Why don't electrons fall into the nucleus, even though they are attracted to the protons in it?
Key idea
Rediscovering the Roots of Atomic Theory
Parmanu and atomos
Over 2,000 years ago, Acharya Kanada (India) and Leucippus and Democritus (Greece) imagined matter divided until reaching indivisible particles — parmanus, or atomos. A brilliant guess, not an experiment.
Dalton's atomic theory (1808)
The first scientific description: all matter is made of indivisible atoms, based on real experiments — the starting point for our modern understanding.
Key idea
J. J. Thomson Discovers the Electron
In 1897, Thomson studied electric current through low-pressure gas, and found rays flowing from the cathode to the anode.
Negatively charged, and inside every atomThese cathode rays were streams of tiny, negatively charged particles — electrons — present in every element, proving atoms have smaller parts after all.
Key idea
Thomson's Model of the Atom
If atoms are neutral, where is the balancing positive charge?
A sphere of positive charge, studded with electrons
Like a watermelon — the positively charged pulp is the sphere, and the electrons are seeds scattered through it. The first real attempt to balance an atom's charges.
Key idea
The Gold Foil Experiment
Observation | What it meant
Most α-particles passed straight through the foilMost of the atom is empty space
A few were sharply deflectedSomething small and dense repelled them
A very few bounced straight backThat something carries a concentrated positive charge
Key idea
Rutherford's Nuclear Model
1
A tiny, dense nucleus
Positive charge and most of the mass are packed into a nucleus about 100,000 times smaller than the atom itself. Electrons orbit it like planets around the Sun.
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2
Mostly empty space
If an atom were the size of a cricket ground (~100 m across), its nucleus would be a single pepper grain at the centre.
Key idea
A Missing Piece: Why Don't Atoms Collapse?
The stability problemAn orbiting electron is constantly accelerating (changing direction), so classically it should radiate energy, spiral inward, and crash into the nucleus. But atoms are stable — this model couldn't explain why.
The proton, named by RutherfordThe nucleus's positive charge comes from protons — heavier than electrons, with an equal and opposite charge. A neutral atom has equal protons and electrons.
Key idea
Bohr's Model of the Atom (1913)
Rule | Meaning
Fixed shells (K, L, M, N…)
Electrons follow set circular paths, or energy levels — not random paths
No energy loss in a shell
While in an allowed shell, an electron doesn't lose energy
Energy rises with distance
K (closest, n=1) has the least energy; farther shells have more
Jumps need fixed energy
Moving shells means absorbing or releasing an exact amount of energy
Key idea
James Chadwick Discovers the Neutron (1932)
A helium atom has 2 protonsbut its mass is about 4 times a hydrogen atom's, not double. Something else was adding mass.
Same mass as a proton, but no chargeThe neutron explained the missing mass. Found in the nucleus of every atom except ordinary hydrogen, neutrons also help hold the nucleus together against proton-proton repulsion.
Key idea
The Subatomic Particles
Particle | Symbol | Relative charge | Found where
Electron
e⁻ — −1 — Orbiting the nucleus
Proton
p⁺ — +1 — Inside the nucleus
Neutron
n⁰ — 0 — Inside the nucleus
Key idea
Symbols of Elements
1
From pictures to letters
Dalton first drew pictorial symbols (1803). Berzelius (1813) proposed Latin-based letter symbols — now standardised worldwide by IUPAC.
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The rules
First letter capital, second (if any) lowercase — hydrogen H, aluminium Al, not AL. Some come from Latin/Greek/German names: iron is Fe (ferrum), mercury Hg (hydrargyros), tungsten W (wolfram).
Key idea
Atomic Number and Mass Number
1
Atomic number (Z)
The number of protons in the nucleus — equal to the number of electrons in a neutral atom. It uniquely identifies an element.
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2
Mass number (A)
The total number of protons and neutrons (nucleons). Mass number = protons + neutrons. Carbon: ¹²₆C — atomic number 6, mass number 12.
Key idea
How Electrons Fill Energy Levels
Rule | Detail
Maximum electrons per shell2n² — K holds 2, L holds 8, M holds 18
Maximum in the outermost shell8 electrons (or just 2, if it's the only shell)
Filling orderInnermost first: K, then L, then M… never skip ahead
Key idea
Electron Distribution: A Few Examples
Element | Z | K | L | M
Hydrogen (H)
1 — 1 — – — –
Helium (He)
2 — 2 — – — –
Carbon (C)
6 — 2 — 4 — –
Key idea
Electron Distribution: A Few Examples — continued
1
Neon (Ne)
10 — 2 — 8 — –
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2
Sodium (Na)
11 — 2 — 8 — 1
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3
Chlorine (Cl)
17 — 2 — 8 — 7
Key idea
Valency: The Combining Capacity
Atoms with a full outermost shell (an octet of 8, or 2 for helium) are stable. Others lose, gain, or share electrons to get there.
Element | Valence electrons | Tendency | Valency
Sodium (2, 8, 1)1 — Loses 1 electron — 1
Oxygen (2, 6)6 — Gains 2 electrons — 2
Carbon (2, 4)4 — Shares 4 electrons — 4
Key idea
Isotopes
Same atomic number, different mass number
atoms of the same element with a different number of neutrons.
Hydrogen's three isotopes
Protium (no neutrons, ~99.98%), deuterium (1 neutron), and tritium (2 neutrons) — all with 1 proton and 1 electron, so all share the same chemical properties.
Key idea
Average Atomic Mass
Chlorine's isotopes: 35 u (about 75%) and 37 u (about 25%). A simple average ignores how common each one actually is.
Weighted average = 35.5 u(35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5 u — reflecting that ³⁵Cl is three times more common than ³⁷Cl in nature.
Key idea
Isobars
Different elements can share the same mass number, despite having different atomic numbers.
Calcium, potassium, and argon: all mass number 40
Calcium (Z=20), potassium (Z=19), and argon (Z=18) each have 40 nucleons total, despite different numbers of protons — they are isobars, not isotopes.
Key idea
The Journey of Atomic Models
Dalton Indivisible particle
Thomson Charges embedded in a sphere
Rutherford Dense nucleus, mostly empty
Bohr Fixed energy levels
Modern Electron cloudsstill evolving
Chapter · Key terms to remember
Key Terms
Electron / Proton / Neutron
Charge −1, moving around nucleus / +1, in nucleus / 0, in nucleus.
Nucleus
The tiny, dense, positively charged centre of an atom.
Atomic number (Z)
Number of protons — identifies the element.
Chapter · Key terms to remember
More Key Terms
Mass number (A)
Total protons + neutrons (nucleons).
Electronic configuration
How electrons are distributed among shells.
Valency
Electrons gained, lost, or shared to complete an octet.
Chapter · Key terms to remember
More Key Terms
Isotopes
Same atomic number, different mass number.
Isobars
Same mass number, different atomic number.
Questions for your notebook
Write these down, then discuss
1
Three atomic species X, Y, Z have: X (18 protons, 19 neutrons), Y (17 protons, 18 neutrons), Z (17 protons, 20 neutrons). Explain the relation between (i) Y and Z, (ii) Z and X.
2
What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part, based on the alpha particles that bounced back or deflected sharply?
3
Arrange in the correct chronological order: (i) Bohr's model — fixed orbits with definite energy. (ii) Thomson's model — plum pudding. (iii) Rutherford's model — dense central nucleus. (iv) Dalton's model — indivisible particles.
Project as-is — students copy the questions, then the class discusses answers together.