Journey Inside the AtomClass 9 Science NCERT Solutions
15 Solutions
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Solution 1 of 15
Q1Revise, Reflect, Refine
Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford's gold foil experiment:
(i)
The experiment clearly showed the existence of neutrons in the nucleus.
(ii)
The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
(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.
(iv)
The way alpha particles were deflected showed that electrons move around the nucleus.
Solution
The correct options are (ii) and (iii).
Explanation for each option:
-
(i) Incorrect. The gold foil experiment provided evidence for a dense, positive nucleus but gave no information about neutrons. Neutrons were discovered much later in 1932 by James Chadwick.
-
(ii) Correct. J. J. Thomson's plum pudding model predicted that alpha particles would pass through the gold foil with only minor deflections. The observation that some alpha particles were deflected by large angles, and a few even bounced back, directly contradicted and disproved this model. This led to the new idea of a central nucleus.
-
(iii) Correct. The alpha particles are positively charged. The fact that a few were deflected at large angles meant they encountered a very strong repulsive force. Rutherford concluded this force must come from a massive, positively charged region concentrated in a tiny volume at the atom's centre, which he called the nucleus.
-
(iv) Incorrect. The experiment did not provide direct evidence about the movement of electrons. It focused on the interaction between alpha particles and the positive part of the atom. Rutherford inferred that electrons must be outside the nucleus to make the atom neutral, and he proposed they orbit the nucleus, but the experiment itself did not show this motion.
Q2Revise, Reflect, Refine
Which of the following statements are correct or incorrect according to the Bohr's atomic model? Give a reason for each statement.
(i)
Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
(ii)
Electrons can exist anywhere around the nucleus with no fixed energy.
(iii)
Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
(iv)
Electrons can be found between energy levels as they move around the nucleus.
Solution
Analysis of each statement based on Bohr's atomic model:
-
(i) Incorrect. This statement describes the main limitation of Rutherford's model. Bohr's model specifically postulated that electrons in fixed, stationary orbits (or energy levels) do not lose or radiate energy. This was his solution to the problem of atomic instability.
-
(ii) Incorrect. According to Bohr's model, electrons cannot exist just anywhere. They are restricted to specific, discrete circular paths called orbits or shells, each having a fixed amount of energy.
-
(iii) Correct. This is one of the fundamental postulates of Bohr's model. He proposed that electrons revolve in these special orbits, called 'stationary states', where their energy remains constant, thus explaining the stability of the atom.
-
(iv) Incorrect. Bohr's model states that electrons can only exist in the allowed energy levels, not in the space between them. An electron can jump from one level to another by absorbing or emitting a specific amount of energy, but it cannot exist in an intermediate position.
Q3Revise, Reflect, Refine
The composition of the nuclei of three atomic species X, Y, and Z are given as follows.
X Y Z Number of protons 18 17 17 Number of neutrons 19 18 20
Explain the relation between the following:
(i)
Y and Z
(ii)
Z and X
Solution
(i) Relation between Y and Z
- Atoms Y and Z both have the same number of protons (17). The number of protons defines the atomic number () and identifies the element. Therefore, Y and Z are atoms of the same element (Chlorine).
- However, they have different numbers of neutrons (Y has 18, Z has 20). This means they have different mass numbers ():
- Mass number of Y = Protons + Neutrons =
- Mass number of Z = Protons + Neutrons =
- Atoms of the same element with the same atomic number but different mass numbers are called isotopes. Thus, Y and Z are isotopes of each other.
(ii) Relation between Z and X
- Atom Z has 17 protons and a mass number of 37.
- Atom X has 18 protons and its mass number is Protons + Neutrons = .
- Atoms Z and X have different atomic numbers (17 for Z, 18 for X), which means they are different elements (Chlorine and Argon, respectively).
- However, they have the same mass number (37).
- Atoms of different elements with different atomic numbers but the same mass number are called isobars. Thus, Z and X are isobars.
Q4Revise, Reflect, Refine
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 in the gold foil experiment?
Solution
Based on the observation that a few alpha particles were deflected at large angles and some even bounced back, Rutherford drew the following conclusions about the atom's positively charged part:
- Position: The positively charged part must be concentrated in a very small volume at the center of the atom. He named this central part the nucleus.
- Characteristics:
- Dense and Massive: Since the massive alpha particles were repelled and deflected, the nucleus must contain most of the atom's mass.
- Positively Charged: The repulsion of the positively charged alpha particles indicated that the nucleus itself carries a positive charge.
In summary, Rutherford concluded that the atom is not a uniform sphere as Thomson suggested, but consists of a tiny, dense, massive, and positively charged nucleus at its center, with the rest of the atom being mostly empty space.
Q5Revise, Reflect, Refine
Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.
(i)
Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
(ii)
Thomson's model depicted the atom as a 'plum pudding' with electrons embedded in a sphere of positive charge.
(iii)
Rutherford's model proposed that atoms have a dense central nucleus.
(iv)
Dalton's model described atoms as indivisible particles.
Solution
The correct chronological order of the atomic models is: (iv) → (ii) → (iii) → (i).
Explanation of the evolution:
-
(iv) Dalton's model (1808): John Dalton proposed the first scientific atomic theory, stating that all matter is composed of tiny, indivisible, and indestructible particles called atoms. This model did not include any subatomic particles.
-
(ii) Thomson's model (1904): After his discovery of the electron in 1897, J. J. Thomson proposed the 'plum pudding' model. He described the atom as a uniform sphere of positive charge with negatively charged electrons embedded within it, much like plums in a pudding. This was the first model to include subatomic particles.
-
(iii) Rutherford's model (1911): Based on the results of the gold foil experiment, Ernest Rutherford disproved the plum pudding model. He proposed a nuclear model where the atom has a small, dense, positively charged nucleus at the center, with electrons orbiting it. This model established that the atom is mostly empty space.
-
(i) Bohr's model (1913): To address the instability of Rutherford's model (which predicted that orbiting electrons should lose energy and spiral into the nucleus), Niels Bohr proposed a new model. He suggested that electrons move in fixed, discrete orbits or energy levels around the nucleus without radiating energy. This model successfully explained the stability of atoms.
Q6Revise, Reflect, Refine
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
Solution
Electrons do not fly away from the atom because they are held in their orbits by a strong force of attraction from the nucleus.
Explanation:
- The nucleus of an atom contains protons, which are positively charged particles.
- Electrons are negatively charged particles.
- According to the laws of electrostatics, opposite charges attract each other. Therefore, there is a strong electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus.
This attractive force constantly pulls the electrons towards the nucleus, acting as the centripetal force that keeps them in their circular paths or orbits, preventing them from flying away.
Q7Revise, Reflect, Refine
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. Choose the correct option:
(i)
Both A and R are true, and R is the correct explanation of A.
(ii)
Both A and R are true, but R is not the correct explanation of A.
(iii)
A is true, but R is false.
(iv)
A is false, but R is true.
Solution
The correct option is (ii) Both A and R are true, but R is not the correct explanation of A.
Explanation:
- Assertion (A) is true. The discovery of subatomic particles like the electron (by Thomson), proton, and neutron (by Chadwick) was fundamental to developing modern atomic models. It proved that atoms are divisible and have an internal structure, leading to models by Thomson, Rutherford, and Bohr.
- Reason (R) is true. In a neutral atom, the total positive charge must balance the total negative charge. Since each proton has a +1 relative charge and each electron has a -1 relative charge, their numbers must be equal for the atom to be electrically neutral.
- However, R is not the correct explanation of A. The reason why the discovery of subatomic particles helped in understanding atomic structure is that it revealed the components that make up the atom. Knowing these components (electron, proton, neutron) and their properties (mass, charge) allowed scientists to propose models for how they are arranged. The fact that the number of electrons equals the number of protons is a specific characteristic of a neutral atom's structure, not the overarching reason why the discovery of these particles was helpful.
Q8Revise, Reflect, Refine
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.
Solution
Given:
- Name of the element: Magnesium (Mg)
- Mass number () = 24
- Atomic number () = 12
(i) Number of protons:
The number of protons is equal to the atomic number.
Number of protons =
(ii) Number of neutrons:
The number of neutrons is the mass number minus the number of protons.
Number of neutrons =
(iii) Number of electrons:
For a neutral atom, the number of electrons is equal to the number of protons.
Number of electrons = 12
Arrangement of electrons (Electronic Configuration):
A magnesium atom has 12 electrons. These are distributed in shells according to the rules (K shell: max 2, L shell: max 8, etc.).
- K-shell (n=1): 2 electrons
- L-shell (n=2): 8 electrons
- M-shell (n=3): 2 electrons
The electronic configuration of magnesium is 2, 8, 2.
Illustration:
A diagram of a magnesium atom would show:
- A central nucleus containing 12 protons and 12 neutrons.
- The first electron shell (K-shell) with 2 electrons.
- The second electron shell (L-shell) with 8 electrons.
- The third and outermost electron shell (M-shell) with 2 electrons.
Q9Revise, Reflect, Refine
Find the following information for the elements shown in Fig. 8.17:
(i)
Name of the element
(ii)
Symbol
(iii)
Total number of electrons
(iv)
Number of valence electrons
(v)
Valency of the element
(vi)
Number of protons
(vii)
Atomic number (Note: Fig. 8.17 shows a Sodium atom on the left and a Chlorine atom on the right).
Solution
For the Left Atom (Sodium):
The diagram shows a nucleus with 11 protons and electron shells with 2, 8, and 1 electrons.
(i)
Name of the element: Sodium
(ii)
Symbol: Na
(iii)
Total number of electrons:
(iv)
Number of valence electrons: 1 (the electron in the outermost M-shell)
(v)
Valency of the element: 1 (it tends to lose 1 electron to achieve a stable octet)
(vi)
Number of protons: 11 (given in the nucleus)
(vii)
Atomic number: 11 (equal to the number of protons)
For the Right Atom (Chlorine):
The diagram shows a nucleus with 17 protons and electron shells with 2, 8, and 7 electrons.
(i)
Name of the element: Chlorine
(ii)
Symbol: Cl
(iii)
Total number of electrons:
(iv)
Number of valence electrons: 7 (the electrons in the outermost M-shell)
(v)
Valency of the element: 1 (it tends to gain 1 electron to complete its octet, )
(vi)
Number of protons: 17 (given in the nucleus)
(vii)
Atomic number: 17 (equal to the number of protons)
Q10Revise, Reflect, Refine
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?
Solution
The key difference lies in how each model treated the energy of the orbiting electrons.
Rutherford's Model Failure:
Rutherford's model was based on classical physics. According to classical electromagnetic theory, any accelerating charged particle must radiate energy. An electron moving in a circular orbit is constantly changing its direction, which means it is accelerating. Therefore, an orbiting electron should continuously lose energy, causing it to spiral inward and eventually collapse into the nucleus in a fraction of a second. This prediction contradicted the observed stability of atoms, so the model failed to explain why atoms exist.
Bohr's Model Success:
Bohr's model succeeded by introducing a revolutionary, non-classical idea. He proposed specific postulates to solve the stability problem:
- Stationary Orbits: He stated that electrons do not orbit the nucleus randomly but are restricted to certain fixed circular paths called 'stationary states' or 'energy levels'.
- No Energy Radiation: He postulated that while an electron is in one of these specific orbits, it does not radiate any energy, even though it is accelerating.
By postulating that electrons can exist in stable orbits without losing energy, Bohr's model successfully explained why atoms are stable and do not collapse. It provided a framework where electrons could orbit the nucleus indefinitely as long as they remained in their designated energy levels.
Q11Revise, Reflect, Refine
An atom has 31 electrons. How many neutrons are there in its nucleus?
Solution
Given:
- Mass number () of atom X = 70
- Number of electrons in atom X = 31
To Find:
- The number of neutrons in the nucleus.
Calculation:
-
Find the number of protons: For a neutral atom, the number of protons is equal to the number of electrons. The number of protons is also the atomic number (). Number of protons () = Number of electrons = 31
-
Find the number of neutrons: The mass number () is the sum of the number of protons and neutrons. Mass Number = Number of protons + Number of neutrons Rearranging the formula to find the number of neutrons (): Number of neutrons = Mass Number - Number of protons
Final Answer:
There are 39 neutrons in the nucleus of atom X.
Q12Revise, Reflect, Refine
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
Solution
Given:
- Number of protons () = 79
- Mass number () = 197
Calculation:
(i) Number of neutrons:
The number of neutrons is calculated by subtracting the number of protons (atomic number) from the mass number.
Formula: Number of neutrons = Mass number - Number of protons
Number of neutrons =
(ii) Number of electrons:
Assuming the atom is electrically neutral, the number of electrons must be equal to the number of protons to balance the charge.
Number of electrons = Number of protons = 79
Final Answer:
(i)
The atom has 118 neutrons.
(ii)
The atom has 79 electrons.
Q13Revise, Reflect, Refine
Complete the Table 8.5:
Atomic number Mass number Number of neutrons Number of protons Number of electrons Name of the elements 5 - 6 - - - - 14 - - 7 Nitrogen - 24 - 12 - - 15 - 16 - - - - 1 0 - - -
Solution
Here is the completed table with explanations for each row.
Row 1:
- Atomic number = 5, so Number of protons = 5 and Number of electrons = 5.
- Mass number = Protons + Neutrons = .
- The element with atomic number 5 is Boron.
Row 2:
- Number of electrons = 7, so Number of protons = 7 and Atomic number = 7.
- Number of neutrons = Mass number - Protons = .
- The element is Nitrogen.
Row 3:
- Number of protons = 12, so Atomic number = 12 and Number of electrons = 12.
- Number of neutrons = Mass number - Protons = .
- The element with atomic number 12 is Magnesium.
Row 4:
- Atomic number = 15, so Number of protons = 15 and Number of electrons = 15.
- Mass number = Protons + Neutrons = .
- The element with atomic number 15 is Phosphorus.
Row 5:
- Number of protons = Mass number - Neutrons = .
- Atomic number = 1 and Number of electrons = 1.
- The element with atomic number 1 is Hydrogen.
Completed Table:
| Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the elements |
|---|---|---|---|---|---|
| 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 |
Q14Revise, Reflect, Refine
Aman was discussing the structure of 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:
(i)
How many electrons and protons does element X have?
(ii)
What is its atomic number?
(iii)
Identify the element X.
(iv)
Write its electronic configuration.
(v)
How many valence electrons does it have?
(vi)
What will be the mass number if two neutrons are added to its nucleus?
(vii)
What will be the relation of X with the new atom?
Solution
Given:
- Mass number () = 35
- Number of neutrons = 18
(i) How many electrons and protons does element X have?
First, find the number of protons (which is the atomic number, ).
Number of protons = Mass number - Number of neutrons = .
For a neutral atom, the number of electrons equals the number of protons.
- Protons = 17
- Electrons = 17
(ii) What is its atomic number?
The atomic number () is equal to the number of protons.
- Atomic number = 17
(iii) Identify the element X.
The element with atomic number 17 is Chlorine (Cl).
(iv) Write its electronic configuration.
The atom has 17 electrons. They are arranged in shells as follows:
- K-shell: 2 electrons
- L-shell: 8 electrons
- M-shell: 7 electrons
- Electronic configuration = 2, 8, 7
(v) How many valence electrons does it have?
Valence electrons are the electrons in the outermost shell. The M-shell is the outermost shell.
- Valence electrons = 7
(vi) What will be the mass number if two neutrons are added to its nucleus?
The original mass number is 35. Adding two neutrons will increase the mass number by 2.
- New mass number =
(vii) What will be the relation of X with the new atom?
- The original atom (X) is .
- The new atom has the same number of protons (17) but a new mass number of 37. So, it is .
- Atoms of the same element (same atomic number) but with different mass numbers are called isotopes. Therefore, the new atom is an isotope of element X.
Q15Revise, Reflect, Refine
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:
(i)
Atomic number
(ii)
Atomic mass
(iii)
Mass number
(iv)
Overall charge
Solution
Let's analyze the effect of this hypothetical change on the properties of the atom.
Original Atom:
- Protons = 12
- Neutrons = 12
- Electrons = 12 (since it's a neutral atom)
Hypothetical Atom:
- Protons = 12
- Neutrons = 12
- Hypothetical particles = 12 (each with a charge of -1 and a mass 500 times that of an electron)
(i) Atomic number:
The atomic number is defined by the number of protons in the nucleus. Since the number of protons remains 12, the atomic number will not change.
(ii) Atomic mass:
The atomic mass is the total mass of all protons, neutrons, and electrons (or the hypothetical particles). In a normal atom, the mass of electrons is considered negligible. However, the new particles are 500 times heavier than electrons. This significant increase in the mass of the orbiting particles means the atomic mass will increase significantly.
(iii) Mass number:
The mass number is defined as the total number of protons and neutrons in the nucleus. Since the nucleus is unchanged (12 protons + 12 neutrons), the mass number will not change. It remains .
(iv) Overall charge:
The overall charge is the sum of the charges of all particles. The nucleus has a charge of +12 (from 12 protons). The 12 hypothetical particles have the same charge as electrons, so their total charge is -12. The overall charge is . Therefore, the overall charge will not change; the atom remains neutral.