Structure of the Atom – Study Notes

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1. Are atoms really indivisible?

Earlier, scientists like Dalton thought:
👉 Atom cannot be broken.

But later we discovered:
👉 Atom has even smaller particles inside it!

These are:

  • Electron → tiny, negative charge
  • Proton → positive charge
  • Neutron → no charge

Think of an atom like a mini solar system:

  • Sun = nucleus
  • Planets = electrons

 

2. How did we discover these particles?

J.J. Thomson – Electron

He found a tiny particle with a negative charge → electron.

Goldstein – Proton

He saw rays going the opposite direction (canal rays) and found positively charged particles → protons.

Chadwick – Neutron

He found a particle with no charge → neutron.

 

3. Thomson’s Model (Plum Pudding Model)

Imagine a big sweet laddu

·       The laddu = positive charge

  • Dry fruits inside = electrons

He said:

  • Atom is a ball of positive charge with electrons stuck inside it.
  • Atom is neutral because + and – balance.

But… this idea was later proved wrong.

4. Rutherford’s Famous Gold Foil Experiment

He shot alpha particles (very fast, tiny bullets) at a thin gold foil.

What happened?

  1. Most particles went straight → Atom is mostly empty space.
  2. Some bent a little → Positive part is tiny but strong.
  3. A few bounced back → A very small, dense nucleus exists.

So he said:
👉 Atom has a small, heavy nucleus in the center.

But his model had a problem:

  • If electrons revolve, they should lose energy and fall into the nucleus.
  • But this never happens.

So the model was incomplete.

5. Bohr’s Model – The Correct One

Bohr corrected Rutherford.

He said:

  • Electrons move in fixed paths called shells or energy levels.
  • These shells are K, L, M, N,…
  • Electrons do not fall into the nucleus because they do not lose energy.

Perfectly matches real atoms.

Bohr's Model Of An Atom

6. How are electrons filled in shells?

Bohr & Bury gave rules:

Rule 1:

Max electrons in a shell = 2n²

  • K (n=1) → 2
  • L (n=2) → 8
  • M (n=3) → 18

Rule 2:

Outermost shell can have maximum 8 electrons.

Rule 3:

Inner shells fill first.

Example:
Magnesium → 12 electrons
Filling: 2, 8, 2

 

7. Valency – How an atom reacts

Valency = the combining power of an atom.

Easy trick:

  • Atoms want 8 electrons in their outer shell (octet rule).
  • Metals → lose electrons.
  • Non-metals → gain electrons.

Examples:

  • Sodium (2,8,1) → loses 1 → valency = 1
  • Oxygen (2,6) → gains 2 → valency = 2
  • Chlorine (2,8,7) → gains 1 → valency = 1

 

8. Atomic Number & Mass Number

Atomic Number (Z)

= Number of protons
Also = number of electrons (in a neutral atom)

Example: Carbon
Z = 6 → has 6 protons.

 

Mass Number (A)

= Protons + Neutrons

Example:
Carbon: 6p + 6n = 12
So A = 12

 

9. Isotopes

Same element
Same atomic number
Different mass number

Example:
Hydrogen has 3 isotopes:

  • ¹H (protium)
  • ²H (deuterium)
  • ³H (tritium)

Chemical properties → same
Physical properties → different

Uses:

  • Iodine-131 → goitre treatment
  • Cobalt-60 → cancer treatment
  • Uranium-235 → nuclear reactors

 

10. Isobars

Different elements
Same mass number
Different atomic numbers

Example:
Argon-40 and Calcium-40 → both mass = 40 but different elements.

 

⭐ SUPER QUICK REVISION (30 seconds)

  • Atom = proton + neutron in nucleus, electrons outside.
  • Electrons move in fixed shells.
  • Max electrons = 2n².
  • Valency = electrons lost or gained.
  • Atomic number = protons.
  • Mass number = protons + neutrons.
  • Isotopes = same Z, different A.
  • Isobars = same A, different Z.

 

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