Key relationship
A=Z+N; R≈1.2 A^(1/3) fm; illustrative attractive tail U∝−exp(−r/1.4 fm)/r.
Physics · Class 12
Nuclear size and composition
Protons define an element; neutrons distinguish its isotopes. Nuclear radius grows with the cube root of nucleon number, making average nuclear density approximately constant. The strong interaction is short ranged.
Open the interactive experiment →Nuclear size and composition
புரோட்டான்கள் தனிமத்தை நிர்ணயிக்கும்; நியூட்ரான்கள் ஐசோடோப்புகளை வேறுபடுத்தும். அணுக்கரு ஆரம் A இன் கனமூலத்தைச் சாரும்.
Key notes, relationships and model limits
Remember
Protons define an element; neutrons distinguish its isotopes. Nuclear radius grows with the cube root of nucleon number, making average nuclear density approximately constant. The strong interaction is short ranged.
Common trap
A nucleus is far smaller than its electron cloud; schematic dots are not fixed nucleon orbits.
Model limits
Empirical radius; isotope stability is not predicted. Attractive nuclear-force tail is normalized and omits short-range repulsion and detailed nucleon physics. Cluster positions are illustrative.
What you will learn
Build isotope notation and compare nuclear length scales.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
Iron56 has Z26,N30 and radius≈4.59 fm.
Why this matters
The model connects nuclear size and composition with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
Two isotopes of the same element have the same?
- Neutron number
- Proton number
- Mass number
Read the explanation
Their atomic number Z matches; N may differ.
Course connection: Class XII Chapter 13. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.