Atomic Theory: From Ancient Greece to Modern Physics
First Discussions on the Subject
It was the Greek philosophers who began to question rationally, trying to find explanations or principles to explain the nature of things. For example, Thales of Miletus, who lived between the 5th and 6th centuries BC, postulated that the fundamental substance of the universe was water, from which it was possible to explain the constitution of all things.
A different view was that of Empedocles (5th century BC), who, by adding different traditions, argued that there were four basic substances: water, air, fire, and earth, and the mixture of them could produce all the items in nature.
One of the discussions that arose in the ancient world in relation to the matter was whether this was always divisible (continuous) or if there was a limit (discrete).
While some philosophers postulated that matter was continuous, i.e., could always be further subdivided, others argued that it was discrete, i.e., there was a limit to which it could not be further subdivided. This limit, or fundamental particle, was called atom (without division).
The main proponents of this idea were Democritus of Abdera and Leucippus of Miletus (both Greek philosophers who lived in the 5th century BC).
The Atomists
Leucippus and Democritus believed that matter was a discrete distribution and that the fundamental particles, tiny, indivisible, and with no structures, were the basis of all matter.
Postulates of Atomic Theory
- All matter is made up of atoms, and they are in a vacuum.
- The atoms are small particles characterized by their hardness, shape, and size. Other qualities of matter, such as color, taste, and temperature, would only be subjective impressions.
- The combination of atoms can form the four elements (water, air, fire, and earth), which in turn shapes all elements with ligatures.
The Contribution of Scientific Thought
The ideas of Greek atomistic philosophers have the merit of approaching reality through the attributes of thought, observation, imagination, and reasoning.
However, they missed the great step that scientific thinking itself gave:
Experimentation.
Galileo Galilei was the one who gave priority to the experimental method to test some insights into the nature.
Robert Boyle was outstanding in the field of study of the subject, who established the concept of chemical element as a substance that is not made by others, and suggested that their number should be greater than four.
John Dalton developed the first model of the atom of the scientific age, who, based on mass measurements and proportions of substances, published in 1808 a treaty that you could read the following ideas:
- The chemical elements are made of tiny and indivisible particles.
- All atoms of the same chemical element are equal.
- The masses of atoms of different chemical elements are different.
- Atoms are indestructible and retain their identity in chemical changes.
- The atoms are spherical and are combined with each other in simple ratios (1:2, 2:3. Etc.) to form chemicals.
To summarize their findings, Dalton used mathematical language and also invented a symbolism to write the chemical changes.
First Experiments with Atomic Particles
I. – The Thomson Experiment
During the 19th century, scientists began experimenting with “cathode ray tubes.” These consist of a glass tube with a gas at very low pressure inside, and where two metal plates called cathode and anode are placed, connected to a potential difference.
The English physicist Joseph Thomson studied in depth the so-called “cathode rays” in 1897 using an assembly as that shown in the drawing.
Thomson’s Observations and Conclusions
Experimental evidence observed by Thomson:
- The rays produce shadows.
- The rays are deflected by passing through a magnetic field.
- The rays always go from the negative plate (cathode) to the positive plate (anode).
Thomson’s conclusions:
- They are electrically charged particles.
- They travel in straight lines.
- They have a negative electrical charge.
In short, Thomson found that cathode rays were negatively charged particles and fragments that he considered part of the atom. As the electric charge of the matter is, in general, neutral, he concluded that the other part of the atom had a positive charge.
Finally, in calculations of mass, he found that the greatest amount of mass is in the positive charge.
With all their findings, Thomson established a model of the atom in which electrons are embedded in a mass of positive charge, similar to raisins in a loaf of Easter bread.
II. – The Rutherford Experiment
In 1911, the English physicist Ernest Rutherford, applying the technique of scattering *, conducted the following experiment:
He bombarded a thin gold foil with alpha particles (helium nuclei) as shown by the following scheme.
From a radioactive source came alpha particles, which were headed by a collimator to the gold foil. On the other side of the blade, he set a screen with a fluorescence that recorded the impact of the particles, therefore, bore witness to the trajectory of them after passing through the gold foil.
Rutherford noted that the majority of particles passed straight from the source of alpha particles to a screen, a few were deflected at small angles, while some (rarely) were diverted at a wide angle, even to be returned.
* Scattering: indirect observation technique, is to launch particles into a target and analyze the results
Rutherford’s Model
What most attracted the attention of Rutherford was the fact that some alpha particles were returned by 180° with respect to the release. The scientists said it was as surprising as if you launched cannon balls at a piece of paper and some of them bounced.
The former was assumed to physical most of the mass of the atoms should be confined to a very small space, that the call nucleus. Through their observations, he postulated a model of the atom based on the following characteristics:
- The atom has a nucleus which concentrates more than 99% of the mass.
- The nucleus is positively charged.
- The core radius is a hundred thousand times smaller than the radius of the atom, i.e., about 10^-15 m.
- In the outer shell of the atom are negatively charged electrons and small mass.
- The electrons are moving at high speed around the nucleus.
* As the sizes of atoms are very small, the units that commonly work to measure length (meters or centimeters) are impractical for such small scales. Thus defined called angstrom unit (ǻ), where 1 ǻ = 10^-10 m, since the size of atoms varies between 1 and 2 ǻ.
Rutherford’s Planetary Model
By the fact that the electrons revolve around the nucleus, Rutherford’s model is also known as the planetary model.
Weaknesses of Rutherford’s Model
A scientific model mainly aims to explain the observed phenomena and despite that Rutherford’s model was a major breakthrough compared to Thomson’s, and explaining things Thomson’s model could not, it leaves some unexplained phenomena.
In Rutherford’s atomic model, electrons are kept at some distance from the nucleus, moving around him. To be moving their velocities are changing and, therefore, have centripetal acceleration which implies that this burden should be accelerated and emit electromagnetic radiation.
If we assume that electrons are accelerated, they should emit radiation and, therefore, lose energy and spiral into the nucleus. This would make the atom very unstable, which does not occur in reality.