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400 BCE
Democritus
Democritus was the first contributor to atomic theory and planted the seed for scientists in the future. He supported the atom's existence but was a philosopher so his theories didn't have any real evidence other than his own thoughts and reasoning. He described atoms as such:
-indestructable
-mechanically bound (i.e. hooks)
-always in motion
-everything is made of atoms -
350 BCE
Aristotle
The philosopher Aristotle did not directly support the atomic theory and instead, proposed his own theory based on his own logic. His theory is now know as geocentric theory and goes something like this: everything is made of four elements only water, earth, fire, and air. Different proportions of this matter makes things what they are. He also thought matter was infinitely divisible. Most of his theory was incorrect. -
Isaac Newton
Isaac Newton was the first to propose a theory about light. He thought considering the geometric nature of reflection and refraction, he thought it could only be explained if light was made of particles because waves usually don't travel in straight lines. -
Christiaan Huygens
Christiaan Huygens believed that light was made up of waves that could be visualized much like those created in water. This was important because this distinguished light as a form of energy rather than matter as it doesn't contain atoms. This was important to rule out because light doesn't exhibit properties that only atoms could produce. -
John Dalton
Dalton had experimented for many years before coming to his own model of the atom. He proposed a theory with spherical solid atoms
composed of different elements. He also reaffirmed atoms cannot be created or destroyed and thus, Daltons theories gave scientists some headway in explaining the evidence about atoms. -
Gustav Kirchhoff
Kirchoff can be attributed the achievement of creating spectrum analysis. He demonstrated that every element gives off a unique colour when heated. This light, when separated by a prism, has a pattern of individual wavelengths specific to each and every element. This created a new research tool for scientists testing atoms of different elements. -
James Maxwell
Maxwell introduced the concept of the electromagnetic field theory to the limelight by a demonstration to show the connection between the waves in light and electromagnets. He proved that an electromagnetic wave was in fact possible. -
Heinrich Hertz
Hertz proved the existence of the electromagnetic waves theorized by James Clerk Maxwell's electromagnetic theory of light. The unit of frequency — cycle per second was promptly named after him. Additionally, his development of the photoelectric effect helped explain the emission of electrons when light was being shined on something. -
J. J. Thomson
J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged and using this information, hypothsized that electrons were arranged randomly in a postive spherical atmosphere. This is what is known now as the plum pudding model which Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. -
Max Planck
Planck made the assumption that energy was made of individual units, or quanta. This lead to the development of quantum theory. Although Planck made many contributions to theoretical physics, but his fame as a physicist rests primarily on his role as the originator of quantum theory, which revolutionized human understanding of atomic and subatomic processes. -
Albert Einstein
Albert Einstein became famous for the theory of relativity, which laid the basis for the release of atomic energy. He helped confirm the type of motion seen in atoms which is now known as Brownian motion. Additionally, his research on electron helped confirm most of atomic theory to be correct. -
Ernest Rutherford
Rutherford's Gold Foil Experiment proved the existence of protons and a small positively charged center to atoms, which would later be known as the nucleus of an atom. Other than the nucleus, he concluded that atoms are mostly empty space. Rutherford made a lot of headwat on the atomic model but was unable to explain explain why negatively charged electrons remain in orbit, when they should instantly fall into the positively charged nucleus. -
Niels Bohr
Bohr proposed a theory known as the planetary model. This model depicts the atom with a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. If the electron gains or loses energy, it can jump these levels. This was important because it explained how electrons can have stable orbits around the nucleus. This solved the problem of Rutherford's model. -
Werner Heisenberg
Heisenberg was the scientist who developed the uncertainty principle. This is expressed like so: the position and the velocity of an electron cannot both be measured exactly, at the same time, even in theory. This was an important step in the quantum theory of the atom. -
Erwin Schrödinger
Schrödinger took the atomic model proposed by Niels Bohr and advanced that one step further. He used mathematical equations to describe the probability of finding an electron in a certain position at any given time. This atomic model is now known as the quantum mechanical model is the most updated version of the atomic model. -
James Chadwick
Chadwick was the one who discovered neutrons in the nucleus. From his findings he determined neutrons were different from alpha particles because they repelled electrical forces in the nucleus. The proof of electrons also helped explain the mass of atoms.
