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Democritus was a Greek philosopher (460-370 BCE), who significantly contributed to atomic theory. He had over 73 works. Democritus theorized that all matter came from smaller, invisible building blocks. About 430 BCE Democritus named these building blocks atomos which means invisible. He believed atoms were uniform, solid, hard, incompresible, and indestructible. Democritus' views on atoms now has evolved into modern scientific theory as other scientist improve his theory. -
John Dalton was born in Eaglesfield, England, on September 5th or 6th, 1766. He was a chemist and meteorologist who made pioneering contributions to modern atomic theory. Dalton's theorized that all matter is made up of matter. He also believed atoms could not be destroyed, divided, or created. He was the first to believe all atoms of the same element were the same. But they are different from atoms of all other elements. Dalton's atomic theory earned him the title of "father of chemistry". -
In 1903, John Dalton created the "Solid Sphere" (also called the "Billiard Ball") model, proposing that all matter is made of atoms. This model showed that atoms are featureless -
Dmitri Mendeleev was born in Tobolsk, Russia, on February 8th, 1834. He was a chemist who created the periodic table. In 1869, Mendeleev created the periodic table. This allowed elements to be ordered based on their atomic weight. He left gaps in the periodic table for undiscovered elements. He used his table to predict their properties. Dmitri Mendeleev's work provided a connection between understanding the relationships between elements and predicting their behavior. -
J.J. Thomson was born near Manchester, England, on December 18, 1856. He was a physicist who discovered the electron. In 1897, J.J. Thomson led an experiment by using cathode rays. Thomson found particles that were smaller than the atom that had a negative charge. He later named them electrons. Thomson's discovery proved that atoms were not indivisible, homogeneous particles. He also created a new model of the atom. -
In 1904, after J.J. Thomson's discovery of the electron, the Plum Pudding Model was created. It showed that the atom was one big positively charged sphere while electrons were mixed through it. -
Robert Millikan was born on March 22, 1868, in Morrison, Illinois. He was a physicist who won the Nobel Prize for Physics in 1923. In 1909, he started his oil-drop experiment. He could use oil and a microscope to measure the electric charge of atoms. His experiment proved the fact that atoms were electrically charged. -
Ernest Ruthford was born on August 30th, 1871, in Spring Grove, New Zealand. He was a British physicist considered to be the greatest experimentalist of his time. In 1909, he initiated an experiment to examine each part of the atom. His experiment showed that atoms have a small, dense center, with a positive charge called a nucleus. He theorized that this nucleus was surrounded by moving electrons. He later found out that the nucleus was made up of positively charged particles called protons. -
The Solar System Model, also known as the Rutheford model, was proposed in 1911. It showed a nucleus in the middle surrounded by moving electrons. -
Niels Bohr was born on October 7th, 1885, in Copenhagen, Denmark. He was a physicist who received the Nobel Prize for physics in 1922. Niels Bohr theorized that electrons orbit the nucleus in a circular path at a certain distance from the positively charged nucleus. Bohr's model was revolutionary as it blended classical and quantum concepts. -
Erwin Schrödinger was born on August 12th, 1887, in Vienna, Austria. He was a physicist who shared the 1933 Nobel Prize for physics. Erwin Schrödinger theorized that electrons were more like standing waves rather than tiny balls orbiting a nucleus. He created an equation to allow us to find where the electron waves are most likely to be. His new theory and equation shifted the perspective of atoms. -
In 1926, Erwin Schrodinger created the electron cloud model to show high probability where electrons would be found. -
Werner Heisenberg was born on December 5th, 1901, in Wurzburg, Germany. He was a physicist and philosopher who won a Nobel Prize for physics in 1932. Werner Heisenberg created the uncertainty principle. This principle states that it is impossible to know both the exact location and the exact speed at the same time. He also came up with a new way to explain atoms through quantum mechanics using math called matrices. His discoveries changed the understanding of atoms. -
James Chadwick was born on October 20th, 1891, in Manchester, England. He was a physicist who received the Nobel Prize for Physics in 1935. In 1932, James Chadwick performed a beryllium bombardment experiment and discovered the neutron. Due to his discovery, he created a new model of the nucleus, which now included the neutron. This was a pivotal contribution to atomic theory as it revealed another part of atoms.