History of the Atom: Dalton, Thomson, and Electrons

The history of the atom traces how scientists moved from the idea of indivisible particles to Dalton’s atomic theory (1803 to 1808) and then to J. J. Thomson’s discovery of the electron in 1897, which showed that atoms contain smaller, negatively charged parts. In brief: Dalton pictured atoms as tiny solid spheres, Thomson showed that they contain electrons, and his “plum pudding” model placed those electrons inside a positively charged mass.

Atomic theory developed over more than two thousand years, with each stage built on experiments and corrected the one before. This article describes the early ideas, Dalton’s postulates and the parts that were later changed, the cathode ray experiments that revealed the electron, how its charge was measured, a timeline table, a worked calculation with Thomson’s method and common misunderstandings.

What did ancient thinkers believe about atoms?

In the 5th century BCE, Leucippus and his student Democritus proposed that matter is made of tiny, indivisible particles moving in empty space. The Greek word atomos means “uncuttable”. Aristotle rejected the existence of empty space and favored the idea that matter is continuous, and his view dominated European thought for about two thousand years. The atomic idea was revived in the 17th century by Pierre Gassendi and Robert Boyle, but it remained a philosophical idea until quantitative evidence arrived.

What was Dalton’s atomic theory?

The English schoolteacher and chemist John Dalton developed an atomic theory between 1803 and 1808 to explain laws of chemical combination, such as the law of conservation of mass and the law of definite proportions. His main ideas were:

1. All matter is made of tiny particles called atoms.
2. Atoms of the same element are identical in mass and properties, and atoms of different elements differ.
3. Atoms cannot be created, destroyed or divided in chemical reactions.
4. Atoms of different elements combine in simple whole-number ratios to form compounds.
5. A chemical reaction is a rearrangement of atoms.

Dalton’s theory was remarkably successful, and most of it stands today. Two parts have been modified. Atoms are divisible into smaller particles, as shown by later experiments, and atoms of the same element can have different masses (isotopes). Atoms can also be changed in nuclear reactions.

How did Thomson discover the electron?

In the 19th century, scientists studied the glow produced when a high voltage is applied across electrodes in a sealed glass tube from which most of the air has been removed. The glow starts from the negative electrode (the cathode), so the rays were called cathode rays. In 1897, J. J. Thomson at the Cavendish Laboratory in Cambridge showed that these rays are deflected by electric and magnetic fields in a way that proves they are streams of negatively charged particles.

By balancing the deflection due to an electric field with that of a magnetic field, Thomson could determine the ratio of the charge to the mass (e/m) of the particles. The ratio was the same whatever the gas in the tube or the metal of the cathode, and about 1,800 times larger than the value known for the hydrogen ion. He concluded that the particles are much lighter than any atom and are a universal component of all matter. The particles were later named electrons.

How was the charge of the electron measured?

Between 1909 and 1913, Robert Millikan and Harvey Fletcher measured the charge of a single electron with the oil drop experiment. They sprayed tiny charged oil droplets between two metal plates and adjusted the electric field until a droplet hung motionless, so that the electric force balanced gravity. By repeating the measurement for many droplets, they found that the charges were always whole-number multiples of a smallest charge, about 1.6 × 10⁻¹⁹ coulombs. This is the elementary charge. Combined with Thomson’s e/m ratio, it gave the electron mass of about 9.1 × 10⁻³¹ kg.

What was the plum pudding model?

Atoms are electrically neutral, but they contain negative electrons, so there must be positive charge to balance them. Around 1904, Thomson proposed that the atom is a sphere of positive charge with electrons embedded throughout it, like plums in a pudding. The electrons would be arranged in a stable pattern, and they could vibrate to produce light. The model was tested in 1909 by Rutherford’s students and was replaced by the nuclear atom.

Timeline of early atomic ideas

Worked example: the charge-to-mass ratio

The modern values are: elementary charge e = 1.602 × 10⁻¹⁹ C, electron mass m_e = 9.109 × 10⁻³¹ kg and proton mass m_p = 1.673 × 10⁻²⁷ kg.

1. Electron e/m. 1.602 × 10⁻¹⁹ ÷ 9.109 × 10⁻³¹ = 1.759 × 10¹¹ C/kg. This is close to the value Thomson found, within the accuracy of his apparatus.
2. Hydrogen ion (proton) e/m. 1.602 × 10⁻¹⁹ ÷ 1.673 × 10⁻²⁷ = 9.576 × 10⁷ C/kg.
3. Ratio. 1.759 × 10¹¹ ÷ 9.576 × 10⁷ = 1,837. The electron’s charge-to-mass ratio is about 1,840 times larger than that of the hydrogen ion. Since the charges have the same size, the electron’s mass is about 1/1,840 of the mass of a hydrogen atom.
4. Charge on a droplet. In the oil drop experiment, a droplet carrying 5 excess electrons has a charge of 5 × 1.602 × 10⁻¹⁹ = 8.01 × 10⁻¹⁹ C. Millikan’s data show measured charges in the ratio of small whole numbers.

Which mistakes are common?

Mistake 1: Saying that Dalton discovered atoms

Atoms were proposed in ancient Greece. Dalton gave the idea a scientific basis with atomic masses and chemical evidence.

Mistake 2: Thinking that Dalton’s theory is wrong

Most of it is still valid for chemical reactions. Only the statements about indivisible and identical atoms need modification.

Mistake 3: Believing that the plum pudding model had a nucleus

In that model the positive charge is spread through the whole atom. The nucleus was introduced later.

Mistake 4: Confusing cathode rays with light

Cathode rays are streams of electrons. They are deflected by electric and magnetic fields, which light is not.