GSEB Class 9 Science Solutions Chapter 4 Structure of the Atom

 

Question 1. What are canal rays?
Answer: Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Goldstein in 1886. These rays help us understand the components of an atom.
In simple words: Canal rays are positive rays made of protons, found by Goldstein.

Exam Tip: Remember to mention both the nature of the charge and the particles (protons) when defining canal rays.

 

Question 2. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: An electron is a negatively charged particle, while a proton is a positively charged particle. The size of their charges is equal but opposite. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.
In simple words: No, it won't carry a charge. The negative electron cancels the positive proton, making the atom neutral.

Exam Tip: Always remember that in a neutral atom, the number of electrons equals the number of protons, leading to zero net charge.

 

Question 3. On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson's model of the atom, an atom consists of both negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere. These negative and positive charges are equal in size. Thus, by balancing each other's effect, they make an atom neutral.
In simple words: Thomson's model says an atom is neutral because its equal positive and negative charges cancel each other out.

Exam Tip: When explaining Thomson's model, refer to it as the "plum pudding" model and clearly state the equal magnitude of opposite charges.

 

Question 4. On the basis of Rutherford's model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: On the basis of Rutherford's model of an atom, protons (positively charged particles) are present in the nucleus of an atom.
In simple words: Rutherford's model shows that positively charged protons are inside the atom's nucleus.

Exam Tip: Recall Rutherford's gold foil experiment and how it led to the discovery of the dense, positively charged nucleus containing protons.

 

Question 5. Draw a sketch of Bohr's model of an atom with three shells.
Answer:

In simple words: This picture shows Bohr's atom model, with a central nucleus and three electron shells labeled K, L, and M.

Exam Tip: Ensure your diagram clearly shows the nucleus and distinct, labeled circular orbits (shells) for electrons.

 

Question 6. What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: If the alpha-particle scattering experiment is carried out using a foil of a metal other than gold, there would be no change in the general observation. However, a gold foil was chosen for the alpha-scattering experiment because gold is malleable, and a very thin foil of gold can be easily made. It is difficult to produce such thin foils from other metals. The results would be similar, but less ideal due to foil thickness.
In simple words: The results would be similar, but gold is used because it's easy to make a super-thin foil from it, which is needed for the experiment.

Exam Tip: Remember that while the fundamental observations would be similar, gold's malleability is key for achieving a sufficiently thin foil for accurate results.

 

Question 7. Name the three sub-atomic particles of an atom.
Answer: The three sub-atomic particles of an atom are the electron, proton, and neutron.
In simple words: An atom has three main tiny parts: electrons, protons, and neutrons.

Exam Tip: Be precise in naming all three fundamental particles: electron, proton, and neutron.

 

Question 8. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Answer: The number of neutrons can be calculated by subtracting the number of protons from the atomic mass. Number of neutrons = Atomic mass - Number of protons. Therefore, the number of neutrons in the helium atom = \( 4 - 2 = 2 \). Helium has 2 neutrons.
In simple words: To find neutrons, subtract the number of protons (2) from the atomic mass (4), which gives 2 neutrons for helium.

Exam Tip: Always remember the formula: Number of neutrons = Atomic mass - Number of protons.

 

Question 9. Write the distribution of electrons in carbon and sodium atoms.
Answer:
(i) For Carbon: The atomic number of carbon is 6, which means it has a total of 6 electrons. The first shell (K-shell) will hold 2 electrons. The remaining electrons, which is 4, will be present in the second shell (L-shell). Thus the electron distribution will be:
K L
2 4
(ii) For Sodium: The atomic number of sodium is 11, which means it has a total of 11 electrons. The electron distribution will be:
K L M
2 8 1.
In simple words: Carbon has 2 electrons in the first shell and 4 in the second. Sodium has 2 in the first, 8 in the second, and 1 in the third.

Exam Tip: When writing electron distributions, always remember the maximum capacity of each shell (K=2, L=8, M=18, N=32) and fill them sequentially.

 

Question 10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: The maximum capacity of the K shell is 2 electrons, and the L shell can hold a maximum of 8 electrons. Therefore, if both K and L shells are full, there will be a total of \( 2 + 8 = 10 \) electrons in the atom.
In simple words: If K (2 electrons) and L (8 electrons) shells are completely filled, the atom has 10 electrons.

Exam Tip: Recall Bohr's model rules for shell capacities: K=2, L=8. Add these capacities when shells are full.

 

Question 11. How will you find the valency of chlorine, sulphur and magnesium?
Answer:
(i) For Chlorine: The atomic number of chlorine is 17. Its electron distribution is K=2, L=8, M=7.
Thus, the outermost shell has 7 electrons. It can easily complete its octet by gaining one electron. Hence, its valency = 1.
(ii) For Sulphur: The atomic number of sulphur is 16. Its electron distribution is K=2, L=8, M=6.
Thus, the outermost shell has 6 electrons. It will gain 2 electrons to complete its octet. Hence, its valency = \( 8 - 6 = 2 \).
(iii) For Magnesium: The atomic number of magnesium is 12. Its electron distribution is K=2, L=8, M=2.
Thus, the outermost shell has only 2 electrons. It can easily lose these two electrons to complete its octet. Hence, its valency = 2.
In simple words: Valency is found by checking how many electrons an atom needs to gain or lose to get 8 electrons in its outer shell. Chlorine needs 1, sulphur needs 2, and magnesium loses 2.

Exam Tip: Remember that valency is determined by the number of electrons an atom gains, loses, or shares to achieve a stable octet (or duplet for the first shell).

 

Question 12. If number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the atom? and
(ii) What is the charge on the atom?
Answer:
(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.
(ii) Since the number of both electrons and protons is equal, the charges cancel out. Therefore, the charge on the atom is 0.
In simple words: If an atom has 8 electrons and 8 protons, its atomic number is 8, and it has no overall charge.

Exam Tip: Always associate the atomic number directly with the number of protons, and a neutral charge with an equal number of protons and electrons.

 

Question 13. With the help of table 4.1(Textbook Page 51), find out the mass of number of oxygen and sulphur atom.
Answer: The mass number of an atom is the sum of its protons and neutrons.
Mass number of oxygen = Number of protons + Number of neutrons = \( 8 + 8 = 16 \)
Mass number of sulphur = Number of protons + Number of neutrons = \( 16 + 16 = 32 \)
In simple words: The mass number for oxygen is 16 (8 protons + 8 neutrons), and for sulphur is 32 (16 protons + 16 neutrons).

Exam Tip: The mass number represents the total count of nucleons (protons and neutrons) in the nucleus of an atom.

 

Question 14. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Answer: Here is a table showing the sub-atomic particles for H, D, and T:

In simple words: This table shows that hydrogen (H), deuterium (D), and tritium (T) all have one proton and one electron, but they differ in their number of neutrons.

Exam Tip: Remember that isotopes of the same element have the same number of protons (and electrons in a neutral atom) but different numbers of neutrons.

 

Question 15. Write the electronic configuration of any one pair of isotopes and isobars.
Answer:
Isotopes of chlorine: \(_{17}^{35}\text{Cl}\) and \(_{17}^{37}\text{Cl}\).
Electronic configuration for each of them is K=2, L=8, M=7. Both isotopes have the same electron arrangement.
Isobars: \(_{18}^{40}\text{Ar}\) and \(_{20}^{40}\text{Ca}\).
Electronic configuration of \(_{18}^{40}\text{Ar}\) is K=2, L=8, M=8.
Electronic configuration of \(_{20}^{40}\text{Ca}\) is K=2, L=8, M=8, N=2.
In simple words: Isotopes have the same electron configuration, like chlorine-35 and chlorine-37, both 2, 8, 7. Isobars, like argon-40 and calcium-40, have different electron configurations.

Exam Tip: Recall that isotopes have the same atomic number (same electron configuration) but different mass numbers. Isobars have the same mass number but different atomic numbers (different electron configurations).

 

Gujarat Board Class 9 Science Structure of the Atom Textbook Questions and Answers

 

Question 1. Compare the properties of electrons, protons and neutrons.
Answer: Here is a comparison of the properties of electrons, protons, and neutrons:

In simple words: Electrons are tiny and negative, orbiting the nucleus. Protons are positive and heavy, found in the nucleus. Neutrons are neutral and heavy, also in the nucleus.

Exam Tip: For comparison questions, always create a clear table with distinct categories for each property (charge, mass, location) to ensure all points are covered effectively.

 

Question 2. What are the limitations of J.J. Thomson's model of an atom?
Answer: The limitations of Thomson's model of an atom are:
(i) It could not explain the outcomes of the scattering experiment performed by Rutherford.
(ii) It did not have any experimental proof to support its ideas.
In simple words: Thomson's model couldn't explain Rutherford's experiment and had no proof, making it incomplete.

Exam Tip: Remember that models are often replaced when new experimental evidence contradicts them; Thomson's model failed to account for Rutherford's observations.

 

Question 3. What are the limitations of Rutherford's model of an atom?
Answer: The limitations of Rutherford's Model of an atom are:
1. Rutherford's model of an atom could not explain the stability of the atom. According to classical electromagnetic theory, an electron in a circular orbit would accelerate and continuously emit radiation, losing energy and eventually spiraling into the nucleus, making the atom unstable.
2. Rutherford's model of an atom could not explain how the electrons are distributed in the extra-nuclear portion in an atom. It did not specify discrete energy levels or orbits.
In simple words: Rutherford's model couldn't explain why atoms are stable (electrons should crash into the nucleus) or how electrons are arranged outside the nucleus.

Exam Tip: When discussing Rutherford's limitations, emphasize the issue of atomic stability and the lack of explanation for electron arrangement, which paved the way for Bohr's model.

 

Question 4. Describe Bohr's model of an atom.
Answer: In 1913, Niels Bohr presented a theory concerning the arrangement of electrons in the outer space of an atom. The main principles of the theory are listed:
1. In the outer nuclear part of an atom, the electrons move in well-defined circular paths known as orbits.
2. These circular orbits are also referred to as energy levels or energy shells.
3. These have been named as K, L, M, N, O,... (or as 1, 2, 3, 4, 5,...) based on the amount of energy present.
4. The order of the energy of these energy shells is: K < L < M < N < O <.... or 1 < 2 < 3 < 4 < 5 <....
5. While moving in an orbit, the electron is not in a position to either lose or gain energy. In other words, its energy remains constant. Therefore, these energy states for the electrons are also known as stationary states.
In simple words: Bohr's model states that electrons move in specific energy paths (orbits) around the nucleus. These orbits are called shells (K, L, M) and electrons in them don't lose or gain energy, making the atom stable.

Exam Tip: For Bohr's model, focus on the key postulates: fixed orbits, discrete energy levels (shells), no energy radiation in these orbits, and the sequential naming of shells (K, L, M...).

 

Question 5. Compare all the proposed models of an atom given in this chapter.
Answer:
For Thomson's Model of an atom:
1. It could not explain the outcome of the scattering experiment performed by Rutherford.
2. It did not possess any experimental evidence to support its claims.
For Rutherford's Model of an atom:
1. Rutherford's model of an atom could not explain the stability of the atom. Electrons should spiral into the nucleus.
2. Rutherford's model of an atom could not explain how the electrons are arranged in the extra-nuclear portion of an atom.
For Bohr's Model of an atom:
1. It gave no clear idea about the shapes of the molecules formed by the combination of atoms.
2. According to Bohr's theory, an electron in an atom follows a well-defined circular path called an orbit. However, later studies showed that the path of the electron cannot be followed precisely. It is only of a probable nature and not as exact as stated by Bohr. However, it is not possible to discuss this concept at the current level of the students.
In simple words: Thomson's model failed to explain scattering experiments. Rutherford's model couldn't explain atom stability or electron arrangement. Bohr's model explained stability and energy levels but didn't address molecule shapes or the exact path of electrons.

Exam Tip: When comparing models, highlight what each model successfully explained and what its major shortcomings were, showing how later models improved upon previous ones.

 

Question 6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer: The rules for writing the distribution of electrons in various shells for the first eighteen elements are given below. The following rules are used to fill electrons in different energy levels:
(i) If \( n \) is the given number of the orbit or energy level, then \( 2n^2 \) gives the maximum number of electrons possible in that orbit or energy level. Thus:
First orbit or K-shell will have \( 2n^2 = 2 \times 1^2 = 2 \) electrons.
Second orbit or L-shell will have \( 2n^2 = 2 \times 2^2 = 8 \) electrons.
Third orbit or M-shell will have \( 2n^2 = 2 \times 3^2 = 18 \) electrons.
(ii) If it is the outermost orbit, then it should not contain more than 8 electrons.
(iii) There should be a stepwise filling of electrons in different orbits, meaning electrons are not placed in a given orbit if the earlier orbits or shells are incompletely filled.
In simple words: Electrons fill shells according to these rules: maximum electrons in a shell is \( 2n^2 \), the outermost shell can't have more than 8 electrons, and inner shells must fill before outer ones.

Exam Tip: Be sure to cite the \( 2n^2 \) rule, the octet rule for the outermost shell, and the stepwise filling principle when explaining electron distribution.

 

Question 7. Define valency by taking examples of silicon and oxygen.
Answer: Valency is defined as the number of electrons which an atom can lose, gain, or share with other atoms to complete its octet, which means 8 electrons in the outermost shell.
Examples:
(i) Silicon: The atomic number of silicon is 14. This means 14 electrons are present. Its electronic configuration is K=2, L=8, M=4. Thus, the outermost shell has 4 electrons, which it can share with other atoms to complete its octet. Hence, its valency = 4.
(ii) Oxygen: The atomic number of oxygen is 8. This means 8 electrons are present. Its electronic configuration is K=2, L=6. Thus, the outermost shell has 6 electrons. It will gain 2 electrons to complete its octet. Hence, its valency = 2.
In simple words: Valency tells us how many electrons an atom gains, loses, or shares to get 8 electrons in its outer shell. Silicon's valency is 4 (shares 4), and Oxygen's valency is 2 (gains 2).

Exam Tip: When defining valency, clearly state that it's about achieving stability (an octet) and provide distinct examples of gaining, losing, or sharing electrons.

 

Question 8. Explain with the examples
(i) Atomic number
(ii) Mass number
(iii) Isotopes and
(iv) Isobars.
Give any two uses of isotopes.
Answer:
(i) Atomic number: The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
(ii) Mass number: The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is \( 5 + 6 = 11 \).
(iii) Isotopes: They are atoms of the same element that have the same atomic number but different mass numbers or atomic masses. For example, Carbon: \(_{6}^{12}\text{C}\) and \(_{6}^{14}\text{C}\).
(iv) Isobars: They are atoms of different elements having the same mass number but different atomic numbers. For example, calcium (atomic number 20) and argon (atomic number 18) are isobars and both have a mass number of 40.
The number of electrons in these atoms is different, but the mass number of both these elements is 40. The total number of neutrons is the same in the atoms of this pair of elements.
Two uses of isotopes are as follows:

• An isotope of uranium is used as a fuel in nuclear reactors.
• An isotope of cobalt is used in the treatment of cancer.

In simple words: Atomic number is the count of protons. Mass number is the sum of protons and neutrons. Isotopes are atoms of the same element with different neutron counts. Isobars are atoms of different elements with the same mass number. Uranium isotopes power reactors, and cobalt isotopes treat cancer.

Exam Tip: Clearly differentiate between atomic number (protons) and mass number (protons + neutrons). For isotopes and isobars, emphasize what is the same and what is different, along with relevant examples and practical uses.

 

Question 9. Na+ has completely filled K and L shells. Explain.
Answer: The atomic number of sodium is 11. So, a neutral sodium atom has 11 electrons. The \( \text{Na}^+ \) ion has 10 electrons (one less than neutral sodium). Out of these 10 electrons, the K-shell contains 2 electrons, and the L-shell contains 8 electrons. Thus, \( \text{Na}^+ \) has completely filled K and L-shells, giving it a stable electron configuration.
In simple words: Sodium's atomic number is 11. \( \text{Na}^+ \) loses one electron, leaving 10 electrons. These 10 electrons fill the K-shell (2) and L-shell (8) completely, making it stable.

Exam Tip: When explaining ion electron configurations, always start with the neutral atom's electron count, then adjust for the ion's charge (gain or loss of electrons) to determine the new configuration.

 

Question 10. If bromine atom is available in the form of, say, two isotopes \( _{35}^{79}\text{Br} \) and \( _{35}^{81}\text{Br} \) (50.3%). Calculate the average atomic mass of bromine atom.
Answer:
Percentage of Br isotope with mass number 79 = 49.7%
Percentage of Br isotope with mass number 81 = 50.3%
\( \therefore \) Average atomic mass of Br \( = \frac{(49.7 \times 79) + (50.3 \times 81)}{100} \)
\( = \frac{3926.3 + 4074.3}{100} \)
\( = \frac{8000.6}{100} \)
\( = 80.006 \text{ u} \)
In simple words: To find the average atomic mass, multiply the mass of each bromine isotope by its percentage, add these values, then divide by 100. The result is 80.006 u.

Exam Tip: The average atomic mass is a weighted average of the masses of an element's isotopes, where the weighting factors are the relative abundances of each isotope.

 

Question 11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes \( _{8}^{16}\text{X} \) and \( _{8}^{18}\text{X} \) in the sample?
Answer:
Let the percentage of \( _{8}^{16}\text{X} \) be A.
Then the percentage of \( _{8}^{18}\text{X} = 100 – \text{A} \)
Then, we have the equation for average atomic mass:
\( \frac{(16 \times \text{A}) + (18 \times (100 – \text{A}))}{100} = 16.2 \)
\( 16\text{A} + 1800 – 18\text{A} = 16.2 \times 100 \)
\( -2\text{A} + 1800 = 1620 \)
\( -2\text{A} = 1620 - 1800 \)
\( -2\text{A} = -180 \)
\( \text{A} = \frac{-180}{-2} \)
\( \text{A} = 90 \)
Therefore, \( _{8}^{16}\text{X} \) is 90% and \( _{8}^{18}\text{X} \) is 10%.
In simple words: If the average mass is 16.2u, and we have isotopes with masses 16 and 18, then 90% of the sample is the 16 mass isotope and 10% is the 18 mass isotope.

Exam Tip: Set up an algebraic equation with one variable for the percentage of one isotope (A) and express the other as (100-A) to solve for their abundances.

 

Question 12. If Z = 3, what would be the valency of the element? Also, name the element.
Answer: If Z = 3, meaning the atomic number is 3, then the element is lithium. It has an electron distribution of 2, 1 (2 electrons in the K-shell and 1 in the L-shell). Since it has 1 electron in its outermost shell, lithium tends to lose this electron to achieve a stable duplet configuration. Thus, lithium has a valency of 1.
In simple words: If atomic number is 3, the element is lithium. It has 1 electron in its outer shell, so its valency is 1 because it wants to lose that electron.

Exam Tip: Remember that Z (atomic number) directly identifies the element and its number of protons/electrons, which helps in determining its electron configuration and valency.

 

Question 13. Composition of the nuclei of two atomic species X and Y are given as under

Give the mass numbers of X and Y. What is the relation between the two species?
Answer:
Mass number of X = Number of protons + Number of neutrons = \( 6 + 6 = 12 \)
Mass number of Y = Number of protons + Number of neutrons = \( 6 + 8 = 14 \)
These two atomic species X and Y have the same atomic number (6 protons) but different mass numbers (12 and 14). Hence, they are isotopes.
In simple words: X has a mass number of 12, and Y has 14. Since they both have 6 protons but different neutrons, X and Y are isotopes.

Exam Tip: Remember that mass number is the sum of protons and neutrons. If two atoms have the same number of protons but different numbers of neutrons, they are isotopes.

 

Question 14. For the following statements, write T for True and F for False.
(a) J. J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together.
(c) The mass of an electron is about 1/2000 times that of a proton.
(d) Isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer:
(a) False
(b) False
(c) True
(d) False
In simple words: (a) False, (b) False, (c) True, (d) False.

Exam Tip: Review the contributions of J.J. Thomson (electron, plum pudding model), the composition of neutrons (not electron+proton combination), the relative masses of subatomic particles, and common uses of isotopes to verify such true/false statements.

 

Question 15. Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer: (a) Atomic nucleus
In simple words: Rutherford's alpha-particle experiment helped discover the atomic nucleus.

Exam Tip: Link Rutherford's experiment directly to the discovery of the dense, positively charged nucleus within an atom.

 

Question 16. Isotopes of an element have
(a) the same physical properties
(b) the same chemical properties
(c) different number of neutrons
(d) different atomic numbers
Answer: (c) different number of neutrons
In simple words: Isotopes of an element are atoms that have the same number of protons but a different count of neutrons.

Exam Tip: The defining characteristic of isotopes is a variation in neutron count, leading to different mass numbers but identical atomic numbers and chemical behavior.

 

Question 17. Number of valence electrons in \( \text{Cl}^- \) ion is:
(a) 16
(b) 8
(c) 17
(d) 18
Answer: (b) 8
In simple words: A neutral chlorine atom has 7 valence electrons. The \( \text{Cl}^- \) ion gains one electron, so it has 8 valence electrons.

Exam Tip: For ions, adjust the electron count based on the charge (add for negative, subtract for positive) before determining the valence electrons.

 

Question 18. Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
Answer: (d) 2, 8, 1
In simple words: Sodium has 11 electrons, so its correct electron arrangement is 2 in the first shell, 8 in the second, and 1 in the third.

Exam Tip: Remember to fill the electron shells sequentially according to their maximum capacities (K=2, L=8, M=18).

 

Question 19. Complete the following table.
Answer: Here is the completed table with the missing information filled in. The calculations for mass number and particle counts are shown below the table.

*Explanation for each row:*
First Line (Fluorine):
Mass number = Atomic number (9) + No. of neutrons (10) = 19
No. of protons = Atomic number = 9
No. of electrons = Atomic number = 9
Name of species = Fluorine (F)
Second Line (Sulphur):
No. of neutrons = Mass number (32) - Atomic number (16) = 16
No. of protons = Atomic number = 16
No. of electrons = Atomic number = 16
Name of species = Sulphur (S)
Third Line (Magnesium):
Atomic number = No. of protons = 12
No. of neutrons = Mass number (24) - Atomic number (12) = 12
No. of electrons = Atomic number = 12
Name of species = Magnesium (Mg)
Fourth Line (Deuterium):
Atomic number = No. of protons = 1
No. of neutrons = Mass number (2) - Atomic number (1) = 1
No. of electrons = Atomic number = 1
Name of species = Deuterium (D)
Fifth Line (Protium):
Atomic number = No. of protons = 1
No. of electrons = Atomic number = 1
Name of species = Protium or hydrogen (H)
In simple words: The table is filled by using the relationships between atomic number (protons, electrons), mass number (protons + neutrons), and the name of the element. For example, Fluorine has 9 protons, 10 neutrons, total mass 19.

Exam Tip: Remember the fundamental definitions: Atomic number = Number of Protons = Number of Electrons (in a neutral atom); Mass number = Number of Protons + Number of Neutrons. Use these to fill in any missing values in such tables.

 

In-Text Activities Solved (Textbook Page 46)

 

Activity 4.1
Answer:
Observation: When a comb is first rubbed with dry hair and then brought near small pieces of paper, the comb attracts the paper pieces towards itself. However, when the comb is simply brought near the tiny paper pieces without rubbing, it has no effect. Similarly, when a glass rod is rubbed with a silk cloth and then brought close to an inflated balloon, the glass rod attracts the balloon.
Conclusion: This shows that when a comb is rubbed with dry hair, it becomes electrically charged, which then exerts an attractive force on the tiny pieces of paper and draws them in. In the same way, when a glass rod is rubbed with a silk cloth, it gains an electric charge which exerts an electric force on the inflated balloon and attracts it. Hence, a substance that has an electric charge is said to be charged or electrified.
In simple words: Rubbing a comb on hair or a glass rod on silk gives them an electric charge, causing them to attract light objects like paper or balloons. Unrubbed objects don't attract. This means they become "electrified."

Exam Tip: When describing experiments, clearly separate observations from conclusions. Focus on cause and effect (rubbing causes charge, charge causes attraction) and use the correct terminology (electrically charged, electrified).

 

Activity 4.2
Answer: Table 4.1: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells

In simple words: This table shows details for the first 18 elements, including their symbols, atomic numbers, counts of protons, neutrons, and electrons, how electrons are arranged in shells, and their valency.

Exam Tip: Be familiar with the basics of electron configuration, atomic number, mass number, and how they relate to the properties of the first 20 elements on the periodic table.