History of the Atom

History of the Atom:

By Jenny Baek and Jack Grimaldi

Did we always know what everything was made of?  Understanding how we developed the current model of the atom is not only important history, but it  teaches us how scientific models are built and helps us make sense of how the periodic table is organized and what we still don’t know. 

Protons, neutrons, and electrons are the three components that make up an atom. The protons and neutrons are what make up the core of the atom, the nucleus, and are the method by which to find the atomic number and atomic mass of any element (atomic number is the number of protons, atomic mass is the number of protons plus the number of neutrons). The electrons are the negatively charged subatomic particles that make up the outer energy levels of an atom. The electrons create a balance with the positively charged protons in the nucleus and create stability throughout the atom. Electrons are also the reason that covalent and ionic bonds can be created by multiple atoms, through sharing, gaining, or losing valence electrons. All of these subatomic particles play key roles in the anatomy of atoms, making it crucial to understand their history.

An atom is the smallest component of the element having chemical properties of the element. It also consists of a nucleus which holds the protons and neutrons, as well as electrons surrounding it.  All these components of the atom and how its electrons, protons, and neutrons behave was not discovered overnight. This information has been discovered and rediscovered for over 1000 years with many different philosophers and chemists. This article will help you understand as well as review the journey of the atom discovery.


The foundation of the atom was first discovered by Leucippus. Not much is known about this philosopher, however, he has originally come up with the atomic theory which states that ” The universe is composed of two elements: the atoms and the void in which they exist and move”



The discovery of the atom is first discovered by Democritus, a Greek philosopher in 456 BC.  As he adopted Leucippus’ theory, he has soon stated that atoms are very small quantities of matter. After this, he has hypothesized that the infinite atoms surrounding us cannot be destroyed, differ in size, and are always moving invisibly.


After Leucippus, Aristotle came to make sense of the atoms. Unlike Leucippus, Aristotle did not believe in the atomic theory and he taught his students in the theory that he believed in. He believed that all materials on earth were not made of atoms, but the four elements: Fire, Earth, Water and Air. (356 BC)



 John Dalton:

John Dalton has given chemistry 4 different theories of the Atom:

  1. All matter is made up of atoms. All atoms are indivisible and indestructible
  2. All atoms are given an element that are identical in mass and properties
  3. Compounds can be formed in a combination of two or more atoms
  4. A chemical reaction is the rearrangement of atoms

Though, these theories remain valid, we have discovered today that atoms can be destroyed by nuclear reactions. Dalton did not know about subatomic particle and this is why his theory is often referred to as the “Billiard ball” model.  His theory thought all atoms were unique and there was nothing common about them.

Dalton’s model is often referred to as the “Billiard ball”  model of the atom


JJ Thompson:

Using the Plum pudding model, JJ Thomson has made a significant contribution to the discovery of the atom. He has discovered electrons, a negatively charged particle, exists within the atom, which is represented by the plums and is attracted to a positively charged particle, which is represented by the pudding. With this model and extraordinary activity, not only the electrons were discovered, but also the protons. (1898)

The electron – J.J. Thomson discovered the electron in 1897. Thomson ran a series of experiments that involved using a cathode ray tube (vacuum tubes that contain two electrodes- one on either side). One of the electrodes, the cathode, would fire electrons at the other electrode, the anode, when voltage was added to the system. When the tube was running, Thomson noticed a beam of light between the two electrodes. When he applied a magnetic field to the beam he discovered that the beam moved towards a positive charge and away from a negative charge. This led to the conclusion that the cathode rays were of negative charge. These cathode rays are now called electrons. Thomson then concluded that atoms are composite objects, made of pieces with positive and negative charge, and that the negatively charged electrons within the atom were very small compared to the entire atom. He therefore proposed that atoms have structure similar to a plum pudding, with tiny, negatively charged electrons embedded in a positively charged substrate. He called this the plum pudding model. This was eventually proven incorrect. Neils Bohr took this one step further when he discovered that electrons can jump from energy level to energy level and stay in stable orbits. Bohr also concluded that the electrons release light energy when moving closer to the nucleus and absorb light energy when moving further away from the nucleus.

Cathode Ray tube used by JJ Thomas[Picture credits: Study.com]




Thomson’s Plum Pudding model (sometimes called raisin bun, chocolate chip cookie… it may be a requirement to be hungry when studying Thomson’s model of the atom)


Ernest Rutherford:

In 1911, to test JJ Thomson’s, Ernest Rutherford conducted E the famous gold foil experiment and discovered the space between the electron and proton. Basically, his experiment is based on shooting alpha rays through a gold foil. He hypothesized that particles will go through the foil and gather at the other side of the foil, all in the centre. To his surprise, the alpha particles were deflected and did not pass through and if they did, they did not all hit one target. This deflection shown that the atom is made up of empty space, and the nucleus in the middle has the largest mass. Through this experiment, the structure of the atom began to appear.

Proton :

The proton was discovered by Ernest Rutherford in 1920. A proton is a subatomic particle with a positive charge that is located in the nucleus of an atom. The nucleus of an atom was also discovered by Rutherford in 1911. Rutherford discovered both the nucleus and, later on, the proton, through the Gold Foil Experiment. This comprised a series of experiments in which a beam of alpha particles was shot at a thin foil of metal and the patterns were measured using a fluorescent light. Alpha particles were spotted bouncing off the metal foil in all directions, and some directly back to the source. This should not have happened as all the alpha particles should have all gone through the foil because, according to JJ Thomson’s model of the atom, there are only positively and negatively charged particles in atoms. This proves that the atom’s positive charge was concentrated in a far smaller volume than electrons. In addition, only a small amount of the alpha particles were deflected by more than 90 degrees, meaning that the tiny spheres of intense positive charge were separated by lots of empty space. In this case, the tiny sphere would be the nucleus, the positive charge would be coming from the protons, and the empty space would be the energy shells.


Niel Bohr:

Enhancing Ernest Rutherford’s discovery, Bohr has created a model for the atom. This model shows the atom as a small positively charged nucleus with the electrons orbiting it. He has concluded that electrons surrounds the nucleus in separate orbital and the outer orbit determines the property of the element.  This discovery allowed the structure of the atom to be discovered. (1913)

Bohr Rutherford Diagram [Credits: StamfordNucelarphysics]
With the combination of Bohr’s and Rutherford’s discoveries, the simple but eloquent Bohr-Rutherford diagram was formed.  (Remember, this model is an oversimplification of our current understanding of the model of the atom)


James Chadwick:

In 1932, Chadwick has also been a student of Rutherford, and believed strongly there are neutral particles. After several attempts to discover the neutron, there was no proof that it actually existed. Because of this, he has conducted an experiment where he used polonium, a radioactive element, to discover the neutron. When he shot the polonium as a source of neutons, to the wx. Protons were released and Chadwick did measurements of the protons behaviour. He believed that when the protons were shot, the energy was transferred to the neutron to the proton and was correct when observing the proton’s behaviour. Thus, the neutron was discovered.


                The neutron was discovered by James Chadwick in 1932, while working under Ernest Rutherford. Chadwick learned that the nucleus of an atom could not be comprised solely of protons, because the atomic number of an element did not match its atomic mass. An experiment was conducted in which alpha particles were shot at Beryllium, releasing a kind of energetic stream. It was thought to be radiation, however when this energetic stream came in contact with something containing a lot of protons, such as paraffin, protons were knocked loose. Chadwick came to the conclusion that this energetic stream was made up of neutrons and discovered that they have relatively the same mass as protons, only with a neutral charge.


*All Images from google images

*All the information of the history of the atom comes from AP Chemistry Edition by Brown and the SCH4U class at VillanovaCollege.