Brief Evolution of The Atomic Model Part 1

*Evolution of The Atomic Model*

What is an atom?

In modern chemistry, one can define an atom as the smallest possible amount of matter which still retains its identity as a chemical element and consisting of a nucleus surrounded by electrons that are in their respective atomic orbital.

Well, this is one of the most plausible and simplest answers nowadays.

For the majority of people, describing things/matter in terms of atoms is the most basic thing to do. But do we really know how did we reach the atomic model that a lot of people refer to?!

Was this atomic model always the same?

How did scientists come to this stage?!

In the next articles, we shall take a leap back in time and see when was the first atomic theory created and how this theory evolved along the timeline.

First Atomic Theory

The first atomic theory was set up in the years 450-460 BC. In these years there were two great Greek philosophers among others, Aristotle and Democritus, who belong primarily to the history of philosophy but also have a place in the history of physical science. Democritus, also known as the laughing philosopher, was the great opponent of the more influential Aristotle.

(Greeks are seen to have laid the foundation of physical science and they are the inventors of the most scientific languages and methods, for example, Pythagorean Theorem, Hippocrates who was known as the father of modern medicine...)

At that period, a debate was set up on the question " In how many small pieces can a grain of sand be divided?"

*Sand*

Aristotle stated that the grain could be divided infinitely and that there was no specific limit to how small the divisions could be. Since Aristotle was one of, if not the most influential philosopher at that time, very few people disagree with him and his statement became a popular belief.

Challenging Aristotle's notion, Democritus came with the following hypothesis that there was a limit to the division of the grains which is called the 'Atomos' and he backed his hypothesis with the following explanations:

• These atomos are infinite in number and can be differentiated by their shape and size. Any change in a thing is based upon a change in the positions of the atoms. (For example, breaking of something is due to a change of the atomic positions prior to an applied force.)
• The motion of these atomos are eternal like the atoms themselves and can only be altered by pressure and percussion. When the atomos collide, they are either repelled of are connected in clusters, held by small hooks on their surfaces.
• They are solids and have no internal structure.

("Atomos" was then coined as "Atoms.")

Even though his explanation seems more plausible, Aristotle disagreed with him and proposed that matter (or everything) consists of 4 elements being air, fire, water and earth and this idea prevailed for a very long time.

It was after around 2000 years that the ideas of Democritus were exploited but unfortunately, his original works were not saved for posterity and his atomic theory was know by the works of others.

--This was the first atomic theory and the long journey starts.

John Dalton

John Dalton

In the year 1800, the English meteorologist and chemist continued the works of Democritus. John Dalton is a pioneer in the development of the modern atomic theory and his proposed theory became the basis for intense study of chemistry. He was the first chemist to completely describe matter in terms of atoms and their properties.

Fundamentally, he based his atomic theory on the law of conservation of mass and the law of constant composition.

The law of conservation of mass basically means that mass can neither be created nor destroyed in chemical reactions (except for nuclear reactions which were later discovered) and thus the mass of reactants will equal the mass of the products.

The law of constant composition states that samples of pure chemicals will consist of the same elements in the same mass proportions or ratios.

For example, water will always consist of Hydrogen and Oxygen in a mass ratio of 1:9 and carbon dioxide contains C and O in a mass ratio of 3:8.

And this law is the basis of stoichiometry in chemistry.

His atomic theory

• Matter is composed of tiny definite particles called atoms. And this clearly explains the laws stated above.
• Atoms are indivisible and indestructible (solid, hard and massy). Well, one should understand that at that time, he did not have the required instruments to better this statement. And of course, I am referring to nuclear reactions.
• Atoms of a particular element share identical properties, including weight. (This statement was later on modified because of the discovery of isotopes that have the same atomic number but different atomic masses. The existence of isotopes was suggested around a century later (1913) by the English Chemist Frederic Soddy.)
• Atoms of different elements have different masses. Although certain elements may possess similar chemical properties, their atoms will always differ in their atomic masses.
• Atoms of different elements combine in a fixed, whole number ratios when forming compounds.

For example, let us consider Hydrogen and Oxygen. One is an explosive gas and the other promotes violent combustion and yet, the combination of both make liquid water which puts out fires.

And water consists of oxygen and hydrogen in an atomic ratio of 2:1.

We all know that the periodic table (the one that is currently used) was developed in 1869 by the Russian Chemist Dimitri Mendeleev. But did you know that John Dalton had also published tables of elements??? To get a quick overview of his tables of elements, click here.

Prior to some misalignments in his atomic theory, some of them were discontinued and revised but a large part still holds today and makes the foundations of modern chemistry.

Micheal Faraday

Micheal Faraday

In 1830, Micheal Faraday, a famous British Physicist, made a significant discovery about the relationship between electricity and atoms.

He placed an anode (positive electrode) and a cathode (negative electrode) in an aqueous solution of a dissolved compound and what he discovered was astounding! (This experiment is now known as electrolysis.)

Simple electrolytic cell                  

(Anodes and cathodes are basically made of inert metal rods such as platinum)

He discovered that one of the elements of the dissolved compound was deposited on one electrode and the other element was deposited on the other electrode.

It was this discovery that led to the idea that atoms are somehow related to electricity.

Joseph John Thompson 1896

Thompson, a British Physicist, is widely recognised as the discoverer of electrons, which he initially called the corpuscles in his Nobel Lecture. (Corpuscles usually refer to small things or bodies)

J.J Thompson

He discovered electrons while he was experimenting with a modified cathode ray tube, which was invented a few years back and was also known as the Crookes tube.

The schematic diagram of a CRT(cathode ray tube) is shown below:

Modified Cathode Ray Tube

In a CRT, mostly all of the air present is evacuated (to basically form a vacuum) so as to minimise collisions of the cathode ray with air molecules.

What happens in a CRT is that when an electric current passes through the cathode which gets heated, the cathode emits a cathode ray which is accelerated towards the positive anode. At that time, the electrical nature of the cathode ray was unknown and it was coined as the cathode ray since it emanates from the cathode.

The far end of the CRT is coated with a phosphorescent material (Usually zinc sulfide).

At first, he conducted the experiment without the addition of the aluminium plates D and E and as expected, he visualised a single glow on the coating. The path of the cathode ray without the application of an electric field is shown in the above diagram.

So as to investigate further about the nature of the cathode ray, Thompson added two aluminium plates D and E connected in a circuit so that plate D is positive and plate E negative and carried the experiment again.

He then observed that the glow was higher in position than usual (the path of the cathode ray is shown in red) and as opposite charges attract and the cathode ray path seemed to deviate towards the positive metal plate D, he deemed that the cathode ray must surely contain negatively charged species.

To delve further in his investigations, he also conducted magnetic experiments and in combination with other data, he finally made the following conclusions:

• Cathode rays are composed of negatively charged particles.
• These particles must be part of the atom since the mass of one of these particles is only 1/2000 times that of a Hydrogen atom.
• These subatomic (part of an atom) particles can be found within the atoms of all elements. He came to this conclusion by varying the metal used as the cathode (Al, Fe, Cu, Zn...) and he found out that all these metals gave similar cathode rays. He then concluded that all atoms contained these fundamental negatively charged particles which he called corpuscles. And gradually, these corpuscles were renamed as electrons.

His conclusions did not stop here.

He eventually came with an atomic model commonly known as the Plum Pudding Model.

The Plum Pudding Model

Since atoms are neutrally charged, so as to counter the negative charges of the electrons, Thompson thought that there must be a source of positive charges in the atoms.

So he analogised the Plum Pudding as an atom, where the plums were the negatively charged electrons and the dough is positively charged.

However, he did not figure out the mass to charge ratio of the electrons. This is where Robert Andrews Millikan came in.

Robert Andrews Millikan

In 1909, Millikan came up with the Millikan's Oil Drop Experiment so as do determine the charge of an electron.

Robert Millikan

For those of you who are not familiar with the Oil Drop experiment, here is a brief:

The setup of this experiment is as follows:

Oil drop Experiment

In this experiment, an atomiser/nebuliser is used to produce very fine oil droplets which by gravity falls down through the small aperture.

During this process, the oil droplets lose electrons to become positively charged and the amount of positive charge depends on the number of electrons lost.

Now, as the droplet passes through the aperture into an electric field (which is set up between the two metal plates so that the upper plate is negatively charged) there are two forces acting on the positively charged droplet.

There is the gravitational force pulling the oil droplet down and there is the electric force pulling the positively charged drop towards the negative metal plate D.

The voltage of the electric current is varied so that the electric force is equal to the gravitational force and this causes the oil drop to float, which is seen through the microscope.

The gravitational force on the droplet is dependent on the mass of the droplet and the electric force needed to counterbalance the gravitational force is dependent on the charge and the voltage between the metal plates. They could determine the mass of the oil droplet and the voltage between the plates easily. After obtaining those two variables, the charge of the droplet could be easily determined. 

This procedure was repeated for a large number of masses of oil droplets and what they found out was very astonishing!

After their calculations, they found out that the differences between the charges of the different masses had something very similar. The differences were all multiples of a specific number which was calculated to be 1.60 x 10^-19.

Since the droplet had lost an electron to gain this amount of positive charge, they then concluded that the charge on an electron is -1.60 x 10^-19 !!!!

Amazing right?!

Well, this was the first experiment that was used to determine the charge on an electron.

There is more to come, stay tuned!!!

"Never lose a holy curiosity." - Albert Einstein