Structure of Atom

Lesson

Part 1 — from the electron and the nucleus to the first quantum model of the atom.

In this lesson you will

Describe the subatomic particles and the Thomson and Rutherford models with their limitations
Use c = νλ and E = hν for electromagnetic radiation and the photoelectric effect
Interpret the hydrogen line spectrum and apply Bohr’s model to energies, radii and transitions

Table of charge and mass of the electron, proton and neutron
Particle	Charge / C	Mass / kg	Mass / u
Electron	$-1.602 \times 10^{-19}$	$9.109 \times 10^{-31}$	0.000549
Proton	$+1.602 \times 10^{-19}$	$1.673 \times 10^{-27}$	1.0073
Neutron	0	$1.675 \times 10^{-27}$	1.0087

Concept

Discovery of subatomic particles

Cathode rays showed that atoms contain electrons; canal rays revealed protons; Chadwick found the neutron in 1932. Thomson measured the charge-to-mass ratio of the electron, $e/m_\text{e} = 1.76 \times 10^{11}\ \text{C kg}^{-1}$ , and Millikan’s oil-drop experiment gave its charge.

Table of who discovered each subatomic particle and the evidence
Particle	Discovered by	Evidence
Electron	J. J. Thomson (1897)	Cathode rays deflected by electric and magnetic fields
Proton	Goldstein, Rutherford	Canal rays; hydrogen nucleus
Neutron	Chadwick (1932)	Beryllium bombarded with alpha particles
Charge on electron	Millikan	Oil-drop experiment

Concept

Thomson’s model

Thomson pictured the atom as a positive sphere with electrons embedded like seeds in a watermelon or raisins in a pudding: electrically neutral, with charge spread uniformly. It could not explain what Rutherford observed.

Table comparing Thomson’s model with Rutherford’s nuclear model
Feature	Thomson (1904)	Rutherford (1911)
Positive charge	Spread through the whole sphere	Concentrated in a tiny nucleus
Electrons	Embedded in the sphere	Outside the nucleus
Mass	Spread out	Almost all in the nucleus
Explains alpha scattering?	No	Yes

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