Atomic Theory and the Atom September 30, 2014

Early Ideas About Matter – Greek Philosophers Democritus (460 -370 BC) Matter is composed of atoms, which move through empty space Atoms are solid, homogeneous, indestructible, and indivisible Different kinds of atoms have different sizes and shapes Size, shape, and movement of atoms determine the properties of matter Aristotle (384 -322 BC) Empty space can not exist Matter is made of earth, fire, air, and water Ideas were widely accepted and were not challenged for two thousand years

John Dalton – English (1766 – 1844) Marks the beginning of the development of modern atomic theory Dalton and Democritus are similar Dalton’s research is known as Dalton’s Atomic Theory which was proposed in 1803.

Dalton’s Atomic Theory Matter is composed of extremely small particles called atoms. Atoms are indivisible and indestructible. Atoms of a given element are identical in size, mass, and chemical properties. Atoms of a specific element are different from those of another element. Different atoms combine in simple whole-number ratios to form compounds. In a chemical reaction, atoms are separated, combined, or rearranged.

Defining The Atom Atom – the smallest particle of an element that retains the properties of the element Atoms are made up of Electrons – negatively charged particles that are part of all forms of matter Protons – subatomic particle carrying a positive charge Neutrons – subatomic particle that has a mass nearly equal to that of a proton, but carries no electric charge

Cathode Ray Tube Sir William Crookes Cathode Rays were a stream of charged particles The particles carried a negative charge JJ Thomson (English Physicist 1856 – 1940) Cathode Ray Tube experiments late 1890s Charge-to-mass ratio Identified the first subatomic particle Received the Noble Peace Prize in 1906

Oil Drop Experiment – late 1910s Robert Millikan (American Physicist 1868-1953) Charge of an electron 1.602 x 10-19 coulombs Which we say is a single unit of negative charge or 1- Mass of an electron Using the mass-to-charge ratio Millikan calculated the mass of an electron to be 9.1 x 10-28 g or 1/1840 the mass of a hydrogen atom

Plum Pudding Model JJ Thomson proposed the model Spherically shaped atom Composed of a uniformly distributed positive charge Where individual negatively charged electrons resided

Ernest Rutherford (1871 – 1937) 1911 – began to study how positively charged alpha particles interacted with solid matter Gold Foil Experiment Rutherford expected the alpha particles to pass through the gold atoms. He expected a few to be slightly deflected. The results observed showed that some alpha particles were deflected at large angles Rutherford concluded that the plum pudding model was incorrect because it could not explain his results.

Rutherford’s Model Rutherford concluded that almost all the atom’s positive charge and almost all of its mass were contained in a tiny, dense region in the center of the atom, which he called the nucleus. By 1920 Rutherford had refined the concept of the nucleus and concluded that it contained protons A proton has a charge of 1+ A proton has a relative mass of 1

James Chadwick - English Physicist (1891 – 1974) 1932 – Chadwick (coworker of Rutherford) showed that the nucleus also contained another subatomic particle called a neutron Neutrons – neutral, and with a relative mass of 1 Chadwick received a Nobel prize in 1935