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492 BCE
Demcritus
Democritus was a Greek philosopher who lived between 470-380 B.C. He developed the concept of the 'atom', Greek for 'indivisible'. Democritus believed that everything in the universe was made up of atoms, which were microscopic and indestructible. -
Antoine Lavoisier
He recognized the elements hydrogen and oxygen and made radical advances in chemistry. He discovered Hydrogen and he clarified that burning is combining objects with oxygen. He led to the theory of the law of conservation of mass. -
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. -
Dalton "Billiard Ball" Model
Because 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 arranged the elements in order of increasing relative atomic mass . When he did this he noted that the chemical properties of the elements and their compounds showed a periodic trend . -
Cathode Ray Tube
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. -
JJ Thomson
J.J. Thomson announced his discovery that atoms were made up of smaller components called the electrons . -
Plum Pudding Atomic Model
Is an atom model brought up by JJ Thomson. As in a plum the "pudding" is the positively charged atoms and the "dotting" was the negatively charged atoms. -
Gold Foil Experiment
The gold-foil experiment (Created by Rutherford) showed that the atom consists of a small, massive, positively charged nucleus with the negatively charged electrons being at a great distance from the center. -
Robert Millikan
Robert Millikan was able to determine the mass of an electron by using charged oil drops. Electrons have such a small mass, they were believed to be massless. Millikan's experiment determined that the electrons did, indeed, have mass. -
Ernest Rutherford
Ernest Rutherford found that the atom is mostly empty space, with nearly all of its mass concentrated in a tiny central nucleus. The nucleus is positively charged and surrounded at a great distance by the negatively charged electrons. -
Rutherford Planetary Model
Rutherford's model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set, predictable paths. -
Henry Moseley
He used self-built equipment to prove that every element's identity is uniquely determined by the number of protons it has. His discovery revealed the true basis of the periodic table and enabled Moseley to predict confidently the existence of four new chemical elements, all of which were found. -
Niels Bohr
Niels Bohr proposed a model of the atom in which the electron was able to occupy only certain orbits around the nucleus. This atomic model was the first to use quantum theory, in that the electrons were limited to specific orbits around the nucleus -
Bohr Model
The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus (think of planets orbiting around the sun) -
Gilbert Lewis
Gilbert Newton Lewis published his seminal paper suggesting that a chemical bond is a pair of electrons shared by two atoms. Lewis was instrumental in developing a bonding theory based on the number of electrons in the outermost “valence” shell of the atom -
Lewis Dot Structure
A Lewis electron dot diagram is a representation of the valence electrons of an atom that uses dots around the symbol of the element. The number of dots equals the number of valence electrons in the atom. -
Erwin Schrodinger
Assuming that matter (e.g., electrons) could be regarded as both particles and waves, in 1926 Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms. -
James Chadwick
Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. With the discovery of the neutron, an adequate model of the atom became available to chemists.