Complete Class 9 Science Exploration Chapter 8 question answers covering Think It Over, Pause and Ponder, Think as a Scientist, What if, Revise Reflect Refine, numericals, tables and the concluding enquiry.
NCERT Solutions
Think It Over
Q1Question 1. Are atoms the smallest indivisible particles?
No, atoms are not the smallest indivisible particles. Earlier, scientists thought that atoms were indivisible, but later experiments proved that atoms are made up of smaller particles called subatomic particles.
The three main subatomic particles are:
Electron — negatively charged
Proton — positively charged
Neutron — neutral particle
Therefore, atoms are divisible and have an internal structure.
Q2Question 2. Why do electrons not fall into the nucleus even though they are attracted to protons in it?
Electrons are negatively charged and the nucleus contains positively charged protons, so electrons are attracted towards the nucleus.
According to Bohr’s model of the atom, electrons move around the nucleus only in fixed circular paths called shells or energy levels. While moving in these fixed shells, electrons do not lose energy.
Since electrons do not lose energy in their allowed shells, they do not spiral inward and fall into the nucleus. This explains the stability of atoms.
Q3Question 3. Why did scientists keep modifying atomic models?
Scientists kept modifying atomic models because new experiments gave new evidence that older models could not explain.
For example:
Thomson’s model could not explain the results of Rutherford’s gold foil experiment.
Rutherford’s model could not explain why electrons do not fall into the nucleus.
Bohr’s model explained atomic stability better, but later it was also found to have limitations.
Thus, atomic models were changed and improved as scientific knowledge increased.
NCERT Solutions
Pause and Ponder — Page 143
Q1(i)Question 1. Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: Question 1(i). The positive charge on the clay is lesser than the total negative charge of the beads?
If the positive charge on the clay is less than the total negative charge of the beads, the model will have more negative charge than positive charge.
Therefore, the atom will not be neutral. It will become negatively charged.
This means the model will not correctly represent a neutral atom.
Q1(ii)Question 1. Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: Question 1(ii). By mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
No, the model would not represent a neutral atom.
In Thomson’s model, the clay represents the positive charge of the atom and the beads represent electrons. If the clay itself also carries negative charge, then the total negative charge will increase.
For the atom to be neutral:
Total positive charge = Total negative charge
But if the clay also becomes negatively charged, the balance will be disturbed. Therefore, the model will not be neutral.
Q2Question 2. Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson’s idea and where does it fall short?
Yes, an orange or a lemon can be used as a simple comparison for Thomson’s model, but only partly.
It matches Thomson’s idea because:
The soft pulp can represent the positively charged sphere.
The seeds inside the pulp can represent electrons embedded in the atom.
But this comparison falls short because:
The pulp of an orange or lemon is not electrically positive.
The seeds are not negatively charged particles.
The seeds are not spread uniformly everywhere in the pulp.
A real atom is extremely tiny and cannot be compared exactly with a fruit.
So, an orange or lemon gives a rough idea, but it is not a perfect model.
Q3Question 3. Why did Thomson conclude that electrons are present in all atoms?
Thomson studied cathode rays using different gases and different cathode materials.
He found that the nature of cathode rays remained the same, no matter which gas or cathode material was used.
This showed that the particles present in cathode rays were common to all atoms. These particles were later called electrons.
Therefore, Thomson concluded that electrons are present in all atoms.
NCERT Solutions
Think as a Scientist — Page 144
QuestionQuestion. Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would expect if the gold foil in the experiment were made thicker. Also, draw a simple diagram to show the observations you expect.
If the gold foil were made thicker, alpha particles would pass through many more layers of gold atoms.
Expected observations:
Fewer alpha particles would pass straight through.
More alpha particles would be deflected.
Some alpha particles might be deflected through larger angles.
More alpha particles may bounce back compared to a thin foil.
This would happen because in a thicker foil, alpha particles have a greater chance of coming close to or hitting the positively charged nuclei of gold atoms.
Simple Diagram
Conclusion
A thicker foil would increase the chances of collision or close interaction between alpha particles and nuclei. Therefore, scattering would increase.
NCERT Solutions
Pause and Ponder — Page 144
Q4Question 4. What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?
Alpha particles are positively charged, so they are repelled by the positively charged nucleus.
If alpha particles were replaced with negatively charged particles, they would be attracted towards the positively charged nucleus instead of being repelled.
Expected results:
Negatively charged particles may bend towards the nucleus.
Some may be strongly attracted and move closer to the nucleus.
The scattering pattern would be different from alpha particles.
So, the experiment would not show the same kind of repulsion-based deflection seen with alpha particles.
Q5Question 5. Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?
According to Thomson’s plum pudding model, the positive charge of the atom was spread uniformly throughout the atom.
If this were true, alpha particles should have passed through the atom with only slight deflection. They should not have bounced back sharply.
But Rutherford observed that a few alpha particles bounced back. This meant that they had hit or come very close to a very small, dense, positively charged region.
This proved that:
The positive charge is not spread throughout the atom.
It is concentrated in a small central region called the nucleus.
Most of the atom is empty space.
Thus, the sharp bouncing back of alpha particles completely ruled out Thomson’s plum pudding model.
Q6Question 6. If you could ask Rutherford one question about his work, what would it be?
I would ask Rutherford:
“How did you feel when you observed that a few alpha particles bounced back, even though Thomson’s model predicted that they should pass through the foil?”
This question is important because that unexpected observation led to the discovery of the nucleus and changed our understanding of atomic structure.
NCERT Solutions
Pause and Ponder — Page 145
Q7Question 7. Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere. 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.
Correct option: (ii) Both A and R are true, but R is not the correct explanation of A.
Explanation
Assertion is true because Rutherford concluded from the gold foil experiment that most of the mass and positive charge of an atom are concentrated in a small central region called the nucleus.
Reason is also true because Thomson’s model described the atom as a positively charged sphere with electrons embedded in it.
However, Thomson’s model does not explain Rutherford’s conclusion. Rutherford’s conclusion came from the gold foil experiment, not from Thomson’s model.
Therefore, option (ii) is correct.
NCERT Solutions
What if — Page 147
QuestionQuestion. An atom had no empty space? How would this have affected the size of various objects?
If atoms had no empty space, all the particles inside atoms would be packed very closely together.
As a result:
Atoms would become much smaller.
Objects made of atoms would also become much smaller.
Matter would become extremely dense.
Large objects like books, tables, houses, and even our bodies would occupy much less space.
Rutherford’s experiment showed that most of an atom is empty space. If this empty space did not exist, the size of objects around us would reduce greatly, while their mass might remain almost the same.
NCERT Solutions
Pause and Ponder — Page 149
Q8Question 8. Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules.
Suppose I discovered a new element and named it Vaibhavium.
I would choose the symbol Vb.
This symbol follows IUPAC rules because:
The first letter V is capital.
The second letter b is small.
The symbol has one or two letters.
It is based on the name of the element.
Therefore, Vb is a suitable symbol for the imaginary element Vaibhavium.
Q9Question 9. What problems could arise if every scientist used different symbols for the same element?
If every scientist used different symbols for the same element, many problems would arise.
Scientists from different countries would not understand each other clearly.
Chemical formulas would become confusing.
Students and teachers would find chemistry difficult to study.
There could be mistakes in experiments and chemical reactions.
Scientific communication would become slow and inaccurate.
Therefore, standard symbols approved by IUPAC are important for clear and universal communication.
NCERT Solutions
Pause and Ponder — Page 150
Q10Question 10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Given:
Atomic number = 26
Number of nucleons = 56
Atomic number = Number of protons
So,
Number of protons = 26
For a neutral atom:
Number of electrons = Number of protons = 26
Mass number = Number of nucleons = 56
Number of neutrons = Mass number − Atomic number
= 56 − 26
= 30
Therefore:
Electrons = 26
Protons = 26
Neutrons = 30
Q11Question 11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Given:
Number of protons = 20
Mass number = 41
Number of neutrons = Mass number − Number of protons
= 41 − 20
= 21
Therefore, the atom has 21 neutrons.
Q12Question 12. An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Given:
Number of neutrons = 18
Atomic number = 17
Atomic number = Number of protons
So,
Number of protons = 17
Mass number = Number of protons + Number of neutrons
= 17 + 18
= 35
Therefore, the mass number is 35.
Q13Question 13. An atom ²³A has 11 electrons. Find the number of neutrons in it.
Given:
Mass number = 23
Number of electrons = 11
For a neutral atom:
Number of electrons = Number of protons
So,
Number of protons = 11
Number of neutrons = Mass number − Number of protons
= 23 − 11
= 12
Therefore, the atom has 12 neutrons.
NCERT Solutions
Pause and Ponder — Page 152
Q14(i)Question 14. Identify the number of electrons in the outermost shell of the following elements: Question 14(i). ¹²₆C
Atomic number of carbon = 6
Electronic configuration = 2, 4
Outermost shell has 4 electrons.
Therefore, carbon has 4 valence electrons.
Q14(ii)Question 14. Identify the number of electrons in the outermost shell of the following elements: Question 14(ii). ¹⁹₉F
Atomic number of fluorine = 9
Electronic configuration = 2, 7
Outermost shell has 7 electrons.
Therefore, fluorine has 7 valence electrons.
Q14(iii)Question 14. Identify the number of electrons in the outermost shell of the following elements: Question 14(iii). ²⁸₁₄Si
Atomic number of silicon = 14
Electronic configuration = 2, 8, 4
Outermost shell has 4 electrons.
Therefore, silicon has 4 valence electrons.
Q15Question 15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Atomic number 12
Element = Magnesium
Number of electrons = 12
Electronic configuration = 2, 8, 2
Atomic number 16
Element = Sulfur
Number of electrons = 16
Electronic configuration = 2, 8, 6
Atomic number 18
Element = Argon
Number of electrons = 18
Electronic configuration = 2, 8, 8
Q16Question 16. Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.
Given:
Mass number = 23
Number of protons = 11
Atomic number = Number of protons = 11
The element with atomic number 11 is sodium (Na).
Number of neutrons = Mass number − Number of protons
= 23 − 11
= 12
Therefore, the atom is sodium, and it has 12 neutrons.
One Similar Riddle
I am an atom with atomic number 17.
I have 18 neutrons.
I am a greenish-yellow gas and combine with sodium to form common salt.
Who am I?
Chlorine
Mass number = 17 + 18 = 35
NCERT Solutions
Pause and Ponder — Page 156
Q17Question 17. Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?
Both atoms have 11 protons.
Atomic number = Number of protons
So, both atoms have atomic number 11.
Atom 1:
Protons = 11
Neutrons = 12
Mass number = 11 + 12 = 23
Atom 2:
Protons = 11
Neutrons = 13
Mass number = 11 + 13 = 24
Both atoms have the same atomic number but different mass numbers.
Therefore, they are atoms of the same element but different isotopes.
The element with atomic number 11 is sodium. So these are isotopes of sodium.
Q18Question 18. If a bromine atom is available in the form of, say two isotopes, ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of the bromine atom.
Given:
Mass of first isotope = 79 u
Abundance = 49.7%
Mass of second isotope = 81 u
Abundance = 50.3%
Average atomic mass:
= 79 × 49.7/100 + 81 × 50.3/100
= 39.263 + 40.743
= 80.006 u
Therefore, the average atomic mass of bromine is approximately:
80.0 u
So, the average atomic mass of bromine is 80 u.
NCERT Solutions
Revise, Reflect, Refine — Question Answers for Class 9
Q1Question 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: (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.
The correct options are:
(ii) and (iii)
Explanation of each option
(i) Incorrect
Rutherford’s gold foil experiment did not show the existence of neutrons. Neutrons were discovered later by James Chadwick in 1932.
(ii) Correct
The experiment disproved Thomson’s plum pudding model because if positive charge were spread throughout the atom, alpha particles would not have bounced back sharply. The experiment led to the idea of a small, dense nucleus at the centre of the atom.
(iii) Correct
A few alpha particles were deflected through large angles or bounced back. This showed that most of the positive charge and mass of the atom are concentrated in a very small central region called the nucleus.
(iv) Incorrect
The deflection of alpha particles gave evidence for the nucleus, not for the movement of electrons around the nucleus.
Q2Question 2. 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.
(i) Incorrect
According to Bohr’s model, electrons do not lose energy while moving in fixed orbits. Therefore, they do not fall into the nucleus.
(ii) Incorrect
Bohr’s model says that electrons can revolve only in certain fixed orbits or shells. They cannot exist anywhere around the nucleus with any random energy.
(iii) Correct
According to Bohr’s model, electrons revolve around the nucleus in fixed energy levels or shells without losing energy.
(iv) Incorrect
Bohr’s model states that electrons cannot exist between two energy levels. They can move from one shell to another only by absorbing or releasing a fixed amount of energy.
Q3Question 3. The composition of the nuclei of three atomic species X, Y, and Z are given as follows. Explain the relation between the following: (i) Y and Z (ii) Z and X
(i) Relation between Y and Z
Y has:
Protons = 17
Neutrons = 18
Mass number = 17 + 18 = 35
Z has:
Protons = 17
Neutrons = 20
Mass number = 17 + 20 = 37
Y and Z have the same number of protons but different numbers of neutrons. Therefore, they have the same atomic number but different mass numbers.
So, Y and Z are isotopes of the same element.
(ii) Relation between Z and X
Z has:
Protons = 17
Neutrons = 20
Mass number = 17 + 20 = 37
X has:
Protons = 18
Neutrons = 19
Mass number = 18 + 19 = 37
Z and X have the same mass number, 37, but different atomic numbers.
So, Z and X are isobars.
| Atomic species | X | Y | Z |
|---|---|---|---|
| Number of protons | 18 | 17 | 17 |
| Number of neutrons | 19 | 18 | 20 |
Q4Question 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 in the gold foil experiment?
Rutherford concluded that the positively charged part of the atom is not spread throughout the atom. It is concentrated in a very small region at the centre of the atom.
This central region is called the nucleus.
He also concluded that:
The nucleus is very small.
The nucleus is dense.
The nucleus contains positive charge.
Most of the mass of the atom is concentrated in the nucleus.
Most of the atom is empty space.
Q5Question 5. 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 ʻplum puddingʼ 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.
The correct chronological order is:
(iv) → (ii) → (iii) → (i)
Explanation
1. Dalton’s model
Dalton described atoms as indivisible particles. He believed atoms were the smallest units of matter.
2. Thomson’s model
Thomson discovered electrons and proposed the plum pudding model. According to him, electrons were embedded in a positively charged sphere.
3. Rutherford’s model
Rutherford’s gold foil experiment showed that an atom has a dense, positively charged nucleus at the centre.
4. Bohr’s model
Bohr improved Rutherford’s model by suggesting that electrons revolve around the nucleus in fixed orbits with definite energy.
Q6Question 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. The nucleus contains positively charged protons.
Opposite charges attract each other. Therefore, the negatively charged electrons are attracted towards the positively charged nucleus.
This electrostatic force of attraction keeps electrons bound to the atom and prevents them from flying away.
According to Bohr’s model, electrons move in fixed shells or energy levels around the nucleus without losing energy.
Q7Question 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. 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.
Correct option: (ii) Both A and R are true, but R is not the correct explanation of A.
Explanation
Assertion is true because the discovery of electrons, protons, and neutrons helped scientists understand the structure of the atom.
Reason is also true because in a neutral atom, the number of electrons is equal to the number of protons.
However, the reason does not correctly explain why the discovery of subatomic particles helped in understanding atomic structure. Therefore, option (ii) is correct.
Q8Question 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:
Mass number = 24
Atomic number = 12
Atomic number = Number of protons
So,
Number of protons = 12
For a neutral atom:
Number of electrons = Number of protons
So,
Number of electrons = 12
Number of neutrons = Mass number − Atomic number
= 24 − 12
= 12
Therefore:
(i) Protons = 12
(ii) Neutrons = 12
(iii) Electrons = 12
Electronic configuration of magnesium
Magnesium has 12 electrons.
Electron arrangement:
K shell = 2
L shell = 8
M shell = 2
So, electronic configuration = 2, 8, 2
Simple illustration
Electronic configuration: 2, 8, 2
Q9Question 9. 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
For a neutral atom, the total number of electrons equals the number of protons and the atomic number. Valence electrons are the electrons in the outermost shell, and valency is found from the number of electrons lost, gained or shared to achieve a stable shell.
The correct information for the four electronic configurations in Fig. 8.17 is given in the table below.
| Figure | Electronic configuration | Name of element | Symbol | Total electrons | Valence electrons | Valency | Protons | Atomic number |
|---|---|---|---|---|---|---|---|---|
| (a) | 2, 1 | Lithium | Li | 3 | 1 | 1 | 3 | 3 |
| (b) | 2, 5 | Nitrogen | N | 7 | 5 | 3 | 7 | 7 |
| (c) | 2, 8, 3 | Aluminium | Al | 13 | 3 | 3 | 13 | 13 |
| (d) | 2, 7 | Fluorine | F | 9 | 7 | 1 | 9 | 9 |
Q10Question 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?
Rutherford’s model said that electrons revolve around the nucleus like planets around the Sun. But according to classical physics, an electron moving in a circular path should continuously lose energy.
If electrons lose energy, they should spiral inward and fall into the nucleus. This would make atoms unstable. But atoms are actually stable. Therefore, Rutherford’s model failed to explain atomic stability.
Bohr solved this problem by proposing that electrons revolve only in fixed orbits or shells of definite energy. While moving in these fixed shells, electrons do not lose energy.
Therefore, Bohr’s model explained why atoms are stable.
Q11Question 11. An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
Given:
Mass number = 70
Number of electrons = 31
For a neutral atom:
Number of electrons = Number of protons
So,
Number of protons = 31
Number of neutrons = Mass number − Number of protons
= 70 − 31
= 39
Therefore, the number of neutrons is 39.
Q12Question 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
Number of neutrons = Mass number − Number of protons
= 197 − 79
= 118
So, number of neutrons = 118
(ii) Number of electrons
For a neutral atom:
Number of electrons = Number of protons
So, number of electrons = 79
Therefore:
Neutrons = 118
Electrons = 79
Q13Question 13. Complete the Table 8.5:
Explanation
Mass number = Protons + Neutrons
Atomic number = Number of protons
For a neutral atom, electrons = protons
| 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 | – | – | – |
| 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 |
Q14Question 14. 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?
Given:
Mass number = 35
Number of neutrons = 18
Number of protons = Mass number − Number of neutrons
= 35 − 18
= 17
(i) How many electrons and protons does element X have?
Number of protons = 17
For a neutral atom:
Number of electrons = Number of protons = 17
So, element X has 17 protons and 17 electrons.
(ii) What is its atomic number?
Atomic number = Number of protons
So, atomic number = 17
(iii) Identify the element X.
The element with atomic number 17 is chlorine.
So, X is chlorine (Cl).
(iv) Write its electronic configuration.
Chlorine has 17 electrons.
Electronic configuration = 2, 8, 7
(v) How many valence electrons does it have?
The outermost shell has 7 electrons.
So, chlorine has 7 valence electrons.
(vi) What will be the mass number if two neutrons are added to its nucleus?
Original mass number = 35
If 2 neutrons are added:
New mass number = 35 + 2 = 37
(vii) What will be the relation of X with the new atom?
The original atom has atomic number 17 and mass number 35.
The new atom has atomic number 17 and mass number 37.
They have the same atomic number but different mass numbers.
Therefore, they are isotopes of chlorine.
Q15Question 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: (i) Atomic number (ii) Atomic mass (iii) Mass number (iv) Overall charge
Given:
Protons = 12
Neutrons = 12
Electrons are replaced by hypothetical particles having the same charge as electrons but 500 times heavier.
(i) Atomic number
Atomic number depends only on the number of protons.
Since the number of protons is still 12, the atomic number will not change.
Atomic number = 12
(ii) Atomic mass
Atomic mass will increase because the new hypothetical particles are 500 times heavier than electrons.
Although ordinary electrons have negligible mass, these heavier particles will add more mass to the atom.
So, atomic mass will increase.
(iii) Mass number
Mass number depends only on the number of protons and neutrons.
Mass number = Protons + Neutrons
= 12 + 12
= 24
Since the number of protons and neutrons does not change, the mass number will remain the same.
Mass number = 24
(iv) Overall charge
The hypothetical particles have the same charge as electrons, that is, negative charge.
If the number of these particles is equal to the number of protons, the total negative charge will balance the total positive charge.
So, the atom will remain electrically neutral.
Overall charge = 0
NCERT Solutions
The Quest Continues
QuestionQuestion. Is it possible to completely understand everything that happens inside an atom?
At present, it is not possible to completely understand everything that happens inside an atom.
Scientists have discovered many important things about atoms, such as electrons, protons, neutrons, nucleus, energy levels, isotopes, and isobars. However, the atomic world is extremely tiny and complex.
Earlier models like Dalton’s, Thomson’s, Rutherford’s, and Bohr’s models helped us understand atoms step by step. Later, scientists developed the quantum mechanical model, which gives a deeper explanation of atomic structure.
Even today, research continues in atomic and nuclear science. Scientists are still trying to understand more about the behaviour of particles inside atoms.
So, we can say that our understanding of atoms has improved greatly, but the journey of completely understanding the atom is still continuing.
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