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The Atomic Theory John Dalton English chemist Early 1800’s He revived the theory of the atom as the smallest piece of matter. Was able to explain how chemical substances behave with one another. Five main points of Dalton’s theory: Elements are made of tiny particles called atoms. All atoms of a given element are identical. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights. Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together J.J. Thomson English physicist Believed in the atomic theory Discovered the electron in 1897 Nobel prize in 1906 for physics Developed the muffin or raisin bun model of the atom Ernest Rutherford Student of Thomson Discovered the nucleus in 1911 Later discovered the proton ▪ This was with the alpha scattering experiment using alpha particles and gold foil Nobel prize in 1908 Neils Bohr Danish physicist Nobel prize in 1922 Discovered that electrons travel in orbits around the nucleus (shells) Later worked on the Manhattan Project James Chadwick Discovered the neutron in 1932 Nobel prize for physics in 1935 } Nucleus Shell Atomic Number Refers to the number of protons in an atom Refers to the number of electrons also, if the atom is neutral Each element has its own atomic number Atomic Mass The total number of all neutrons and protons in the nucleus of the atom i.e. Carbon = 12 amu (atomic mass units) Isotopes: Atoms of the same element that have different masses and different numbers of neutrons