-
500 BCE
The Alchemists
Through the study of alchemy, Alchemists have succeeded in obtaining real world elements. By Breaking down the chemical composition of the 4 basic elements of its period, Fire, Earth, Wind, And water eventually evolved into The Periodic table we use today. -
430 BCE
Democritus
The theory of Democritus held that everything is composed of "atoms," which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms. -
427 BCE
Plato
Plato introduced the atomic theory in which ideal geometric forms serve as atoms, according to which atoms broke down mathematically into triangles, such that the form elements had the following shape: fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube). -
330 BCE
Solar System Model
Plato first proposed that the planets followed perfect circular orbits around the Earth. Later, Heraclides (330 B.C.) developed the first Solar System model, placing the planets in order from the Earth it was is now called the geocentric solar system model. -
322 BCE
Aristotle
Aristotole. Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
Robert Boyle
Robert Boyle's major contribution to the atomic theory was that he helped develop a definition of an element ( any substance that can be broken into 2 or more substances is not an element) and helped with " the death" of the four elements. Also he helped change the way people think of science. -
Lavoisier
Antoine Lavoisier determined that oxygen was a key substance in combustion, and he gave the element its name. He developed the modern system of naming chemical substances and has been called the “father of modern chemistry” for his emphasis on careful experimentation. He discovered oxygen and was a chemist. -
John Dalton
Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. -
Solid Sphere of “Billiard Ball” Model
The Solid Sphere model is an atomic model proposed by John Dalton in 1803 stating that all objects are made of particles called atoms. Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. Dalton added these so the model atoms could be joined together with hooks and used to model compounds. -
Dmitri Mendeleev
Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table. -
“Plum Pudding” Model
Thomson's model showed an atom that had a positively charged medium, or space, with negatively charged electrons inside the medium. Soon after its proposal, the model was called a "plum pudding" model because the positive medium was like a pudding, with electrons, or plums, inside. -
The Curies
In 1898 French physicists Pierre and Marie Curie discovered the strongly radioactive elements polonium and radium, which occur naturally in uranium minerals. Marie coined the term radioactivity for the spontaneous emission of ionizing, penetrating rays by certain atoms. -
J.J. Thomson
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup." -
Albert Einstein
Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability. Atomic theory says that any liquid is made up of molecules (invisible in 1905). Furthermore, these molecules are always in random, ceaseless motion. -
Robert Millikan
His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. In 1910 Robert Millikan succeeded in precisely determining the magnitude of the electron's charge. -
Ernest Rutherford
Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. Ernest Rutherford postulated the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. He received the Nobel Prize in Chemistry in 1908. -
Niels Bohr
The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element. -
Henry G. J. Mosely
Physicist Henry Moseley discovered the atomic number of each element using x-rays, which led to more accurate organization of the periodic table. We will cover his life and discovery of the relationship between atomic number and x-ray frequency, known as Moseley's Law. -
The Electron Cloud Model
The electron cloud model was developed in 1925 by Erwin Schrödinger and Werner Heisenberg. The model is a way to help visualize the most probable position of electrons in an atom. The electron cloud model is the current accepted model of an atom. -
Werner Heisenberg
Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly.
