Somewhere in 460-370 BCE

According to Democritus, atoms are indivisible and indestructible particles that make up all matter. He thought that these atoms were different in size and form and that they moved constantly in the empty space. His conclusion was not supported by experimental data, but rather by philosophical reasoning.

Pros:

*Laid the foundation for the concept of atoms.

*Introduced the idea of matter being composed of indivisible particles

Cons:

*Democritus' atomic model lacked evidence to support its claims.

*Democritus' model was overly simplistic in that it proposed only the existence of atoms

1808.

John Dalton is known for his discovery of his atomic model. He discovered that all matter is composed of particles called atoms. All atoms of the same element are identical in mass and properties. Atoms are not created, destroyed, or converted into other kinds of atoms during chemical reactions, they are rearranged into new compounds. Compounds are formed by combining two or more atoms.

Pros:

*Atoms are not created, destroyed, or converted into other kinds of atoms

Cons :

*Dalton's model gave the idea that all atoms of an element are identical in mass and properties.

*Dalton's model did not include subatomic particles like protons, neutrons, and electrons

1897

J.J. Thomson is best known for his experiments with cathode rays, which led to the discovery of the electron. In 1897, he conducted experiments using a cathode ray tube, a vacuum tube with electrodes at each end, and filled with a low-pressure gas. This helped him establish that the cathode ray particles were significantly lighter and smaller than atoms. As a result, he proposed the existence of a subatomic particle that is now known as the electron but which he called the "corpuscle".

Pros:

• It successfully explained the electron neutrality of an atom.

• It is a very simple visual model of an atom.

• It was the first attempt to identify the structure of an atom.

Cons:

• Failed to explain how the positive charge holds on the electrons inside the atom.

• The theory did not mention anything about the nucleus of an atom.

• It was unable to explain the scattering experiment of Rutherford.

1911

To set up his experiment, Rutherford placed a uranium source inside a lead block that had an opening in it, causing the particles to escape in just one direction, and then used the lead block with the uranium source to point it at a thin piece of gold foil that he had pounded. Ernest Rutherford observed that while most alpha particles(protons) passed through the gold foil with little to no deflection, a small fraction of them bounced back in the opposite direction. This led Rutherford to conclude that the atom was mostly empty space with a small, dense, positively charged nucleus at its center.

Pros:

*Introduced a small, dense, positively charged nucleus at the center of an atom

*Allowed scientists to predict and understand the behavior of atomic particles

Cons:

*Failed to describe energy levels.

*It didn't explain how the arrangement of electrons in these shells determines an element's chemical behavior.

1913

Bohr studied Rutherford's nuclear model and felt it needed further editing. Bohr studied why the negatively charged electrons did not just crash into the positively charged nucleus, one of the nuclear model's unexplained problems. Bohr experimented with the atomic emission spectrum of hydrogen and concluded that when electricity is applied to electrons, they leap to a high energy level and release the energy as light when they return to their original level. He came to the conclusion that electrons must orbit the nucleus of the atom at distinct levels, which would explain variations in the color of light emitted since the electrons are in various orbital combinations for different elements.

Pros:

• He led the way for other scientists to carry on his work, eventually, the electron orbital problem was solved

• It can explain the stability of the atom

  
  

Cons:

• When attempting to model atoms with more than one electron, he failed.

• His objective of explaining the colorful emission spectral lines and why electrons were not drawn into the nucleus was fundamentally correct, his prediction of the circular orbital shape was wrong.

1932

James Chadwick is best known for his experiments that led to the discovery of the neutron. Chadwick was convinced that alpha particles did not have enough energy to produce such powerful gamma rays. He performed the beryllium bombardment experiments himself and interpreted radiation as being composed of particles of mass approximately equal to that of the proton but without electrical charge, neutrons.

Pros:

• Significant impact on nuclear science.

• Set the stage for later advancements such as our knowledge of nuclear processes.

• The creation of nuclear technologies.

Cons:

• Failed to explain why individual atoms produce discrete line spectra.

• Discovered that the total mass of an atom was greater than the sum of the protons and electrons.

1926

Erwin Schrödinger's created the quantum mechanical model which describes the behavior of atoms. It provides a mathematical description of how the function of atoms and molecules changes over time. Erwin Schrödinger was able to formulate an equation to accurately calculate the position of electrons in atoms by combining equations for the behavior of waves and the de Broglie equation.

Pros:

• Accurately describes the behavior of quantum systems, which is important for understanding the behavior of particles at the atomic levels.

Cons:

• Schrödinger's equation can be complex and difficult to solve

“The Nobel Prize in Physics 1935.” NobelPrize.Org, www.nobelprize.org/prizes/physics/1935/chadwick/biographical/. Accessed 30 Oct. 2023.

“James Chadwick.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 16 Oct. 2023, www.britannica.com/biography/James-Chadwick.