Study Material

What are anode rays?

anode rays
The anodic rays originate when the electrons that go from the cathode towards the anode, collide with the atoms of the gas enclosed in the Crookes tube.

As the particles of the same sign repel each other, the electrons that go towards the anode tear away the electrons present in the crust of gas atoms in their path.

Thus, the atoms that have remained positively charged – that is, they have been transformed into positive ions (cations) – are attracted to the cathode (negatively charged).


canal ray tubeIt was the German physicist Eugen Goldstein who discovered them, observing them for the first time in 1886.

Later, the work carried out on anodic rays by the scientists Wilhelm Wien and Joseph John Thomson ended up assuming the development of mass spectrometry.

Properties of anodic rays

The main properties of anodic rays are the following:

– They have a positive charge, the value of their charge being an integer multiple of the charge of the electron (1.6 ∙ 10 -19 C).

– They move in a straight line in the absence of electric fields and magnetic fields.

– They deviate in the presence of electric fields and magnetic fields, moving towards the negative zone.

– Thin layers of metals can penetrate.

– They can ionize gases.

– Both the mass and the charge of the particles that make up the anode rays vary depending on the gas enclosed in the tube. Normally their mass is identical to the mass of the atoms or molecules from which they are derived.

– They can cause physical and chemical changes.

A little history

Prior to the discovery of anode rays, the discovery of cathode rays took place, which occurred throughout the years 1858 and 1859. The discovery is due to Julius Plücker, a German mathematician and physicist.

Later, it was the English physicist Joseph John Thomson who studied in depth the behavior, characteristics and effects of cathode rays.

Joseph John Thomson
Joseph John Thomson
For his part, Eugen Goldstein – who had previously carried out other investigations with cathode rays – was the one who discovered anode rays. The discovery took place in 1886 and he made it when he realized that discharge tubes with the perforated cathode also emitted light at the end of the cathode.

However, it was not he, but Wilhelm Wien who later made extensive studies of anode rays. Wien, together with Joseph John Thomson, ended up establishing the basis for mass spectrometry.

Eugen Goldstein’s discovery of anode rays constituted a fundamental pillar for the later development of contemporary physics.

Thanks to the discovery of anode rays, swarms of atoms in rapid and orderly movement became available for the first time, the application of which was very fertile for different branches of atomic physics.

The anode ray tube

In the discovery of anode rays, Goldstein used a discharge tube that had the cathode perforated. The detailed process by which anodic rays are formed in a gas discharge tube is as follows.

anode ray tube

By applying a large potential difference of several thousand volts to the tube, the electric field that is created accelerates the small number of ions that are always present in a gas and that are created by natural processes such as radioactivity.

These accelerated ions collide with atoms in the gas, ripping electrons from them and creating more positive ions. These ions and electrons in turn attack more atoms again, creating more positive ions in what is a chain reaction.

Positive ions are attracted to the negative cathode and some pass through holes in the cathode. By the time they reach the cathode, they have already accelerated fast enough that when they collide with other atoms and molecules in the gas, they excite the species to higher energy levels.

When these species return to their original energy levels, the atoms and molecules release the energy they had previously gained; energy is emitted in the form of light.

This process of light production, called fluorescence, causes a glow to appear in the region where the ions emerge from the cathode.

The proton

Although Goldstein obtained protons with his experiments with anodic rays, the truth is that he is not the one who is credited with the discovery of the proton, since he was not able to identify it correctly.

The proton is the lightest of the positive particles produced in anode ray tubes. The proton is produced when the tube is charged with hydrogen gas. In this way, when hydrogen ionizes and loses its electron, protons are obtained.

The proton has a mass of 1.67 ∙ 10 -24 g, almost the same as that of the hydrogen atom, and has the same charge but with the opposite sign as that of the electron; that is, 1.6 ∙ 10 -19 C.

Mass spectrometry

Mass spectrometry

Mass spectrometry, developed from the discovery of anodic rays, is an analytical procedure that makes it possible to study the chemical composition of the molecules of a substance based on their mass.

It allows both to recognize unknown compounds, to count compounds that are known, as well as to know the properties and structure of the molecules of a substance.

For its part, the mass spectrometer is a device with which the structure of different chemical compounds and isotopes can be analyzed in a very precise way.

The mass spectrometer allows you to separate atomic nuclei based on the relationship between mass and charge.

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