Journey Inside the Atom — Class 9 Science
Exploration · Chapter 8
CBSE Class 9 community
View all
6- Mathematics Part I
- Mathematics Part II
- + 4 more
- Formula cards
- Graph reading
- 16 traps corrected
2- Mathematics Part I
- Mathematics Part II
- Timelines
- Comparison tables
- Article numbers
- 16 texts summarised
- Poetry devices
- Grammar rules
- 12 पाठ
- अलंकार और समास
- मुहावरे
Chapter summary
Key ideas & terms
- Atomic number (Z)
- The number of protons in the nucleus of an atom; it identifies the element and equals the number of electrons in a neutral atom.
- Mass number (A)
- The total number of protons and neutrons (nucleons) in the nucleus of an atom.
- Electron, proton, neutron
- The three subatomic particles, with relative charges of −1, +1 and 0 respectively; protons and neutrons lie in the nucleus and electrons revolve around it.
- Bohr’s shells (energy levels)
- Fixed orbits K, L, M, N… (n = 1, 2, 3…) in which electrons move without losing energy, each holding up to
electrons. - Valency
- The combining capacity of an atom, equal to the number of electrons it gains, loses or shares to complete its octet.
- Isotopes
- Atoms of the same element with the same atomic number but different mass numbers, e.g. protium, deuterium and tritium of hydrogen.
Extra questions & answers
What did Thomson discover and what model did he propose?
He discovered the electron from cathode rays and proposed the plum-pudding model, in which electrons are embedded in a sphere of positive charge so the atom is neutral.
What did the gold-foil experiment reveal about the atom?
Most alpha particles passed straight through (atom is mostly empty space), some were deflected and a few bounced back, showing a tiny, dense, positively charged nucleus at the centre.
Why did Rutherford’s model fail to explain stability?
An electron moving in a circular orbit is accelerating and should continuously radiate energy, causing it to spiral into the nucleus; but real atoms are stable, so the model was incomplete.
How did Bohr’s model account for stability?
Bohr proposed that electrons revolve only in certain fixed energy levels (shells) where they do not lose energy, so they neither radiate nor spiral into the nucleus.
Why was the neutron’s discovery important?
Chadwick’s neutral neutron explained why an atom’s mass is greater than that of its protons alone (e.g. helium is four times hydrogen’s mass, not twice).
Differentiate atomic number and mass number.
Atomic number (Z) is the number of protons and identifies the element, while mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus.
State the rules for filling electrons in shells.
A shell n holds at most
How is the valency of an element determined from its configuration?
It is the number of electrons gained, lost or shared to reach a stable octet; if valence electrons are fewer than four the atom tends to lose them, if more than four it tends to gain.
What are isotopes? Give an example.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, such as the carbon isotopes carbon-12, carbon-13 and carbon-14.
Why is the average atomic mass of chlorine 35.5 u?
Chlorine has isotopes of mass 35 u and 37 u in about 3:1 ratio; the abundance-weighted average is
Think It Over
Are atoms the smallest indivisible particles?
No. Atoms are not indivisible. Experiments showed that atoms are made of smaller subatomic particles — electrons, protons and neutrons. Even protons and neutrons are made of still smaller particles, but at the Class 9 level we say the atom is divisible into electrons, protons and neutrons.
Why do electrons not fall into the nucleus even though they are attracted to protons in it?
In Rutherford’s planetary model electrons should lose energy and spiral into the nucleus, but this does not happen. Bohr explained that electrons revolve only in allowed fixed orbits called stationary states or energy levels. While in one such orbit an electron does not lose energy, so it does not fall in. It can only change orbit by absorbing or emitting a fixed amount of energy.
Why did scientists keep modifying atomic models?
Science progresses with new evidence. Each model explained the experiments known at that time, but new experiments gave results that the old model could not explain. For example, the discovery of electrons changed Dalton’s indivisible atom to Thomson’s model, the gold foil scattering changed it to Rutherford’s nuclear model, and the problem of stability led to Bohr’s model. Each modification made the model fit new observations better.
Pause and Ponder
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: (i) the positive charge on the clay is lesser than the total negative charge of the beads?
The model would no longer be neutral. It would have a net negative charge equal to the excess negative charge of the beads, unlike a real atom which is electrically neutral because total positive charge equals total negative charge.
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: (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
No. In Thomson’s model the clay must represent only positive charge to balance the negative beads. If the clay itself is negative, the whole model becomes negative and does not represent a neutral atom. The positive sphere and negative electrons must balance to make the atom neutral.
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?
It matches partly: the soft pulp is like the sphere of positive charge and the seeds scattered inside are like electrons embedded in it. It falls short because in a real atom the positive charge and electrons are not visible pieces like pulp and seeds, electrons are extremely tiny and distributed throughout the sphere, the atom is spherical and uniform, and the model does not show the relative sizes or electrical nature correctly. Also the orange has a skin which has no counterpart in the atom.
3. Why did Thomson conclude that electrons are present in all atoms?
Thomson found that the properties of cathode rays — their charge and mass ratio — were the same regardless of the material of the cathode or the gas in the tube. Since the same negatively charged particles came from different elements, he concluded electrons are a fundamental constituent present in atoms of all elements.
Think as a Scientist
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. Hint: Compare thin foil vs thick foil. How does the thickness affect the chances of hitting a nucleus?
With a thicker foil the alpha particles would have to pass through many more atoms and thus have more chances to come close to a nucleus. So fewer particles would go straight through undeflected, more particles would be deflected through small and large angles, and more particles would bounce back. The diagram would show the same incoming beam but with a larger fraction of arrows scattered at wide angles and backwards compared to the thin foil case, where most arrows pass straight.
Pause and Ponder
4. What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?
Negatively charged particles would be attracted by the positively charged nucleus instead of being repelled. Instead of being scattered backwards by repulsion, they would be pulled towards the nucleus and might be deflected in a different way or even captured. They would also be repelled by the electrons around the nucleus, so the pattern of mostly undeflected particles and a few large-angle scatterings would not be observed. The experiment depends on repulsion between two positive charges.
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?
In Thomson’s model the positive charge is spread uniformly throughout the atom, so the electric field is weak and cannot exert a strong enough repulsive force to turn a fast, heavy positively charged
6. If you could ask Rutherford one question about his work, what would it be?
One thoughtful question could be: How did you feel when you saw a few alpha particles bouncing back, and what made you confident that this meant a tiny dense nucleus existed rather than an experimental error? Any genuine curiosity-based question about his observation or his new planetary model is acceptable.
Pause and Ponder
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.
Option (ii) is correct. Assertion (A) is true — Rutherford’s scattering showed mass and positive charge are concentrated in the nucleus. Reason (R) is also true as a description of Thomson’s model, but it is not the explanation for (A). In fact Rutherford’s conclusion was reached because Thomson’s model could not explain the large-angle scattering.
Pause and Ponder
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.
Example: I name the element pranavium after myself. I give it the symbol Pa only if Pa is not already used, but Pa is already protactinium, so I could choose Pr or Pv. The IUPAC rules are: symbol is one or two letters, first letter capital and second letter small, and it should be derived from the name and not duplicate an existing symbol. For example, if my name is Anya, I could propose Anyasium with symbol Ay — first letter A capital, second letter y small, and Ay is not used for any existing element, so it follows the rules.
9. What problems could arise if every scientist used different symbols for the same element?
Communication would become confusing and errors would occur. Scientists in different countries could not understand each other’s formulas and equations, students would be confused, and sharing research, buying chemicals, or writing chemical equations would lead to mistakes. A common internationally approved symbol system avoids language barriers and ensures everyone means the same element.
Pause and Ponder
10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Atomic number
11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Protons
12. An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Atomic number
13. An atom
The notation
Pause and Ponder
14. Identify the number of electrons in the outermost shell of the following elements: (i)
14. Identify the number of electrons in the outermost shell of the following elements: (ii)
14. Identify the number of electrons in the outermost shell of the following elements: (iii)
15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Using the rules
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.
Protons 11 is sodium, symbol Na, mass number 23. Neutrons =
Pause and Ponder
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 have protons 11, so atomic number
18. If a bromine atom is available in the form of, say two isotopes,
Average mass = weighted average:
Revise, Reflect, Refine
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.
Incorrect. Rutherford’s experiment in 1911 showed a dense positively charged nucleus. Neutrons were discovered much later in 1932 by Chadwick. The scattering of
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: (ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
Correct. The observation that most
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: (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 tiny, massive, highly charged centre could repel a heavy fast
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: (iv) The way alpha particles were deflected showed that electrons move around the nucleus.
Incorrect. Rutherford’s scattering tells us about the nucleus, not about electron motion. Electrons are too light to deflect heavy
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.
Incorrect according to Bohr. Bohr postulated that while in a stationary orbit (energy level) an electron does not lose energy, so it stays in that orbit and does not spiral into the nucleus.
2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (ii) Electrons can exist anywhere around the nucleus with no fixed energy.
Incorrect. Bohr said electrons can exist only in allowed fixed orbits with definite energy values, not anywhere between them.
2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
Correct. This is Bohr’s central postulate — each shell K, L, M... has a fixed energy and electrons in it remain stable without radiating energy.
2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (iv) Electrons can be found between energy levels as they move around the nucleus.
Incorrect. Bohr stated electrons are found only in the allowed levels, not in between. Jumping between levels requires absorbing or emitting exact energy difference.
3. 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
Y has
3. 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: (ii) Z and X
Z has
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?
He concluded that the entire positive charge and almost all the mass of the atom are concentrated in a very small, dense region at the centre called the nucleus. The nucleus is about
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 ‘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.
Correct chronological order: (iv) Dalton 1808 — indivisible atoms → (ii) Thomson 1897-1904 — plum pudding with electrons in positive sphere → (iii) Rutherford 1911 — nuclear planetary model → (i) Bohr 1913 — fixed energy orbits. Each step was driven by new experiments that the previous model could not explain.
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 and the nucleus contains positively charged protons. The opposite charges attract each other with an electrostatic force. This attraction provides the centripetal force that keeps electrons orbiting and prevents them from flying away, similar to how gravity keeps planets orbiting the sun.
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.
Option (ii) is correct. Both statements are true — discovering electrons, protons and neutrons revealed the internal structure, and in a neutral atom electrons equal protons. However R is not the reason for A; the understanding came from discovering that atoms are divisible and studying the properties of those particles, not simply from charge balance.
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,
Atomic number
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 (ii) neutrons,
Mass number
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 (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
In a neutral atom electrons = protons = 12. Arrangement by Bohr-Bury rules: K shell
9. Find the following information for the elements shown in Fig. 8.17: (i) Name of the element
Figure 8.17 shows atomic diagrams. Count electrons (which equals protons and atomic number) to identify: For example, if a diagram shows 3 electrons it is lithium Li, 8 electrons it is oxygen O, etc. The name is determined by the atomic number that the total electron count gives.
9. Find the following information for the elements shown in Fig. 8.17: (ii) Symbol
Symbol is the IUPAC one-letter or two-letter abbreviation for the name found in (i). For instance lithium is Li, oxygen is O, sodium is Na, chlorine is Cl.
9. Find the following information for the elements shown in Fig. 8.17: (iii) Total number of electrons
Total electrons are counted as dots or circles in the diagram. That number equals the atomic number for a neutral atom. For example diagram (a) with 3 dots has 3 electrons, (b) with 11 dots has 11 electrons, etc.
9. Find the following information for the elements shown in Fig. 8.17: (iv) Number of valence electrons
Valence electrons are those in the outermost shell. From the configuration, e.g.
9. Find the following information for the elements shown in Fig. 8.17: (v) Valency of the element
Valency is electrons needed to gain/lose/share to get octet. For valence 1, valency 1 (lose 1); for valence 7, valency 1 (gain 1); for valence 4, valency 4. Calculated from valence count.
9. Find the following information for the elements shown in Fig. 8.17: (vi) Number of protons
In a neutral atom protons = total electrons = atomic number. So count from (iii).
9. Find the following information for the elements shown in Fig. 8.17: (vii) Atomic number
Atomic number
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 allowed electrons to orbit at any radius like planets. A accelerating charged particle should radiate energy and spiral into the nucleus, so atoms should collapse — but they don’t. Bohr solved this by postulating stationary orbits with fixed energy where electrons do not radiate. Energy is only absorbed or emitted when jumping between orbits, so the atom remains stable.
11. An atom
Electrons 31 means in neutral atom protons
12. An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons,
Neutrons
12. An atom has 79 protons and a mass number of 197. Calculate (ii) the number of electrons.
For a neutral atom electrons = protons = 79. If ionised the number would differ, but as atom it is 79 electrons. This is gold Au.
13. Complete the Table 8.5: Atomic number 5, Mass number –, Number of neutrons 6, Number of protons –, Number of electrons –, Name of the elements –
Row 1:
13. Complete the Table 8.5: Atomic number –, Mass number 14, Number of neutrons –, Number of protons –, Number of electrons 7, Name of the elements Nitrogen
Row 2: electrons 7 means
13. Complete the Table 8.5: Atomic number –, Mass number 24, Number of neutrons –, Number of protons 12, Number of electrons –, Name of the elements –
Row 3:
13. Complete the Table 8.5: Atomic number 15, Mass number –, Number of neutrons 16, Number of protons –, Number of electrons –, Name of the elements –
Row 4:
13. Complete the Table 8.5: Atomic number –, Mass number 1, Number of neutrons 0, Number of protons –, Number of electrons –, Name of the elements –
Row 5:
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?
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: (ii) What is its atomic number?
Atomic number
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: (iii) Identify the element X.
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: (iv) Write its electronic configuration.
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: (v) How many valence electrons does it have?
Valence electrons are in outermost M shell = 7.
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: (vi) What will be the mass number if two neutrons are added to its nucleus?
New neutrons =
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: (vii) What will be the relation of X with the new atom?
Both have same atomic number 17 but different mass numbers 35 and 37, so they are isotopes of chlorine (
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
No effect. Atomic number is number of protons (
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: (ii) Atomic mass
Atomic mass would increase noticeably. Normally electrons contribute almost negligible mass, about
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: (iii) Mass number
No effect. Mass number
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: (iv) Overall charge
No effect. Charge depends on number of positive and negative charges, not their mass. 12 protons positive and 12 negative particles still balance, so atom remains neutral.