Discovery of Electron, Proton, and Neutron - Ninth Grade Science
For nearly a century following John Dalton's 1808 treatise, the atom was revered as the ultimate, indivisible building block of the physical universe. Dalton's billiard-ball model explained the laws of chemical combination, but it left a critical mystery unanswered: why do atoms bond, and what produces electrical phenomena? In the late nineteenth and early twentieth centuries, a series of cathode ray and canal ray experiments shattered the concept of the indivisible atom. Through the work of J.J. Thomson, Eugen Goldstein, and James Chadwick, science penetrated the interior of the atom, uncovering three fundamental subatomic particles: the negatively charged electron, the positively charged proton, and the neutral neutron. In this chapter, you will trace the discovery of these subatomic particles, analyze their physical properties, and explore the internal electrical balance of the atom.
Static Electricity and the First Clues of Subatomic Charge
The first hints that atoms contained internal electrical charges emerged not from particle accelerators, but from simple electrostatic observations.
Electrostatic Evidence of Subatomic Charges
Comb Rubbed on Dry Hair Glass Rod Rubbed on Silk
[ Comb - ] [ Glass Rod + ]
| |
v v
Attracts tiny pieces Attracts tiny neutral
of neutral paper! pith balls!
When an amber rod is rubbed with wool, or a plastic comb is passed through dry hair, the comb acquires the ability to attract small bits of neutral paper. How could a neutral object acquire an electrical charge simply through mechanical friction? If an atom were an indivisible, unchargeable sphere, friction could not induce electrical polarity. The phenomenon proved that atoms must contain separable electrical charges that can be transferred from one material to another.
The Discovery of the Electron: J.J. Thomson (1897)
The first subatomic particle was unmasked through experiments with gas discharge tubes (Crookes tubes) by British physicist J.J. Thomson at the Cavendish Laboratory in Cambridge.
The Cathode Ray Tube Experiment (1897)
Perforated Anode (+) Fluorescent Screen (ZnS)
Cathode (-) | |
+----+ +----+ |
| | ========>| |--------------------->* (Glowing Green Spot)
+----+ +----+ |
| | |
| High Voltage | |
| (10,000 V) | Electric Plates |
+-------[====]---+ [ + + + + ] |
^ |
Beam Deflects UPWARD toward (+) Plate!
[ - - - - ]
Cathode Ray Tube Experiments
A cathode ray tube is a sealed glass cylinder fitted with two metal electrodes at opposite ends and evacuated to an ultra-low pressure of approximately $0.001 ext{ mm Hg}$ ($0.13 ext{ Pa}$). When a high electrical voltage (roughly 10,000 volts) is applied across the electrodes, a mysterious stream of radiation emanates from the negative electrode (the cathode) and travels in straight lines toward the positive electrode (the anode). These streams were named cathode rays.
Properties of Cathode Rays and the Identity of the Electron
Thomson subjected cathode rays to electric and magnetic fields, establishing key physical properties:
1. Negative Electrical Charge: When passed between two electrically charged metal plates, the cathode ray beam deflected sharply toward the positive plate and away from the negative plate. This proved that cathode rays are composed of negatively charged particles.
2. Mechanical Mass and Momentum: When a tiny, lightweight mica paddle wheel was mounted on rails inside the tube, the cathode ray beam struck the paddles and caused the wheel to rotate, demonstrating that the rays consist of material particles possessing mass, velocity, and momentum.
3. Universal Invariance: Thomson tested different metals for the cathode (iron, copper, platinum, aluminum) and filled the tube with different gases (hydrogen, oxygen, nitrogen, air). Regardless of the electrode metal or gas employed, the charge-to-mass ratio ($e/m$) of the particles remained strictly identical:
\frac{e}{m} \approx 1.76 \times 10^{11}\text{ Coulombs per kilogram (C/kg)}
Because these negative particles were produced by every element tested, Thomson concluded that they are fundamental subatomic constituents of all atoms. Irish physicist George Johnstone Stoney named these particles electrons.
The Discovery of the Proton: Eugen Goldstein (1886)
Because atoms are electrically neutral overall, the discovery of negatively charged electrons implied the existence of a corresponding positive charge within the atom.
Canal Rays (Anode Rays) in a Perforated Tube
Perforated Cathode (-)
|
v Holes (Canals)
Anode (+) +-+-+
+---+ | : |====> Positive Stream (Canal Rays)
| |============>| : |====> Glowing Red Spot on Glass!
+---+ +-+-+
Canal Rays (Anode Rays)
In 1886, German physicist Eugen Goldstein modified the cathode ray tube by using a perforated metal cathode containing tiny holes (canals). When high voltage was applied, he observed faint luminous rays passing backward through the perforations in the cathode, traveling in the opposite direction to cathode rays. Because these rays passed through the canals of the cathode, they were named canal rays (or anode rays).
Properties of Canal Rays and the Proton
- Positive Charge: Canal rays deflected toward the negative plate of an electric field, proving they carry a positive electrical charge.
- Gas-Dependent Mass: Unlike cathode rays, the charge-to-mass ratio ($e/m$) of canal rays was not universal; it varied depending on the gas present in the tube. Canal rays are not fundamental rays; they are positively charged gas ions formed when high-speed electrons collide with and knock electrons out of neutral gas atoms.
- The Proton: When the tube was filled with the lightest gas—hydrogen—the resulting canal ray particles were the lightest and smallest positive particles observed. In 1919, Ernest Rutherford confirmed this fundamental positive subatomic particle and named it the proton.
The Discovery of the Neutron: James Chadwick (1932)
By 1920, physicists realized that the mass of electrons and protons could not account for the total mass of atoms. For example, a helium atom contains two protons and two electrons; its electrical charge is neutral. However, the mass of a helium atom is approximately four times greater than the mass of a hydrogen atom (which contains one proton). Where was the missing mass? Rutherford hypothesized that atomic nuclei must contain a third, electrically neutral particle with a mass comparable to a proton.
Chadwick's Discovery of the Neutron (1932)
Polonium Alpha Source Beryllium Target Paraffin Wax Target
(He-4) (Be-9) (Hydrocarbon)
+---+ +---+ +---+
| o | === [ Alpha ] ===> | | === [ Neutron ] =>| | ===> Protons
+---+ +---+ (Neutral Beam) +---+ Knocked Out!
The Beryllium Bombardment Experiment
In 1932, British physicist James Chadwick resolved the mystery. Chadwick bombarded a thin sheet of beryllium metal with high-energy alpha particles ($\alpha$, helium nuclei) emitted by radioactive polonium. The beryllium target emitted a highly penetrating radiation that was not deflected by electric or magnetic fields, proving it carried zero electrical charge. When this neutral radiation struck a paraffin wax block rich in hydrogen atoms, it knocked protons out of the wax with high kinetic velocity. By measuring the recoil velocities of these ejected protons, Chadwick calculated that the neutral radiation consisted of particles possessing a mass slightly greater than a proton. Chadwick named this neutral subatomic particle the neutron.
Comparative Properties of the Three Subatomic Particles
+-----------------------+-------------------+-------------------+--------------------+
| Subatomic Particle | Electron | Proton | Neutron |
+-----------------------+-------------------+-------------------+--------------------+
| Symbol | e^- | p^+ | n^0 |
| Discoverer | J.J. Thomson | Eugen Goldstein / | James Chadwick |
| | (1897) | Rutherford (1919) | (1932) |
| Absolute Charge (C) | -1.6022 * 10^-19 | +1.6022 * 10^-19 | 0 (Neutral) |
| Relative Charge | -1 | +1 | 0 |
| Absolute Mass (kg) | 9.109 * 10^-31 kg | 1.6726 * 10^-27 kg| 1.6749 * 10^-27 kg |
| Relative Mass (u) | 0.000548 u | 1.00727 u | 1.00866 u |
| | (~ 1/1836 u) | (~ 1 u) | (~ 1 u) |
| Location in Atom | Extranuclear | Inside Atomic | Inside Atomic |
| | electron orbitals | Nucleus (Nucleon) | Nucleus (Nucleon) |
+-----------------------+-------------------+-------------------+--------------------+
Mass Comparison
The proton and neutron are heavy subatomic particles, together known as nucleons, each possessing an approximate mass of one unified atomic mass unit ($1 ext{ u}$). The neutron is slightly heavier than the proton by roughly 0.14%. In contrast, the electron is light: its mass is approximately $\frac{1}{1836} ext{th}$ of the mass of a hydrogen atom (proton). Consequently, virtually the entire mass of an atom (greater than 99.95%) is concentrated in the nucleus, while electrons contribute negligibly to total mass.
Chapter Exercises
Exercise 1: Contrast the nature and origin of cathode rays with canal rays (anode rays) in terms of their electrical charge, the universality of their charge-to-mass ratio ($e/m$), and the direction in which they travel within a gas discharge tube.
Exercise 2: An uncharged, neutral atom of sodium possesses 11 electrons and 12 neutrons. State the number of protons present in the sodium atom, determine the net electrical charge of the nucleus, calculate the approximate total atomic mass in unified atomic mass units, and explain why the overall sodium atom exhibits zero net electrical charge.
Exercise 3: Explain why James Chadwick had difficulty detecting the neutron compared to the earlier discoveries of the electron and the proton. Identify the physical property of the neutron that made its detection challenging.
Solutions and Explanations
Solution 1: Cathode rays are streams of negatively charged electrons that originate from the cathode and travel toward the anode; their charge-to-mass ratio ($e/m$) is universal ($1.76 imes 10^{11} ext{ C/kg}$), remaining identical regardless of the electrode material or residual gas in the tube, because electrons are identical components of all atoms. Canal rays (anode rays) are streams of positively charged gaseous ions that travel in the opposite direction—from the anode through perforations in the cathode; their charge-to-mass ratio is not universal, varying depending on the identity and molar mass of the gas in the tube, because canal rays are formed by residual gas atoms that have lost electrons, resulting in different ionic masses.
Solution 2: Because the sodium atom is electrically neutral overall, the total number of positive charges must equal the total number of negative charges; therefore, with 11 electrons, the atom must contain exactly 11 protons. The nucleus contains 11 protons (each with a relative charge of $+1$) and 12 neutrons (charge 0), giving the nucleus a net charge of $+11$ (or $+1.76 imes 10^{-18}$ coulombs). The atomic mass is the sum of the protons and neutrons: $11 + 12 = 23 ext{ u}$ (electrons contribute negligible mass, roughly $11 imes (1/1836) \approx 0.006 ext{ u}$). The atom as a whole is electrically neutral because the $+11$ positive charge of the 11 nuclear protons is canceled by the $-11$ negative charge of the 11 orbiting electrons ($+11 + (-11) = 0$).
Solution 3: James Chadwick encountered difficulty detecting the neutron because it possesses zero electrical charge ($q = 0$). Both electrons and protons carry electrical charges, allowing them to be deflected by magnetic and electric fields, to ionize gas molecules along their paths, and to register on photographic plates and cloud chambers. In contrast, the neutron has no electrical charge, passing through the electron clouds and electrostatic force fields of atoms without suffering electromagnetic deflection or leaving ionization tracks, making it invisible to conventional detectors. Chadwick detected the neutron by bombarding a paraffin wax target with the neutral radiation and measuring the recoil velocities of the charged protons knocked out of the wax through direct mechanical collisions.