Historical Development of Atomic Theory: Dalton to Rutherford

Key Ideas

• Dalton's atomic theory (1803–1808): matter is made of indivisible atoms; atoms of a given element are identical; atoms combine in small whole-number ratios to form compounds; chemical reactions rearrange atoms without creating or destroying them.
• Law of conservation of mass and law of definite proportions — the two laws Dalton's theory was built to explain.
• Thomson's cathode-ray tube: a beam in an evacuated tube was deflected by electric and magnetic fields toward the positive plate, proving the beam was made of negative particles. The same e/m ratio appeared regardless of the metal or gas used, so these particles are a universal constituent of all atoms — the electron.
• Thomson's "plum pudding" model: electrons embedded in a diffuse sphere of positive charge.
• Millikan's oil-drop experiment: tiny charged oil droplets were suspended between charged plates; balancing gravity against the electric field let him compute the charge on each droplet, which always came in whole-number multiples of 1.6 × 10⁻¹⁹ C — the charge of one electron.
• Rutherford's gold-foil experiment: alpha particles (He²⁺) fired at thin gold foil mostly passed straight through, but a tiny fraction bounced sharply back. Conclusion: the atom is mostly empty space with a small, dense, positively charged nucleus.

Equations and Variables

This topic is largely qualitative, but two quantitative results are central:

Charge of the electron: e = 1.602 × 10⁻¹⁹ C (coulomb), measured by Millikan.

Charge-to-mass ratio (Thomson): e/m ≈ 1.759 × 10¹¹ C/kg. Combined with Millikan's e, this yields the electron mass mₑ ≈ 9.109 × 10⁻³¹ kg.

Variables: e = elementary charge (C); m = electron mass (kg). (These constants appear again in the Subatomic Architecture note.)

How It Works or Problem-Solving Method

When asked to trace a piece of evidence to a conclusion, follow the cause-and-effect chain:

1. Cathode ray + electric field → beam bends toward + plate ⇒ the beam carries negative charge (Thomson).
2. Same deflection for every metal/gas ⇒ electrons are present in all atoms ⇒ atoms are divisible (contradicts Dalton's "indivisible").
3. Oil drops carry charges in whole-number multiples of 1.6 × 10⁻¹⁹ C ⇒ charge is quantized; the smallest unit is one electron's charge (Millikan).
4. Most alpha particles pass straight through; a few ricochet ⇒ atom is mostly empty space with a concentrated positive mass (nucleus) (Rutherford).

For model-comparison questions, map each model to its flaw that the next experiment exposed.

Worked Example

Problem: Interpret Rutherford's data — in a gold-foil experiment, about 1 in every 20,000 alpha particles was deflected back. What does this imply about the atom?

Reasoning: Alpha particles are positively charged and relatively massive. If Thomson's "plum pudding" (spread-out positive charge) were correct, every alpha particle should pass through with only slight deflection, like a bullet through fog. The fact that a small fraction bounced straight back means those particles hit something much more massive and positively charged than themselves. Therefore nearly all the atom's mass and positive charge is concentrated in a tiny central nucleus, and the vast majority of the atom is empty space through which the rest of the alpha particles sail unimpeded. This single observation replaced the plum-pudding model with the nuclear atom.

Problem (quantitative): An oil drop of mass 3.2 × 10⁻¹⁵ kg is held stationary in an electric field of 1.96 × 10⁵ N/C. What charge does it carry, and how many excess electrons is that? (Take g = 9.8 m/s².)

Balance forces: electric force = gravitational force, so qE = mg.

q = mg/E = (3.2 × 10⁻¹⁵ kg × 9.8 m/s²) / (1.96 × 10⁵ N/C) = 3.136 × 10⁻¹⁴ / 1.96 × 10⁵ = 1.6 × 10⁻¹⁹ C.

Number of electrons = q/e = (1.6 × 10⁻¹⁹ C) / (1.6 × 10⁻¹⁹ C) = 1 electron.

Common Confusions

• "Dalton said atoms are indivisible, so his theory was simply wrong." Wrong — most of Dalton's theory survives; only the "indivisible" and "all atoms of an element are identical" postulates were later modified (subatomic particles; isotopes).
• "Rutherford proved electrons orbit the nucleus in fixed circular paths." Wrong — Rutherford established the nucleus; the fixed-orbit (planetary) refinement came from Bohr, and even that was later replaced by quantum mechanics.
• "Thomson discovered the proton." Wrong — Thomson discovered the electron (negative). The proton's positive charge balancing the atom was worked out later (with Rutherford and others).
• "Millikan invented the electron." Wrong — Thomson discovered it; Millikan measured its charge.