Discovery of Subatomic Particles

Why the indivisible atom broke apart

The word atom descends from the Greek a-tomio, meaning uncut-able or non-divisible, and for over two thousand years it carried that meaning literally. John Dalton placed the idea on a firm scientific footing in 1808, treating the atom as the ultimate particle of matter. His atomic theory accounted handsomely for the law of conservation of mass, the law of constant composition and the law of multiple proportion.

Dalton's model nonetheless left ordinary observations unexplained. Substances such as glass or ebonite, when rubbed with silk or fur, acquired an electric charge — behaviour an indivisible neutral particle could not produce. The decisive evidence came from a different direction: passing electricity through gases. As Michael Faraday had shown in 1830, electricity driven through an electrolyte solution liberated and deposited matter at the electrodes, hinting that electricity itself was particulate. To follow what comes next, hold one rule in mind throughout this subtopic.

Like charges repel each other and unlike charges attract each other. Every deflection in every tube discussed below is read through this single statement.

Discovery of the electron: cathode rays

From the mid-1850s, scientists studied electrical discharge in partially evacuated glass tubes fitted with two metal electrodes — the cathode-ray discharge tube. Discharge occurs only at very low pressures and very high voltages, both controlled by evacuating the tube. When a sufficiently high voltage is applied, a current begins to flow as a stream of particles travelling from the negative electrode (cathode) towards the positive electrode (anode). These were named cathode rays.

The rays are themselves invisible, but their path is revealed by coating the tube behind a perforated anode with a phosphorescent material such as zinc sulphide; a bright spot appears where the rays strike. Careful observation produced a tight set of conclusions.

The last point is the conceptual hinge. Because the rays were identical no matter which gas filled the tube or which metal formed the electrodes, the electron could not belong to any one element — it had to be a universal building block of matter.

Thomson and the charge-to-mass ratio

In 1897 the British physicist J.J. Thomson made the cathode ray quantitative. He applied an electric field and a magnetic field perpendicular to each other and to the path of the electrons. With only the electric field on, the beam deflected to one point on the tube; with only the magnetic field on, it struck a different point. By tuning the two field strengths against each other he could return the beam to the path it followed with no field at all.

Thomson reasoned that the amount of deflection depends on three quantities: the magnitude of the charge on the particle, the mass of the particle, and the strength of the applied field. A larger charge or a smaller mass means a larger deflection. From accurate deflection measurements he extracted the ratio of charge to mass of the electron:

$$\dfrac{e}{m_e} = 1.758820 \times 10^{11}\ \text{C kg}^{-1}$$

One subtlety carries straight into examinations: Thomson measured a ratio, not the charge and not the mass on their own. The value above pins the electron's charge and mass together but leaves either one unknown until a second, independent experiment supplies one of them.

Millikan's oil-drop experiment

R.A. Millikan determined the charge on the electron directly in his oil-drop experiment, carried out between 1906 and 1914. Fine oil droplets from an atomiser drifted through a hole into the space between two charged plates of an electrical condenser. The droplets were watched through a telescope fitted with a micrometer eyepiece, and from their rate of fall under gravity Millikan deduced the mass of each drop.

A beam of X-rays passing through the chamber ionised the air, and the droplets picked up charge by colliding with these gaseous ions. By applying a voltage of chosen polarity and strength, Millikan could slow a falling drop, speed it up, or hold it perfectly stationary against gravity. Three forces act on a drop in the chamber: gravity pulling it down, the electrostatic force from the field, and a viscous drag when it moves.

The result was twofold. First, the charge on the electron is $-1.6 \times 10^{-19}\ \text{C}$, the present accepted value being $-1.602176 \times 10^{-19}\ \text{C}$. Second, and just as important, the charge $q$ on any drop was always an integral multiple of one basic value:

$$q = n\,e, \qquad n = 1, 2, 3, \ldots$$

This integral-multiple result is the experimental proof that electric charge is quantised — it comes in discrete packets, never a continuous smear. Combining Millikan's charge with Thomson's ratio finally fixed the electron's mass.

Discovery of the proton and neutron

An atom is electrically neutral, so the negative electrons had to be balanced by positive charge. That charge announced itself in a modified discharge tube as canal rays — positively charged particles travelling opposite to the cathode rays. Their behaviour was instructive precisely because it differed from that of the electron.

The smallest, lightest positive ion came from hydrogen and was named the proton; it was characterised in 1919. With protons and electrons known, a problem remained. The measured atomic masses were larger than the protons and electrons alone could supply. The NIOS supplement (§2.1) puts the discrepancy plainly: helium was expected to weigh twice as much as hydrogen but in fact weighs almost four times as much. A neutral particle of mass comparable to the proton had to exist.

Sir James Chadwick supplied it in 1932. By bombarding a thin sheet of beryllium with $\alpha$-particles he produced a stream of electrically neutral particles whose mass was slightly greater than that of the proton. He named them neutrons. The reaction can be written compactly as:

$$\ce{^9_4Be + ^4_2He -> ^12_6C + ^1_0n}$$

With this, the trio was complete: every atom is made of electrons, protons and neutrons. The neutron's neutrality is why it was the last to be found — carrying no charge, it left no track in the electric and magnetic fields that had exposed the electron and the proton.

Properties of the fundamental particles

The three discoveries converge on a single comparison table, reproduced from NCERT Table 2.1. The relative charges are $-1$, $+1$ and $0$; the proton and neutron are nearly equal in mass and each roughly 1840 times heavier than the electron, as the NIOS supplement notes. These values recur verbatim in PYQs and must be known to the figures given.

Two reminders follow directly from the table. The electron mass is roughly $\tfrac{1}{1840}$ that of the proton, so almost the entire mass of an atom sits in its nucleus. And the neutron is marginally heavier than the proton ($1.00867\ \text{u}$ versus $1.00727\ \text{u}$), a small but examinable distinction.