Class 9 · ScienceNCERT 2026 Revised Syllabus
Chapter 8: Journey Inside the Atom (0906CH08)
Exploration · Atomic models — Thomson, Rutherford, Bohr, electronic configuration.
Chapter code: 0906ch08 · Book: Exploration · Class 9 Science
Find complete NCERT solutions for Class 9 Science, Exploration Chapter 8. Every question has a clear, step-by-step answer for chapter code 0906CH08.
Pause and Ponder(18 questions)
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? (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
School Level Solution
Ans.
An atom is neutral only when total positive charge equals total negative charge. If the positive charge on the clay is smaller, the model has a net negative charge. If the clay itself is also negatively charged, the total negative charge increases further, so the model is not 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?
School Level Solution
Ans.
The analogy works only in the limited sense that small objects are embedded within a larger body. It falls short because Thomson’s model had positive charge spread uniformly through the atom with electrons embedded in it; citrus seeds are discrete structures with a very non-uniform distribution.
3
Why did Thomson conclude that electrons are present in all atoms?
School Level Solution
Ans.
Cathode rays behaved similarly when produced using different materials. Thomson identified the particles as identical, negatively charged particles and concluded that electrons are common constituents of atoms.
4
What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?
School Level Solution
Ans.
Because the nucleus is positively charged, negatively charged particles would experience electrostatic attraction toward the nucleus. Their trajectories would therefore bend toward the nucleus rather than being repelled as positively charged alpha particles are.
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?
School Level Solution
Ans.
A few alpha particles were deflected through very large angles or even backward. Such large deflections require a small, dense, positively charged region containing most of the mass — the nucleus — which is incompatible with Thomson’s uniformly spread positive charge.
6
If you could ask Rutherford one question about his work, what would it be?
School Level Solution
Ans.
A suitable question would be: “How did you estimate the size and charge concentration of the nucleus from the observed scattering angles?” Other scientifically relevant questions about the experimental setup or interpretation are also acceptable.
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.
School Level Solution
Ans.
Rutherford’s conclusion (A) is true. The statement about Thomson’s model (R) is also true, but it does not explain Rutherford’s conclusion; rather, Rutherford’s scattering results showed that Thomson’s model was inadequate.
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.
School Level Solution
Ans.
Example: “Sarition” → symbol Sa. The symbol follows the standard rule: the first letter is capitalised and, if a second letter is used, it is lowercase. The proposed name should also be distinct from existing element names/symbols.
9
What problems could arise if every scientist used different symbols for the same element?
School Level Solution
Ans.
A common symbolic system prevents ambiguity. Different symbols for the same element would make chemical equations, data tables, safety information, research papers and international communication difficult to interpret reliably.
10
An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
School Level Solution
Ans.
Atomic number Z = number of protons = 26. A neutral atom has 26 electrons. Neutrons = nucleons − protons = 56 − 26 = 30.
11
The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
School Level Solution
Ans.
Mass number = protons + neutrons. So neutrons = 41 − 20 = 21.
12
An atom has 18 neutrons and an atomic number of 17. What is its mass number?
School Level Solution
Ans.
Mass number A = protons + neutrons = 17 + 18 = 35.
13
An atom ²³A has 11 electrons. Find the number of neutrons in it.
School Level Solution
Ans.
For a neutral atom, electrons = protons = 11. Neutrons = mass number − protons = 23 − 11 = 12.
14
Identify the number of electrons in the outermost shell of the following elements: (i) ¹²₆C (ii) ¹⁹₉F (iii) ²⁸₁₄Si.
School Level Solution
Ans.
Carbon (Z=6): configuration 2,4 → 4 valence electrons. Fluorine (Z=9): 2,7 → 7. Silicon (Z=14): 2,8,4 → 4.
15
Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
School Level Solution
Ans.
Distribute electrons among shells: 12 → 2,8,2; 16 → 2,8,6; 18 → 2,8,8.
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.
School Level Solution
Ans.
11 protons means atomic number 11, which is sodium. Neutrons = 23 − 11 = 12.
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?
School Level Solution
Ans.
Atomic number = protons = 11 for both. Mass numbers are 11+12=23 and 11+13=24. Therefore they are the same element with different mass numbers — isotopes.
18
If a bromine atom is available in the form of two isotopes, ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass.
School Level Solution
Ans.
Average atomic mass = (79×49.7/100) + (81×50.3/100) = 39.263 + 40.743 = 80.006 u.