Structure of the Atom
~9 min read
- Three particles: Proton (+, mass ~1u), Neutron (neutral, ~1u), Electron (-, ~1/1836 u).
- Atomic number Z: Number of protons. Defines element. Mass number A = protons + neutrons.
- Models: Dalton (1803) → Thomson plum pudding (1897) → Rutherford nuclear (1911) → Bohr orbits (1913) → Quantum mechanical (1920s).
Atomic structure is the foundation of chemistry. NDA tests subatomic particles, atomic number, mass number, and the evolution of atomic models.
Subatomic Particles
| Particle | Symbol | Charge | Mass (u) | Discovered by |
|---|---|---|---|---|
| Electron | e⁻ | -1 | 1/1836 | J.J. Thomson (1897) |
| Proton | p | +1 | 1 | Goldstein (1886), named by Rutherford (1920) |
| Neutron | n | 0 | 1 | James Chadwick (1932) |
Atomic Number, Mass Number, Isotopes
- Atomic number (Z): Number of protons. Equal to electrons in neutral atom. Defines element. (Hydrogen Z=1, Carbon Z=6, Oxygen Z=8.)
- Mass number (A): Total protons + neutrons.
- Notation: ${}_Z^A X$ (e.g., ₆¹²C).
- Atomic mass unit (u or amu): 1 u = 1/12 mass of C-12 atom = 1.66 × 10⁻²⁷ kg.
Atomic Models — Evolution
| Year | Scientist | Model |
|---|---|---|
| 1803 | John Dalton | Atoms are tiny indivisible spheres. Each element has its own kind of atoms |
| 1897 | J.J. Thomson | "Plum pudding" — sphere of positive charge with electrons embedded |
| 1911 | Rutherford | Gold foil experiment — discovered the nucleus. Atom is mostly empty space with small dense positive nucleus and electrons orbiting |
| 1913 | Niels Bohr | Electrons occupy fixed energy levels (orbits). Jump between levels emits/absorbs photons. Could not explain many-electron atoms |
| 1920s | Schrödinger, Heisenberg | Quantum mechanical — electrons as probability clouds (orbitals) |
Electron Shells and Configuration
- Electrons distributed in shells labelled K, L, M, N (or n=1, 2, 3, 4...).
- Maximum capacity: 2n². K=2, L=8, M=18, N=32.
- Valence shell: Outermost shell — determines chemistry.
- Octet rule: Atoms tend to have 8 electrons in outer shell (or 2, for the first shell). Drives bonding.
- Electron configurations (Bohr-Bury):
- Hydrogen (Z=1): 1.
- Carbon (Z=6): 2,4.
- Sodium (Z=11): 2,8,1.
- Chlorine (Z=17): 2,8,7.
- Argon (Z=18): 2,8,8.
NDA PYQ Examples
Q: Who discovered the neutron?
(a) Rutherford (b) Thomson (c) Chadwick (d) Bohr
Answer: (c) James Chadwick — 1932.
Q: Atomic number of an element is the number of:
(a) Protons (b) Neutrons (c) Electrons + protons (d) Nucleons
Answer: (a) Protons.
Q: In Rutherford's gold foil experiment, most alpha particles:
(a) Were absorbed (b) Passed straight through (c) Were reflected back (d) Spiralled into the gold
Answer: (b) Passed straight through — proving atom is mostly empty.
Q: Maximum number of electrons in M shell is:
(a) 2 (b) 8 (c) 18 (d) 32
Answer: (c) 18 (= 2 × 3²).
Drill Structure of the Atom for NDA
NDA-pattern items on Structure of the Atom with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is an atom electrically neutral?
Equal number of protons (+) and electrons (-). The atom as a whole has zero net charge. In ions, this balance is broken — gain electrons = negative ion; lose electrons = positive ion.
Why didn't Rutherford's model survive?
Classical physics predicted that an orbiting electron would lose energy and spiral into the nucleus — an atom couldn't exist. Bohr fixed this by postulating quantised orbits where electrons could exist without radiating.
What is an orbital?
In quantum mechanical model, an orbital is a 3D region around the nucleus where probability of finding an electron is high (~90%). Replaces Bohr's neat orbits. Shapes: s (sphere), p (dumbbell), d (more complex), f (most complex).
What is the modern definition of element?
A substance whose atoms all have the same atomic number (same number of protons). Element identity comes from protons, not neutrons or electrons. Carbon = anything with 6 protons.
Why is the Bohr model still taught despite being incomplete?
It's a good first approximation, explains hydrogen spectrum, and introduces quantisation. Quantum mechanical model is mathematically demanding. Bohr's circular orbits, while wrong in detail, give correct results for hydrogen and one-electron systems.