Therefore, the number of valence electrons is 7.
(vi) What will be the mass number if two neutrons are added to its nucleus?
Answer:
New number of neutrons = 18 + 2 = 20
New Mass Number = Number of Protons + Number of Neutrons
= 17 + 20
= 37
New Mass Number = 37
(vii) What will be the relation of X with the new atom?
Answer:
The new atom has the same atomic number (17) but a different mass number (37).
Therefore, the new atom is an isotope of element X.
| Property | Original Atom | New Atom |
|---|---|---|
| Element | Chlorine | Chlorine |
| Atomic Number | 17 | 17 |
| Mass Number | 35 | 37 |
| Relation | Isotopes | |
Question 15
In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom's:
Given:
- Protons = 12
- Neutrons = 12
- Electrons are replaced by negatively charged particles having the same charge but 500 times greater mass.
(i) Atomic Number
Answer:
The atomic number depends only on the number of protons.
Since the number of protons remains 12, the atomic number remains 12.
(ii) Atomic Mass
Answer:
Normally, electrons have negligible mass. However, the new particles are 500 times heavier. Therefore, they will contribute significantly to the total mass of the atom.
Hence, the atomic mass will increase.
(iii) Mass Number
Answer:
Mass number depends only on the total number of protons and neutrons.
Mass Number = 12 + 12 = 24
Since neither the number of protons nor neutrons changes, the mass number remains 24.
(iv) Overall Charge
Answer:
The new particles have the same negative charge as electrons and their number remains equal to the number of protons.
Therefore, the total positive charge and total negative charge remain equal.
Hence, the overall charge of the atom remains neutral (zero).
| Property | Effect | Reason |
|---|---|---|
| Atomic Number | Unchanged (12) | Depends only on the number of protons. |
| Atomic Mass | Increases | The new negatively charged particles are much heavier. |
| Mass Number | Unchanged (24) | Depends only on protons and neutrons. |
| Overall Charge | Neutral (0) | Equal numbers of positive and negative charges are present. |