The Discovery of Electrons

Some objects exhibit a property called electric charge, which can be positive (+) or negative (-). Positive and negative charges attract, neutralize each other, while two positive or two negative charges repel each other.

All material objects are made up of charged particles.

An electrically neutral object has an equal number of positively and negatively charged particles and carries no net charge.
If the number of positive charges is greater than the number of negative charges, the object has a net positive charge.
If the number of negative charges exceeds the number of positive charges, the object has a net negative charge.
It can be observed that when we rub one substance against another, as when we comb our hair, a static electric charge is produced, which implies that the rubbing separates some positive and negative charges. In addition, when a positive charge is produced somewhere, an equivalent negative charge also appears somewhere else so that the charge is balanced.

The discovery of electrons

The abbreviation for cathode ray tubes, CRT, has become agreed upon as a common acronym. The CRT is the heart of computer monitors and television sets. The first cathode ray tube was built by Michael Faraday (1791-1867) about 150 years ago. By passing electricity through glass tubes under vacuum, Faraday discovered cathode rays, a type of radiation emitted by the negative pole or cathode passing through the evacuated tube to the positive pole or anode. Scientists later found that cathode rays travel in a straight line and have properties that are independent of the cathode material (i.e., whether it is iron, platinum, etc.).

A cathode ray tube consists of a high-voltage source of electricity that creates a negative charge on the electrode on the left (cathode) and a positive charge on the electrode on the right (anode). The cathode rays are directed from the cathode (C) to the anode (A) which is perforated to allow a narrow beam of cathode rays to pass through. The rays are only visible through the fluorescent green color they produce on a zinc sulfide coated screen. They are invisible in the rest of the tube.

The cathode rays produced in the CRT are invisible, and can only be detected by the light emitted by the materials with which they collide. These so-called phosphorescent materials are used as paint at the end of the CRT, so that the path of the cathode rays can be seen. (Fluorescence is the term used to describe the emission of light by a phosphorescent substance when it receives energetic radiation). Another important observation about cathode rays is that they are deflected by electric and magnetic fields in the way expected for negatively charged particles.

Deflection of cathode rays by an electric field

The cathode ray beam is deflected as it travels from left to right in the field created by the electrically carded capacitor plates (E). The deflection corresponds to that expected for negatively charged particles.

Deflection of cathode rays in a magnetic field

The cathode ray beam is deflected when it travels from left to right in a magnetic field (M). The deflection corresponds to that expected for negatively charged particles.

In 1897, J. J. Thomson (1856-1940) established the relationship between the mass (m) and charge (e) of cathode rays, i.e. mass/charge (m/e), by the method described in the Figure below. Thomson also concluded that cathode rays are negatively charged fundamental particles of matter found in all atoms. (The properties of cathode rays are independent of the composition of the cathode). Subsequently, cathode rays were given the name electrons, a term proposed by George Stoney in 1874.

Determination of the mass-to-charge ratio for cathode rays

The cathode ray beam collides with the screen at the end of the tube without deflection if the forces exerted on the beam by the electric and magnetic fields are counteracted. By knowing the electric and magnetic field strength, together with other data, the mass/charge value can be obtained. The most accurate measurements give a value of -5.6857 × 10-9 grams per coulomb. (Since cathode rays are negatively charged, the sign of the charge-to-mass ratio is also negative).

Robert Millikan (1868-1953), determined the electronic charge (e) by a series of experiments with oil droplets (1906-1914) described in the Figure below. The currently accepted value of the electronic charge, expressed with five significant figures is:

-1,6022 × 10-19 C.

Using this value and an exact value of the mass-to-charge ratio for an electron, it is obtained that the mass of an electron is :

9,1094 × 10-28 g.

Millikan’s droplet experiment

Ions, charged atoms or molecules, are produced by the action of energetic radiation known as X-rays. Some of these ions become attached to small oil droplets, giving them a net charge. The rate at which a droplet falls in the electric field between the plates of the condenser increases or decreases depending on the magnitude and sign of the droplet’s charge. By analyzing data from a large number of droplets. Millikan concluded that the magnitude of the charge, (q) of a droplet is an integer multiple of the electronic charge, (e). That is, q = n e (where n = 1, 2, 3,…).

Once the electron was considered as a fundamental particle of matter existing in all atoms, atomic physicists began to speculate on how these particles were incorporated within atoms. The commonly accepted model was that proposed by J. J. Thompson, who thought that the positive charge needed to counteract the negative charges of the electrons in a neutral atom was in the form of a diffuse cloud. He suggested that the electrons floated in this diffuse cloud of positive charge, resembling a mass of jelly with the electrons as “fruits” embedded in it. This model was given the name plum pudding because of its resemblance to a well-known English dessert.

Video about the Discovery of Electrons