Model Test – Solved ഉത്തരങ്ങൾ
Every question of the model paper, in the same order, with the full answer, the marking scheme, an animated orbit diagram or picture for the longer questions, and a link to the page where the idea is taught. Try each one again before you open the answer, and mark your own paper honestly.
As in the textbook, the nucleus (ന്യൂക്ലിയസ്) is a dark-red disc showing the numbers of protons and neutrons (for example 12p / 14n), the shells (ഷെല്ലുകൾ) K, L, M, N are orange rings, and the electrons are green dots. The drawings are not to scale – a real nucleus is far, far smaller than the atom. Every number in the animations is worked out with Z = p = e, A = p + n, n = A − Z and 2n².
💡 Which particle did Chadwick call “neutral”?
The neutron is a neutral particle (നിർവീര്യ കണം) in the nucleus, with a mass of about 1 u. The electron has charge −1, the proton +1, and the α particle is positive. James Chadwick discovered the neutron in 1932.
Marking: 1 mark for (c).
📖 Review this on Page 7 · Table 1.2 · Page 6 · The neutron
💡 Hydrogen atom minus its electron = ?
Canal rays (കനാൽ രശ്മികൾ) depend on the gas in the tube. With hydrogen the positive particles are the smallest and lightest; Rutherford identified them as a subatomic particle and named them protons.
Marking: 1 mark.
📖 Review this on Page 5 · Canal rays and the proton
💡 M is shell number 3. Use 2n².
M is the third shell, so n = 3 and the maximum is 2n² = 2 × 3² = 2 × 9 = 18. (K: 2, L: 8, N: 32.)
Marking: 1 mark for (c).
📖 Review this on Page 9 · Table 1.3, 2n²
💡 Who discovered the neutron, and in which year?
J. J. Thomson’s discharge-tube experiments proved that the electron exists (Nobel Prize in Physics, 1906). In 1932 James Chadwick found the neutral particle in the nucleus – the neutron.
Marking: 1 mark.
📖 Review this on Page 6 · Chadwick and the neutron · Page 4 · Thomson and the electron
💡 The top number is A, the bottom number is Z.
Number of neutrons = mass number − atomic number = A − Z = 37 − 17 = 20. (This chlorine atom has 17 protons and 17 electrons.)
Marking: 1 mark.
📖 Review this on Page 8 · n = A − Z
💡 Look at the bottom numbers.
168O, 178O and 188O all have atomic number 8 – they are isotopes of oxygen. 147N has atomic number 7: it is nitrogen, a different element.
Marking: ½ mark for the nitrogen atom, ½ mark for the reason.
📖 Review this on Page 10 · Isotopes, key box 6
💡 Recall the “mass of the electron” box: the e/m ratio, the charge and the mass are three different numbers. Which one is the charge?
Millikan’s oil-drop experiment gave the charge of the electron, 1.6 × 10−19 coulomb (negative). 1.76 × 1011 is Thomson’s e/m ratio in C/kg, and 9.1 × 10−31 is the mass of the electron in kg, not a charge.
Marking: 1 mark for (c).
📖 Review this on Page 4 · Mass of the electron box · Page 3 · Powers of ten
💡 Table 1.6 – an isotope of a metal found in common salt.
Table 1.6: sodium-24 – detecting leaks in pipelines; iodine-131 – the thyroid gland; cobalt-60 – cancer treatment; iron-59 – diagnosing anaemia; uranium-235 – fuel in nuclear power stations.
Marking: 1 mark.
📖 Review this on Page 10 · Table 1.6 · Page 13 · Applications
💡 Rule 3 of electron configuration.
Given: N is the fourth shell, n = 4
Formula: maximum number of electrons = 2n²
Substitution: 2 × 4² = 2 × 16
Answer: (a) 32 electrons
(b) A shell can hold 2n² electrons, but the outermost shell of an atom can hold at most 8. In 2, 8, 10 the outermost shell M would have 10. So when M has 8, the last 2 electrons go into the next shell, N: 2, 8, 8, 2 (textbook Fig 1.12).
Marking: (a) formula ½, answer ½; (b) 1 mark for “the outermost shell can have at most 8 electrons”.
📖 Review this on Page 9 · The three rules, Table 1.3 · Page 10 · Fig 1.12
(i) sodium (Z = 11): 2, 9 (ii) magnesium (Z = 12): 8, 4
💡 How many electrons can K and L hold?
(i) The L shell (n = 2) can hold at most 2 × 2² = 8 electrons, not 9. Sodium: 2, 8, 1.
(ii) The K shell (n = 1) can hold at most 2 × 1² = 2 electrons, not 8 – and it must be filled first. Magnesium: 2, 8, 2.
Check: 2 + 8 + 1 = 11 and 2 + 8 + 2 = 12 electrons.
Marking: For each: ½ mark for the mistake, ½ mark for the correct configuration.
📖 Review this on Page 9 · Table 1.4, shell filler
💡 Think of the charge and of which electrode each comes from.
| Cathode rays | Canal (anode) rays | |
|---|---|---|
| Charge | negative – bend towards the positive plate | positive |
| Where they come from | from the cathode, travelling towards the anode | from the region of the anode; they pass through the holes of a perforated cathode |
| Change of gas | the same for every gas and electrode metal | depend on the gas; with hydrogen the particles are the lightest (protons) |
| Particles | electrons | positive particles (protons with hydrogen) |
Marking: 1 mark for each correct difference (any two).
📖 Review this on Page 4 · Cathode rays · Page 5 · Canal rays
💡 Plums in a pudding – what is the pudding, what are the plums?
(a) The atom is a positively charged sphere in which the negatively charged electrons are embedded, like the fruit pieces in a plum pudding (പ്ലം പുഡിങ് മാതൃക).
(b) The total positive charge of the sphere is equal to the total negative charge of the electrons, so they cancel and the atom is electrically neutral.
The model could not explain many experimental results (for example the gold foil experiment), so it was set aside.
Marking: (a) 1 mark; (b) 1 mark.
📖 Review this on Page 5 · Plum pudding model
💡 A moving charge gives out energy …
(a) According to electromagnetic theory (വൈദ്യുതകാന്തിക സിദ്ധാന്തം) a moving charged particle continuously gives out energy. So an electron revolving round the nucleus should lose energy, move closer and closer and finally fall into the nucleus. That does not happen – atoms are stable.
(b) Bohr said that electrons revolve only in definite orbits with a definite energy; while an electron stays in an orbit its energy does not change (stationary energy levels, സ്ഥിരോർജനിലകൾ). So it does not spiral into the nucleus. Energy is absorbed or given out only when it jumps from one orbit to another.
Marking: (a) 1 mark; (b) 1 mark.
📖 Review this on Page 6 · Limitations of Rutherford’s model · Page 7 · Bohr model, key box 3
| Observation | Inference |
|---|---|
| (i) Most of the α particles passed straight through the gold foil without any deflection. | …………… |
| (ii) Some α particles were deflected by small angles. | …………… |
| (iii) Very few α particles (about 1 in 20 000) bounced back at 180°. | …………… |
💡 Empty space → straight; near a positive part → a little bend; head-on with a tiny positive core → back.
(i) Most of the space in an atom is empty.
(ii) The positive α particles that passed near the positive part of an atom were repelled a little – the atom has a positively charged part.
(iii) All the positive charge is concentrated in a very small central region; the α particles that hit it head-on were pushed straight back. Rutherford called it the nucleus (ന്യൂക്ലിയസ്). (Key box 2: almost all the mass is there too.)
Marking: 1 mark for each inference.
📖 Review this on Page 6 · Gold foil lab · Page 12 · Let us assess
| Atom | Atomic number (Z) | Mass number (A) | Protons | Electrons | Neutrons |
|---|---|---|---|---|---|
| 94Be | 4 | 9 | (i) …… | 4 | (ii) …… |
| F | (iii) …… | (iv) …… | 9 | 9 | 10 |
| Ca | (v) …… | 44 | 20 | 20 | (vi) …… |
💡 Z = p = e; A = p + n; n = A − Z.
| Atom | Z | A | Protons | Electrons | Neutrons |
|---|---|---|---|---|---|
| 94Be | 4 | 9 | (i) 4 | 4 | (ii) 5 = 9 − 4 |
| 199F | (iii) 9 | (iv) 19 = 9 + 10 | 9 | 9 | 10 |
| 4420Ca | (v) 20 | 44 | 20 | 20 | (vi) 24 = 44 − 20 |
Use Z = number of protons = number of electrons; A = protons + neutrons; neutrons = A − Z.
Marking: ½ mark for each blank.
📖 Review this on Page 8 · Z, A and the table of atoms
💡 Number of electrons = number of protons.
Given: protons = 12, neutrons = 14
Formula: Z = number of protons; A = protons + neutrons
Substitution: A = 12 + 14
Answer: (a) Z = 12, A = 26 – the atom is 2612Mg, an atom of magnesium
(b) 12 electrons: K takes 2, L takes 8, the remaining 2 go to M → 2, 8, 2.
(c) Draw a nucleus marked 12p / 14n and three rings around it with 2, 8 and 2 electrons (see the animation).
Marking: (a) ½ + ½; (b) 1 mark; (c) 1 mark (nucleus labelled ½, electrons in the right rings ½).
📖 Review this on Page 8 · Atomic and mass number · Page 9 · Orbit diagrams
P: 16 protons, 16 neutrons · Q: 16 protons, 20 neutrons · R: 18 protons, 18 neutrons
(a) Write the mass number of each atom. (b) Which two are isotopes? Why? (c) Which two are isobars? Why?
💡 Isotopes: same Z. Isobars: same A.
(a) A = protons + neutrons: P = 16 + 16 = 32; Q = 16 + 20 = 36; R = 18 + 18 = 36.
(b) P and Q are isotopes (ഐസോടോപ്പുകൾ): same atomic number (16 – both are sulphur) but different mass numbers (32 and 36).
(c) Q and R are isobars (ഐസോബാറുകൾ): same mass number (36) but different atomic numbers (16 and 18 – sulphur and argon).
Marking: (a) 1 mark (all three; two correct ½); (b) ½ for the pair, ½ for the reason; (c) ½ + ½.
📖 Review this on Page 10 · Isotopes and isobars, sorter
💡 n = A − Z; isotopes share Z, isobars share A.
(a) Isotopes are different atoms of the same element with the same atomic number and different mass numbers.
Given: Z = 27, A = 60
Formula: protons = Z; neutrons = A − Z
Substitution: 60 − 27
Answer: (b) 27 protons, 33 neutrons
(c) Carbon-13 (only about 1.1 % of all carbon) is used to study life processes. Carbon-12 is the most abundant and the most stable. (Carbon-14 is the radioactive one used for dating.)
(d) They are isobars. Neutrons: 146C → 14 − 6 = 8; 147N → 14 − 7 = 7.
(e) Heavy water (ഘനജലം, D2O) is the oxide of deuterium, 21H; one deuterium atom has 1 neutron (and 1 proton).
Marking: 1 mark for each part ((b), (c), (d), (e): ½ + ½).
📖 Review this on Page 10 · Isotopes, Table 1.6, isobars · Page 13 · Applications
💡 Read the nucleus first; count the green dots ring by ring.
(a) The nucleus shows 14p and 14n: Z = 14, A = 14 + 14 = 28. (X is silicon.)
(b) 2, 8, 4; the outermost shell (M) has 4 electrons.
(c) The M shell – the energy of the shells increases with distance from the nucleus, and M is the farthest shell of X.
(d) It absorbs energy: moving from a lower energy level (L) to a higher one (M) needs energy. (Coming back from M to L it would give out energy.)
(e) 2n² = 2 × 3² = 18. But X has only 14 electrons: K takes 2 and L takes 8, so only 14 − 10 = 4 are left for M.
Marking: 1 mark for each part ((a), (b), (e): ½ + ½).
📖 Review this on Page 9 · Orbit diagrams · Page 7 · Energy ladder · Page 8 · Z and A
Marks summary
| Section | Questions | Marks each | Total | Topics |
|---|---|---|---|---|
| A | 1–8 | 1 | 8 | neutron, protons from canal rays, 2n² for M, Chadwick, n = A − Z, isotopes of oxygen, charge of the electron, sodium-24 |
| B | 9–13 | 2 | 10 | 2n² and the outer-shell rule, correcting configurations, cathode vs canal rays, plum pudding model, Rutherford’s limitation and Bohr |
| C | 14–17 | 3 | 12 | gold foil observations and inferences, Z / A / p / e / n table, orbit diagram of 12p 14n, isotopes and isobars |
| D | 18–19 | 5 | 10 | case study on radioactive isotopes and heavy water, reading an orbit diagram and energy levels |
| Total | 40 | |||
- Cool-off (15 min) – read the whole paper. Mark the counting questions (Q5, Q15, Q16, Q17, Q18, Q19) and note what each gives you: protons, neutrons, A or Z.
- Section A – about 10 minutes. One word or one letter; give the reason in Q6.
- Section B – about 20 minutes. In Q9 and Q10 write 2n² with the shell number; check that your electrons add up to Z.
- Section C – about 25 minutes. In Q16 draw the orbit diagram neatly: label the nucleus “12p 14n” and draw the electrons on the rings.
- Section D – about 25 minutes. Answer every part (a)–(e); show n = A − Z each time.
- Checking – keep 10 minutes: A on top, Z below? n = A − Z (not A + Z)? K has only 2? Outermost shell 8 or fewer?
Count your marks: 32 or more – excellent; 24–31 – good, revise the pages linked under the questions you missed; below 24 – go back through pages 4 to 10 with their labs and do the scored quiz again. Then return to the chapter home and tick off what you have learned.