✨ Class 8 Science Notes ✨
Chapter 5: Inside the Atom

20 Most Important Words (with Hindi Meaning)

Important Notes

• Atom is the smallest, indivisible unit of matter, retaining all properties in physical and chemical changes.
• Dalton’s theory: Atoms are solid, indivisible spheres. Thomson’s model: Positive charge spread all over, with electrons embedded (like plum pudding).
• Rutherford’s gold foil experiment discovered most of atom is empty space, nucleus is tiny and massive, positive, electrons revolve outside.
• Bohr’s model: Electrons move in fixed shells/orbits at set distances; energy is constant within same orbit.
• Three main subatomic particles: Proton (+, in nucleus), Neutron (0, in nucleus), Electron (-, in shells).
• Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons.
• Neutral atom: number of electrons = number of protons.
• Electron arrangement: Shells named K, L, M, N ... Max number in shell is given by 2n².
• Isotopes: Atoms with same atomic number but different mass numbers (due to neutrons).
• Atomic mass is almost entirely due to protons and neutrons.
• Valency is related to number of electrons in the outermost shell.
• Electronic configuration: Numerical (e.g. 2,8,1 for sodium) and diagrammatic forms.
• Nuclear reactors use controlled fission of isotopes (e.g., U-235); moderators and control rods manage speed and number of neutrons.
• Isotopes have many uses—in medicine (cancer treatment), industry, dating ancient objects (C-14).
• Stable atoms have a full duplet/octet in the valence shell (e.g. Helium, Neon).

20 Most Important One-Word Answer Questions

1. Smallest unit of matter? Atom
2. Nucleus charge? Positive
3. Negatively charged subatomic particle? Electron
4. Location of protons in atom? Nucleus
5. Atomic number represented by? Z
6. Discovered neutron? Chadwick
7. Scientist for nuclear atom model? Rutherford
8. Method for electronic configuration? 2n² formula
9. Charge of neutron? Zero
10. Maximum electrons in L shell? 8
11. Total number of protons + neutrons? Mass number (A)
12. Isotope: Number of protons? Same
13. Plum pudding model by? Thomson
14. Atomic mass unit symbol? u or amu
15. Outermost shell called? Valence shell
16. K shell can hold? 2 electrons
17. Unit for atomic mass? Dalton (u)
18. Charge on alpha particles? Positive
19. Stable configuration in inert gases? Octet/duplet
20. Fuel for nuclear reactors? Uranium-235

20 Very Short Answer Questions

1. Define atom. Smallest particle of an element retaining its chemical identity.
2. What is an isotope? Atoms with same atomic number but different mass numbers.
3. Formula for electron capacity in shell? 2n²
4. Largest part of atom’s mass is? Nucleus
5. Who proposed ‘paramanu’? Kanad (Indian philosopher)
6. What is valency? Capacity to combine; determined by valence electrons.
7. Complete the duplet means? 2 electrons in K shell (full shell)
8. Maximum electrons in M shell? 18
9. First shell is called? K shell
10. Location of electrons? Revolving in shells/orbits
11. What is mass number of carbon with 6 protons, 6 neutrons? 12
12. Electronic configuration of sodium? 2,8,1
13. Charge of proton? +1
14. Who discovered electron? J.J. Thomson
15. Atomic number of sodium? 11
16. State two isotopes of carbon. \({}^{12}C\), \({}^{14}C\)
17. What is the symbol for neutron? n
18. Define mass number. Sum of protons and neutrons.
19. Charge on nucleus? Positive
20. Name the force binding nucleus. Nuclear force

20 Short Answer Questions (2–3 lines)

1. Describe Rutherford’s gold foil experiment and its findings. Alpha particles were projected onto a thin gold foil; most passed through, few deflected, suggesting atom is mostly empty space with tiny, dense nucleus at center.
2. Write the key points of Bohr’s atomic model. Electrons move in fixed shells, each shell has certain energy; electron energy is constant within an orbit; electrons absorb or emit energy when moving between shells.
3. How is atom electrically neutral? Number of electrons (negative) equals protons (positive); their charges cancel resulting in no overall charge.
4. Explain valency using electronic configuration. Valency = number of valence electrons if 4 or less; if more than 4, valency = 8 – valence electrons.
5. Difference between atomic number and mass number. Atomic number = protons; mass number = protons + neutrons.
6. Distinguish between isotopes and isobars. Isotopes: same atomic number, different mass numbers; Isobars: same mass number, different atomic numbers.
7. State uses of isotopes in daily life. Medical treatment (cancer), industry (leak detection), dating objects (C-14), electricity generation (U-235).
8. What is an electronic configuration? Give example. Arrangement of electrons in shells/orbits; e.g., oxygen: 2,6.
9. Why mass of electron is negligible? Mass of electron is about 1/1800 that of proton or neutron.
10. Which force binds protons and neutrons in nucleus? Nuclear force binds nucleons strongly together.
11. Explain the term ‘octet’ in atoms. Eight electrons in the valence shell provide maximum stability (as in inert gases).
12. Give modern notation for magnesium atom (Z=12, A=24). \({}_{12}^{24}Mg\)
13. Difference between Dalton’s and Thomson’s model. Dalton: hard, indivisible sphere; Thomson: electrons embedded in diffused positive charge.
14. State the role of neutron in nucleus. Neutrons hold protons together by nuclear force and stabilize the nucleus.
15. What is atomic mass unit (amu)? Unit for atomic mass (1 u = \(1.66\times10^{-27}\) kg).
16. How do isotopes of an element differ? Same number of protons, different number of neutrons (hence mass differs).
17. Write electronic configuration of chlorine (Z=17). 2,8,7
18. State the significance of mass number A. Indicates sum of protons and neutrons, reflects nucleus’s mass.
19. What is duplet? Example. Two electrons in valence shell giving stability (e.g., helium).
20. Relation: K : 2 :: M : ...? 18

All Textbook Exercise Solutions

• What is the difference in the atomic models of Thomson and Rutherford? Thomson: electrons embedded in positive 'pudding', no nucleus. Rutherford: dense nucleus with electrons revolving outside.
• What is meant by valency? Relationship between number of valence electrons and valency? Valency is combining capacity; if valence electrons ≤4, valency = that number; if >4, valency = 8 - that number.
• What is atomic mass number? Explain C (Z=6, A=12). Mass number = protons + neutrons. For carbon, 6 protons + 6 neutrons = 12.
• Define subatomic particle. Briefly describe three types. Constituents of atom: Proton (+, in nucleus, 1u), Neutron (0, nucleus, 1u), Electron (-, outside nucleus, 1/1800 u).
• Why is all the mass of an atom concentrated in the nucleus? Protons/neutrons (nucleus) are massive; electrons are very light; hence almost all mass is in nucleus.
• Why is atom electrically neutral? Same number of protons (+) and electrons (-); charges cancel.
• Why is atomic mass number always a whole number? Protons/neutrons are whole numbers; sum is whole number.
• Why are atoms stable despite moving electrons? Total positive and negative charges balance, making atom stable.
• Define: Atom, Isotope, Atomic number, Atomic mass number, Moderator in nuclear reactor. Atom: Smallest unit of element. Isotope: Same atomic number, different mass. Atomic number (Z): Protons or electrons. Atomic mass number (A): Protons + neutrons. Moderator: Substance slowing neutrons in reactor.
• Fill in the blanks: Electron, proton, neutron are types of ______ in atom. Subatomic particles
• Electron carries ______ charge. Negative
• K shell is nearest to nucleus.
• Magnesium configuration 2,8,2, valence shell is ______. M
• Fe valency (from Fe₂O₃) is ______. 3
• Proton : Positively charged, Electron : Negatively charged, Neutron : Neutral
• \({}_{11}^{23}Na\) neutrons? 12 (23–11)
• \({}_{6}^{14}C\) mass number? 14
• \({}_{17}^{37}Cl\) protons? 17
• State two uses of isotopes. \(\mathrm{U\text{-}235}\): electricity generation, \(\mathrm{Co\text{-}60}\): cancer treatment, \(\mathrm{I\text{-}131}\): thyroid patient diagnosis.
• Difference: proton/neutron? Proton (+, nucleus), neutron (0, nucleus), mass nearly equal.

📘 End of Notes: Inside the Atom (Class 8) 📘