History of Innovation of Atom, Atomic Structure and Elements

In the history of chemistry and physics, there were (are) lot experiments to find what is atom, what is the atomic structure and what are elements. Different models such as Democritus's atomic theory, Plato and Aristotle's atomic theory, John Dalton's atomic theory (1808) for atomic structure were developed and introduced with time by scientists. Some of these models were rejected and modified by time and we will see the history of finding of atom in this tutorial.


  • Background of finding the element, atom and atomic structure
  • Democritus's atomic theory (400 BC)
  • Plato and Aristotle's atomic theory (440BC)
  • John Dalton's atomic theory (1808)
  • J.J Thomson's atomic model (1904)
  • Ernest Rutherford's atomic model (1911)
  • Niels Bohr atomic model (1913)
  • Erwin Schrodinger atomic model (1926)

Background of finding the element, atom and atomic structure

In the beginning, humankind passed several ages. Hunting era, pastoral age, and agrarian age (3000BC-4000BC) respectively.

When humans entered the agrarian age, they had plenty of time to carefully think and observe the environment. Then humans began to think about how these environmental activities run and what and how these are made. And also they found some pure elements (2000BC) like Au, Fe, S, and Hg because this element is less reactive than others and exist naturally pure stage. And also they observed some of these things converted to other things.


These main things they called "elements". That's how the word element came. And converted things they called compounds.

Belief of Greeks

The second thing is ancient Greeks believed that there were four elements that everything was made up of, earth, water, air, and fire. This theory was suggested around 450 BC.

Democritus's atomic theory (400 BC)

He said that matter is made up of very fine substances that are invisible to the naked eye. He said this without any investigation but he observed some incidents and they are mentioned below.

  1. The spread of smoke from a fire through the air
  2. Absorption of dye dropped on a piece of wood by the wood.

And he also said as matter breaks down, it reaches a point where it cannot be further divided. That thing they called "ATOMOS".

Plato and Aristotle's atomic theory (440BC)

They say matters can be divided until infinity. Because of this, Democritus' opinion about atoms was undermined.

John Dalton's atomic theory (1808)

He said that again matters consist of small particles which are invisible naked eye (Democritus theory). But also he said the size and type of elements vary from element to element. Furthermore, he said that combining elements with other elements do not change their nature. It's just like a solid sphere.

Observations of Dalton

When touching the dolomite that rubs our hand, Dyes disperse through water.

J.J Thomson's atomic model (1904)

He did several experiments by using a Cathode ray tube.

Experiment of Cathode ray tube

When applying a high voltage between the cathode and anode he observed a green color beam travel from cathode to anode. And also when a close magnet is to the cathode ray tube the green color beam reflates oppositely. Magnet repulsion is negatively charged and attracts positively charged, they already know this phenomenon.

Incident of finding of electrons and Plum Pudding Model

Accordingly that Thomson decided that the Stanger beam which starts from the cathode consists of or holds a negative charge. This is the first recorded incident about subatomic particles called "electrons". And he introduces the "plum pudding model". He simply said that negatively charged electrons are embedded within a positively charged soup (solid sphere).

Ernest Rutherford's atomic model (1911)

He introduces quite a different idea about atoms. He said that atoms are not like a solid spheres. Hans Geiger and Ernest Marsden practically conducted Rutherford’s idea. Accordingly, in this experiment, a thin gold foil exposes to an alpha particle beam. And also that area was covered by the fluorescent screen which is sensitive to alpha particles.

Here most alpha particles passed straight through the gold foil. And some of the alpha particles were deflected by small angles. Surprisingly one out of every 12,000 alpha particles appeared to rebound. Accordingly that he stated a few statements about atoms.

  • Atoms consist of largely empty space
  • The Center of the atoms has an extremely dense and positively charged area
  • An electron moves or rotates around that dense positive area.

Accordingly, these statements arise big problems for the science community. They accepted this model but Rutherford was unable to answer that questions/ problems.

What are these questions asked by scientists regarding Ernest Rutherford's experiment

  • How that centered highly dense particles bind together without any repulsive force?
  • How that negatively charged electrons rotate around the positively charged area without any attractions.
  • How do non-generate any electric magnetic radiations (EMR) from the electrons? (Normally when some charged particle moves it result generates EMR.)

Niels Bohr atomic model (1913)

Above questions were somewhat resolved by using the Bohr model. He introduced a theoretical model for the hydrogen atom that explained its emission spectrum. A gas discharge tube was used for this and discharge light was transferred to a Prisma and get the line spectrum. According to the observation. he said,

  • Electrons move like a Rutherford model
  • Electrons do not release energy when it traveling
  • Electrons stay in specific energy levels called orbitals.
  • Electrons excited ground level to an excited state and due to unstable properties electrons again jump to the ground state when electrons come back that release energy as electromagnetic radiation.

However, at that time scientists also have some questions,

  • Why electrons cannot stay at intermediate parts of the energy level?
  • Why do these energy levels have different energy?

Erwin Schrodinger atomic model (1926)

Schrodinger was known as the savior of science society at that time. Because he introduces the Schrodinger equation. Before he introduces this equation, they already know that electrons have both particle and wave behaviors (accordingly to the double slit experiment).

This equation simply said that, of the considered electron,

The total energy of the electron = kinetic energy of the electron + potential energy of the electron

ψ = wave function of the electron.

If someone solves the Schrodinger equation he or she can get / ψ /wave function.

We believe that wave function describes how to behave that consider electron in the atom. But this phenomenon is still a mystery. Because of how electrons behave as a particle and wave same time. Accordingly, double slit experiment we observed, we observed some electrons, that behave as a particle and if we do not observe that same electron they behave as a wave. Schrodinger was also unable to describe, what is the real meaning of ψ.

After that Max Born said that, if we square Y we can get a probability density function. That function describes the areas we can catch some considered electrons. However, it does not say what is ψ. So the entire science world is still waiting for the solutions to this.