Discovery of the Proton: MDCAT Chemistry notes
Discovery of the Proton for MDCAT: Goldstein's canal rays, properties of positive rays, e/m depending on the gas, and Rutherford's proton.
Discharge tube experiments
A discharge tube is a glass tube containing a gas at very low pressure, with two electrodes connected to a high voltage. Cathode rays (streams of electrons) travel from the cathode to the anode. In 1886 E. Goldstein used a tube with a perforated cathode and observed rays passing through the holes (canals) in the cathode and travelling in the opposite direction to cathode rays. He called them canal rays; because they carry positive charge they are also called positive rays.
How positive rays are formed
Fast-moving cathode-ray electrons collide with gas molecules in the tube and knock electrons out of them, producing positive ions:
$$M\rightarrow M^++e^-$$
These positive ions are attracted towards the negatively charged cathode. Some pass through the perforations and appear behind it.
Properties of positive rays
- They move in straight lines, towards the cathode.
- They carry positive charge and are deflected by electric and magnetic fields, in the direction opposite to cathode rays.
- They are material particles, can produce a flash on a zinc sulphide screen and can cause mechanical effects.
- Their e/m ratio depends on the nature of the gas in the tube, unlike cathode rays whose e/m is the same for every gas. This is because each gas gives a different positive ion.
- Their e/m is much smaller than that of electrons, since the ions are much heavier.
The proton
The lightest positive rays, with the largest e/m value, are obtained when hydrogen gas is in the tube. The particle obtained is the hydrogen ion, $H^+$. Rutherford recognised this as a fundamental particle present in all atoms and named it the proton (around 1919-1920).
| Particle | Charge | Relative mass | Mass (kg) |
|---|---|---|---|
| Electron | $-1.602\times10^{-19}$ C | 1 | $9.11\times10^{-31}$ |
| Proton | $+1.602\times10^{-19}$ C | 1836 | $1.6726\times10^{-27}$ |
The proton is about 1836 times heavier than the electron, so the electron's mass is 1836 times less than that of the proton. A hydrogen atom (one proton plus one electron) is about 1837 times heavier than an electron.
Proton number
The number of protons in the nucleus is the atomic number (proton number) $Z$, written as a subscript: in $^{23}_{11}Na$ the proton number is 11 and the mass number is 23 (so 12 neutrons).
Ions in the gas phase
Ionisation energy is defined for isolated gaseous atoms. When a gaseous atom loses an electron, both the atom and the cation formed are isolated and free from external influences, which is why the gas phase is used.
Common MDCAT traps
- Positive rays move towards the cathode, not the anode.
- Goldstein discovered canal rays; Rutherford named the proton.
- e/m of positive rays depends on the gas; e/m of cathode rays does not.
- Proton/electron mass ratio is 1836; H atom/electron is 1837.
- In $^{A}_{Z}X$ the subscript is the proton number.
Quick revision
- Canal rays: through the perforated cathode.
- Hydrogen gives the lightest positive rays (protons).
- Proton charge $+1.602\times10^{-19}$ C.
- Proton mass $1.6726\times10^{-27}$ kg.
- Positive rays are deflected opposite to cathode rays.