The Development of Atomic Theory

Early Philosophers Enter...the ATOM Heraclitus: maintained that everything is in a state of flux. (everything changes) Parminedes: everything is what it is (only permanent things are considered “real”) Enter...the ATOM Democritus: around the 5th Century B.C. proposed that matter is composed of tiny, indestructable units called “atoms.” Atoms themselves do not change Move around and combine with other atoms Properties of an atom are determined by the shape of the atom

Popular Does Not Equal Correct! Because Aristotle completely dismissed the idea of atoms, many philosophers and scientists went along with his idea. The study of atoms was reintroduced in Europe during the Renaissance If philosophers like Democritus had not recorded their ideas in some understandable matter, the idea of atoms may not have been “rediscovered” Did You Know? The word atom comes from the Greek word “atomos” meaning invisible.

John Dalton Early 1800’s Each element has it’s own type of atom The masses of different types of atoms are different Started a system of chemical symbols Arranged them on a chart Theorized that elements combine in definite ratios according to their weight Compounds are made of molecules Atoms determine the composition of matter

J.J. Thomson's Theory ELECTRONS are negatively charged particles that make up an atom Since atoms are neutral and negative charges exist, there must be some positive charge in an atom as well 1904 – Thomson proposed a structure of an atom that consisted of a positive sphere with negative charges distributed throughout the charge This model became known as the “plum-pudding model” Chocolate chip cookie dough anyone?!

Ernest Rutherford Niels Bohr In 1911 he described a small, heavy nucleus with electrons in orbit around the nucleus 1919 – discovered a proton and determined that it exists in the nucleus of an atom Protons are positively charged Niels Bohr 1913 – determined that electrons move in orbital shells The electrons in the outer orbital shells determine the properties and interactions of the atom

Modern Atomic Theory As a result of the work of the scientists listed here, as well as hundreds of others, we now use the “Electron Cloud” model of the atom. An atom consists of: A nucleus Protons and neutrons Positively charged An electron cloud Negatively charged