Structure of the Atom – Very Short Answer Type Questions

CBSE Class 9 Chemistry — Chapter 4: Structure of the Atom — 50 Very Short Q&A

Content Bank (quick reference)

Subatomic Particles (Questions 1–8)

1. Name the three main subatomic particles.
Electron, Proton, Neutron
2. Which subatomic particle has a negative charge?
Electron
3. Which particle defines the atomic number?
Proton
4. Which subatomic particle has approximately 1 u mass and no charge?
Neutron
5. Where are protons and neutrons located in an atom?
Nucleus
6. Where are electrons found?
Outside nucleus (in shells/orbitals)
7. Which particle's number changes for isotopes?
Neutron
8. Which particle determines chemical behaviour?
Electrons (especially valence electrons)

9. Define atomic number (Z).
Number of protons in nucleus
10. Define mass number (A).
Total protons + neutrons
11. How to find neutrons from A and Z?
Neutrons = A − Z
12. For ²³Na, what is Z and A?
Z = 11, A = 23
13. If an atom has 8 protons and 10 neutrons, what's A?
A = 18
14. True/False: Atomic number can change without changing element.
False (Z change = different element)
15. What does the notation ^{A}_{Z}X represent?
Isotope with mass A and atomic number Z of element X
16. An atom with Z = 6 has how many electrons when neutral?
6 electrons

17. Define isotopes.
Atoms of same element (same Z) with different A
18. Give an example of chlorine isotopes.
³⁵Cl and ³⁷Cl
19. Do isotopes have same chemical properties?
Yes (similar chemical behaviour)
20. Do isotopes have different physical properties?
Yes (e.g., mass, density, boiling point)
21. What isotope notation for 17 protons and 18 neutrons?
³⁵₁₇Cl (or ^{35}_{17}Cl)
22. True/False: Isotopes change the position in periodic table.
False (same Z → same position)

23. Who proposed the plum pudding model?
J. J. Thomson
24. Which experiment led to nuclear model?
Rutherford's gold foil experiment
25. What did Rutherford conclude about atom's nucleus?
Small, dense, positively charged centre
26. Whose model introduced fixed electron orbits?
Niels Bohr
27. What is one limitation of Bohr model?
Works well for hydrogen but not multi-electron atoms
28. What discovery required revision of Dalton's indivisible atom idea?
Discovery of electrons (subatomic particles)
29. In Rutherford's experiment, most α-particles passed through — why?
Because most of atom is empty space
30. What did deflection of some α-particles indicate?
Presence of concentrated positive charge (nucleus)

31. Name the shells K, L, M corresponding to which principal quantum numbers?
K = n1, L = n2, M = n3
32. Maximum electrons K shell can hold?
2 electrons
33. Maximum electrons L shell can hold (class 9 practical rule)?
8 electrons
34. Electronic distribution of oxygen (Z = 8)?
2, 6
35. Electronic distribution of sodium (Z = 11)?
2, 8, 1
36. How to write electronic configuration shorthand for neon (Z = 10)?
2, 8
37. What determines valency in main group elements?
Number of electrons in outermost shell
38. Valency of chlorine (2, 8, 7)?
1 (or can be 1 as it gains one electron)
39. Valency of carbon (2, 4)?
4
40. Why do atoms form bonds?
To achieve a stable (noble gas) electron configuration

41. How many electrons in a neutral atom of ²³Na?
11 electrons
42. How many neutrons in ²³Na?
Neutrons = 23 − 11 = 12
43. What is Avogadro's number (symbol & value)?
Nₐ = 6.022 × 10²³
44. True/False: Electrons are heavier than protons.
False (electrons much lighter)
45. Which particle contributes most to atomic mass?
Protons and neutrons (nucleus)
46. How is the atomic mass unit (u) defined?
1/12th mass of ¹²C atom
47. How many protons in ²⁷Al?
13 protons
48. Electronic configuration of magnesium (Z = 12)?
2, 8, 2
49. What is a neutral atom?
Atom with equal protons and electrons (no net charge)
50. Give one practical use of isotopes.
Medical tracers / carbon dating / diagnostic imaging