Atoms were once thought to be the smallest, indivisible particles of matter. Over two centuries of scientific experiments shattered that idea. Today we know atoms have a complex internal structure made of smaller particles called subatomic particles. This chapter traces that remarkable journey of discovery.
Early Atomic Theory — Dalton
John Dalton (1808) proposed that matter is made of tiny, indivisible spheres called atoms. He said atoms of the same element are identical, and atoms combine in fixed ratios to form compounds. While mostly correct, his model was too simple.
Discovery of the Electron — J. J. Thomson (1897)
J. J. Thomson passed electric current through a gas at low pressure in a glass tube (cathode ray tube). The rays that travelled from the cathode to the anode were deflected by electric and magnetic fields, proving they were negatively charged particles. He called them electrons. Thomson concluded that atoms contain electrons embedded in a positive sphere — the plum-pudding model.
Discovery of the Proton and the Nucleus — Rutherford (1911)
- Ernest Rutherford performed the famous gold foil experiment. He fired a stream of positively charged alpha particles at a very thin gold foil.
- Most particles passed straight through.
- A few were deflected at large angles.
- A very few bounced almost straight back.
Rutherford concluded that the atom is mostly empty space, with a tiny, dense, positively charged centre called the nucleus. Electrons revolve around the nucleus at a distance. This is the nuclear model.
Discovery of the Neutron — Chadwick (1932)
James Chadwick discovered the neutron — a particle in the nucleus that has mass similar to a proton but carries no charge.
Subatomic Particles Summary
| Particle | Charge | Relative Mass | Location |
|----------|--------|---------------|----------|
| Electron | -1 | 1/1840 | Outside nucleus |
| Proton | +1 | 1 | Inside nucleus |
| Neutron | 0 | 1 | Inside nucleus |
Atomic Number and Mass Number
Key formulas
Isotopes and Isobars
Key formulas
Bohr's Model of the Atom
Niels Bohr proposed that electrons revolve around the nucleus in fixed circular paths called orbits or shells, labelled K, L, M, N … (or 1, 2, 3, 4 …). Each shell has a fixed energy. The maximum number of electrons a shell can hold is given by 2n2, where n is the shell number: K = 2, L = 8, M = 18, N = 32.
Electronic Configuration
- 1.The arrangement of electrons in shells is the electronic configuration. Rules:
- 2.Fill the innermost shell first.
- 3.Maximum electrons per shell = 2n2.
- 4.Outermost shell holds at most 8 electrons (octet rule).
Example: Sodium (Z = 11): 2, 8, 1.
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Find the number of protons, electrons, and neutrons in Chlorine-35 (Z = 17, A = 35).
Step 1: Protons = Z = 17.
Step 2: Electrons = Protons = 17 (neutral atom).
Step 3: Neutrons = A - Z = 35 - 17 = 18.
Answer: 17 protons, 17 electrons, 18 neutrons.
Write the electronic configuration of Magnesium (Z = 12).
Shell K (n=1): max 2 electrons. Fills as 2.
Shell L (n=2): max 8 electrons. Fills as 8.
Shell M (n=3): remaining = 12 - 2 - 8 = 2.
Electronic configuration: 2, 8, 2.
Identify the isotopes among the following: A (Z=6, A=12), B (Z=7, A=14), C (Z=6, A=13).
A and C both have Z = 6 (both carbon), but different mass numbers (12 and 13). They are isotopes of carbon.
Are Argon (Z=18, A=40) and Calcium (Z=20, A=40) isotopes or isobars?
They have the same mass number (A = 40) but different atomic numbers. They are isobars.
How many electrons can the M shell hold? Using 2n2 with n = 3: 2 x 32 = 2 x 9 = 18. The M shell can hold 18 electrons.
Rutherford fired alpha particles at a gold foil of only 1000 atoms thick. Most particles went straight through. What does this tell us about atomic structure? It tells us that most of an atom is empty space and the positive charge is concentrated in a tiny nucleus.
Hydrogen has three isotopes: Protium (A=1), Deuterium (A=2), and Tritium (A=3), all with Z=1. Deuterium is used in nuclear reactors and Tritium is radioactive. Same chemical behaviour because the number of electrons (and therefore protons) is the same in all three.
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Key Formulas
Key formulas
Common mistakes
- Confusing atomic number with mass number — atomic number is protons only; mass number includes both protons and neutrons.
- Assuming isotopes have different chemical properties — they do NOT; chemical properties depend on electron arrangement.
- Thinking Rutherford proposed the plum-pudding model — that was Thomson; Rutherford proposed the nuclear model.
Summary
The atom has a tiny dense nucleus containing protons and neutrons, surrounded by electrons in energy shells. The atomic number (Z) gives the element's identity; the mass number (A) gives total nucleons. Isotopes share Z but differ in A. Electronic configuration follows the 2n2 rule, with the outermost electrons (valence electrons) determining chemical behaviour.