-
400 BCE
Democritus
In 400 BC, Democritus proposed the first atomic theory. He believed that all matter consists of atoms, atoms are indestructible, they're solid but invisible, and they differ in size, shape, mass, position, and arrangement. Democritus also made his own atomic model. This was the first atomic model ever made and it was merely a sphere with no sign of electrons, protons, or neutrons. -
340 BCE
Aristotle
Aristotle, a greek philosopher, did not believe in the atomic theory that was suggested by Democritus 60 years earlier. Instead, he believed that all matter was made up of one or a combination of the four elements; air, earth, water, and fire. Although his beliefs were wrong, they caused Democritus' theory to be overlooked for about 2000 years. -
Antoine Lavoisier
Lavoisier was acknowledged for his experimental skills. One of his best experiments was turning HgO into Hg+O. He used this experiment to create the Law of Conservation of mass. This law states that matter cannot be created or destroyed. His discovery actually symbolizes the beginning of modern chemistry. -
John Dalton
Dalton created an atomic theory in 1803 that became the basis of theoretical foundation in chemistry. Almost 2000 years after Democritus proposed his atomic theory, Dalton went back on the beliefs he suggested. Dalton stated that all matter is made up of atoms which are invisible and indestructible, all atoms of a specific element have the same mass and properties, compounds are formed from a combination of two or more differing atoms, and that a chemical reaction is a rearrangement of atoms. -
Henri Becquerel
In 1896, Henri Becquerel conducted an experiment that led to the discovery of radioactivity. For his experiment, he brought a crystal comprised with uranium into sunlight. Once the crystal had absorbed some sunlight, Becquerel laid it on a photographic plate. The picture that the crystal imprinted on the plate led Becquerel to believe that the uranium was letting off the energy of the sunlight that had been soaked up in the form of an x-ray. -
J.J. Thomson
In 1897, J.J. Thomson discovered the electron when working with a cathode ray tube. He performed multiple experiments using cathode rays produced from a cathode ray tube in attempt to prove that they were really a string of negatively charged particles. During his experiments. Thomson also went on to create his own atomic model, which he compared to plum pudding. He stated that the raisins in plum pudding were the electrons in an atom, and the dough included the positively charged particles. -
Marie and Pierre Curie
Using Becquerel's discovery of radioactivity as her thesis, Marie worked with other elements to see if they also let of radioactive energy. She found that radioactivity doesn't depend on how atoms are arranged into molecules, but that it generates within the atoms themselves. Together the Curie's discovered -after working at it for four years- two elements, radium and polonium. Polonium was later used by the Joliot-Curie's and James Chadwick during the discovery of the neutron. -
Max Planck
To explain the hues of hot glowing matter, Planck suggested that energy is radiated in extremely small, and sneaky quantized amounts, instead of in a constant stream. Planck named these little pockets of radiated energy, quanta. His quantum theory was a huge contribution to the discovery of the atom, and was used to make other findings by many scientists such as Albert Einstein and Niels Bohr. -
Robert Millikan
Millikan used J.J. Thomson's hypothesis that an electron's mass is at least 1,000 times less than the tiniest atoms', to stimulate his oil-drop experiment. During which, he sprayed oil onto two different plates, one with a negative electric charge, and the other with a positive charge. Some of the droplets, from the sprayed oil, traveled through a pinhole into a space between the two plates. -
Robert Millikan continued
The particles of oil that failed to catch electrons from the x-rays ionized into the center pinhole, were pulled by gravity to the bottom plate. Particles that caught at least one electron floated up to the positive plate or fell to the bottom plate at a much slower pace. From his experiment, Millikan concluded that each drop had a charge that was a multiple of 1.59 x 10^(-19) Coulombs. -
Ernest Rutherford
In 1898, Rutherford discovered two radioactive elements which he called alpha and beta rays. But in 1911, Rutherford made a larger contribution to the atom. He thought that if Thomson's plum pudding model of the atom was correct, then the mass of the atom should be spread all throughout the atom. For his experiment, he shot helium nuclei at a thin gold foil of atoms. -
Rutherford continued
As a result, most of the alpha particles (contained in helium nuclei) went right through the foil but a few particles bounced right back off the foil. Since this wasn't accurate with Thomson's model, Rutherford made his own, which all the protons were in the nucleus and the electrons circled around the nucleus. -
Henry Moseley
In 1910, Henri Moseley began working with Ernest Rutherford at Manchester university. While working with Rutherford, Moseley experimented with radioactive chemical elements. Using these elements Moseley made the first atomic battery, a beta cell, which he called a radium battery. After working with Rutherford and his team to create an atomic battery, Moseley decided to go off and work on his own. -
Henry Moseley continued
He created his own experimental tools, which shot high-energy electrons at different chemical elements and measured the wavelengths and frequencies of the established x-rays. In his experiment, Moseley came to the conclusion that each element lets off differing amount of x-rays at their own special frequencies. He also discovered that he could plot a straight line using the square root of the x-rays and the atomic number. This went on to explain that each elements atomic number is the protons. -
Niels Bohr
After working with both J.J. Thomson and Ernest Rutherford in 1912, Bohr came out with his own atomic model, which combined Rutherford's description of the nucleus and Max Planck's quantum theory. His model became known as the Bohr model, and it showed the atom as a small, positively charged nucleus, encompassed by orbiting electrons. Bohr was the first person to discover that electrons travel in separate orbits around nucleus, this determines the charge of the element. -
Werner Heisenberg
Heisenberg made many contributions to the discovery of atoms and science in general, but his biggest contribution came in 1925 when he created the first version of quantum mechanics. He did this by calculating the behavior of electrons and subatomic particles that make up an atom. This helped clarify the modern view of atoms because scientists could now compare the few number of atoms by their movements of electrons and number of electrons. -
Erwin Schrödinger
Schrödinger furthered the Bohr model when he used mathematical equations to explain the probability of finding an electron in a specific position. His model became known as the quantum mechanical model, and instead of emphasizing the exact line of an electron, it anticipates the odds of the location of an electron. He did this by using an electron cloud, where the cloud was most dense, that's where more electrons would be found, and vise versa. This introduced the idea of sub-energy levels. -
James Chadwick continued
The Joliot-Curies thought they had ejected protons from a sample of wax using gamma rays. Even though the evidence that protons had been hit with ample energy to remove them was there, Chadwick didn't believe that gamma rays had enough power to do this. Instead he thought that the protons had been hit by the neutrally charged particle he had been searching for. So he began working in the Cavendish laboratory with the element polonium (element where gamma rays came from) as neutrons. -
James Chadwick continued
He bombarded wax and found that protons were indeed let off from this process, by the wax. Chadwick made notes of the protons' behaviors, and discovered that the protons behaved the same way they would if they had been hit by neutrally charged particles with a mass close to that of their own. From this experiment, Chadwick had found the neutron. -
James Chadwick
Chadwick worked with/under Rutherford for many years at Cambridge University's Cavendish Laboratory(1910-20s). They both believed that particles with no charge could possibly exists in the nucleus. So Chadwick began experimenting with this idea for about a decade without any success. It wasn't until Chadwick heard of an experiment done by the Joliot-Curies in 1932 that he started to really make some discoveries.
