Class 10 Physics • Chapter 12 (Deep Detail)
Neutron ($n$): Neutral particle in nucleus.
Proton ($p$): Positive particle in nucleus.
Electron ($e$): Negative particle orbiting nucleus.
Isotopes: Atoms of same element having same Atomic Number ($Z$) but different Mass Number ($A$). (e.g., C-12, C-14).
Q: Do isotopes of an element have the same chemical properties?
Ans: Yes. Chemical properties depend on Atomic Number (electrons), which is same.
CONCEPTUAL Define Isobars.
Ans: Atoms of different elements having different atomic numbers but same mass number. e.g., Potassium-40 ($_{19}^{40}K$) and Calcium-40 ($_{20}^{40}Ca$).
A spontaneous process in which unstable nuclei emit radiations ($\alpha, \beta, \gamma$) to become stable. It is a nuclear phenomenon (not affected by temperature, pressure, chemical state).
| Property | Alpha ($\alpha$) | Beta ($\beta$) | Gamma ($\gamma$) |
|---|---|---|---|
| Nature | Helium Nucleus ($_2^4He$) | Fast Electron ($_{-1}^0e$) | EM Wave (Photon) |
| Charge | Positive (+2) | Negative (-1) | Neutral (0) |
| Penetrating Power | Low (Stopped by paper) | Medium (Aluminium foil) | High (Lead block) |
| Ionizing Power | Very High | Medium | Very Low |
Alpha Emission: Mass Number decreases by 4, Atomic Number decreases by 2.
$$ _Z^A X \longrightarrow _{Z-2}^{A-4}Y + _2^4He $$Beta Emission: Mass Number remains same, Atomic Number increases by 1.
$$ _Z^A X \longrightarrow _{Z+1}^{A}Y + _{-1}^0e $$Gamma Emission: No change in Mass or Atomic Number. Nucleus loses excess energy.
CONCEPTUAL Why do Beta particles show more deflection than Alpha particles in an electric field?
Ans: Deflection $\propto$ Charge/Mass. Beta particles (electrons) have very small mass compared to Alpha particles (Helium nuclei). Hence, for same force field, lighter Beta particles deflect much more.
NUMERICAL Complete the reaction: $_{88}^{226}Ra \longrightarrow .... + _2^4He$.
Solution:
Conservation of Mass No:
$226 = A + 4 \implies A = 222$.
Conservation of Atomic No:
$88 = Z + 2 \implies Z = 86$.
Element is Radon ($_{86}^{222}Rn$).
NUMERICAL A carbon nucleus $_{6}^{14}C$ emits a beta particle. Write the equaiton.
Solution: $_{6}^{14}C \longrightarrow _{7}^{14}N + _{-1}^0e$. (Atomic number increases by 1).
NUMERICAL A nucleus X has mass number 238 and atomic number 92. It emits an alpha particle to form Y, which then emits a beta particle to form Z. Find atomic number and mass number of Z.
Solution:
1. $X \xrightarrow{-\alpha} Y$: $A_Y = 238-4 = 234$. $Z_Y = 92-2 = 90$.
2. $Y \xrightarrow{-\beta} Z$: $A_Z = 234$ (No change). $Z_Z = 90+1 = 91$.
Ans: Z has Mass No = 234, Atomic No = 91.
Q: What happens to the atomic number during Alpha decay?
Ans: Decreases by 2.
NUMERICAL Nucleus $_{92}^{238}U$ emits 1 Alpha then 1 Beta. Find final nucleus ($Z, A$).
Solution: $\alpha$: $238-4=234, 92-2=90$. $\beta$: $234, 90+1=91$. Final: $Z=91, A=234$.
CONCEPTUAL Which radiation has highest penetrating power and which has highest ionizing power?
Ans: Penetrating: Gamma ($\gamma$). Ionizing: Alpha ($\alpha$).
REASONING Why should radioactive sources not be touched with bare hands?
Ans: Radiations causing skin burns and genetic damage (mutation) can easily penetrate skin. Long tongs increase distance, reducing intensity (Inverse Square Law).
REASONING Why are lead boxes used to store radioactive samples?
Ans: Lead (high density, high Z) effectively absorbs all radiations including Gamma rays, protecting the surroundings.
Splitting of a heavy nucleus into two lighter nuclei with release of energy.
Example: U-235 bombardment with slow neutron.
Used in: Nuclear Reactor (Controlled), Atom Bomb (Uncontrolled).
Combining of two light nuclei to form a heavy nucleus.
Example: Hydrogen fusion to Helium.
Source of energy in Sun/Stars. Requiring very high temperature ($10^7$ K). Hydrogen Bomb.
Q: Which process releases more energy per unit mass: Fission or Fusion?
Ans: Nuclear Fusion releases much more energy.
IDENTIFICATION Name moderator and fuel in a nuclear reactor.
Ans: Moderator: Graphite or Heavy Water. Fuel: Uranium-235 or Plutonium-239.
APPLICATION Which isotope is used to estimate the age of old wooden artifacts?
Ans: Carbon-14 ($^{14}C$). Living trees absorb C-14. When they die, absorption stops and C-14 decays. The remaining amount indicates age.