Atomic Models

The atomic structure of matter is composed primarily of protons, electrons, and neutrons, which are the building blocks of matter.It is composed of protons and neutrons, which form the nucleus of the atom, which is surrounded by electrons that are also part of the atom. 

The Atomic Theory of Dalton

British chemist John Dalton proposed that all matter is composed of atoms, which were said to be indestructible, indivisible, and unbreakable. According to him, the same element has exactly the same number of atoms, but the atoms of different elements differ in size and mass.

The following are the postulates that underpin his theoretical framework

•Atoms are the building blocks of all matter

•Atoms are unbreakable and undividable

•Specific elements contain only one type of atom, which is unique to them

•Each atom has a constant mass that varies from element to element, but is the same for all of them

•During a chemical reaction, the arrangement of atoms is altered

•Atoms can neither be created nor destroyed, but they can be transformed from one form to another through the process of transformation

•Dalton’s atomic theory was successful in explaining the laws of thermodynamics and chemical reactions

Demerits of Dalton’s Atomic Theory 

•The theory was unable to account for the existence of isotopes in the universe

Nothing about the structure of an atom was explained in an appropriate manner.

•Following this discovery, scientists discovered particles within the atom, demonstrating that atoms are divisible

•When particles inside atoms were discovered, it opened the door to a better understanding of chemical species, these particles inside the atoms are referred to as subatomic particles

Model of the Thomson Atomic Structure

In the early 1900s, the English chemist Sir Joseph John Thomson published his model of the atomic structure, which was widely accepted at the time.

He conducted an experiment known as the cathode ray experiment. The following is the set-up for how the experiment will function in practise:

Experiment with Cathode Rays

This apparatus consists of a glass tube with two openings, one for the vacuum pump and the other for the inlet, through which gases are introduced into the apparatus.

The vacuum pump’s primary function is to maintain a “partial vacuum” within the glass chamber during operation. A high voltage power supply is connected to the glass tube through the use of electrodes, i.e. cathode and anode, which are fitted inside the tube.

Observations

1. When a high voltage power supply was turned on, rays could be seen emerging from the cathode and travelling towards the anode. The ‘fluorescent spots’ on the ZnS screen that was used to confirm this were visible. These rays were referred to as “Cathode Rays.”
2. When an external electric field is applied, the cathode rays are deflected in the direction of the positive electrode, whereas when there is no external electric field, the cathode rays travel in a straight line to the positive electrode.
3. It appears that rotor Blades are rotating when they are placed in the path of the cathode rays. This demonstrates that cathode rays are made up of particles with a specific mass, and therefore possess some amount of energy.

Conclusions

1. Based on the results of his cathode ray experiment, Thomson hypothesised that the atomic structure consisted of a positively charged sphere with negatively charged electrons embedded within it.
2. It is commonly referred to as the “plum pudding model” because it can be visualised as a plum pudding dish in which the pudding represents the positively charged atom and the plum pieces represent the electrons, with the plum pieces representing the electrons.