2 atomicmodels tvl.pdf

The Historical Development of Atomic Models  Vocabulary Proton Neutron Electron Atom Nucleus Democritus Leucippus The first idea of atoms came from two Greek philosophers, Democritus and Leucippus in the 5th cent...

The Historical Development of Atomic Models  Vocabulary Proton Neutron Electron Atom Nucleus Democritus Leucippus The first idea of atoms came from two Greek philosophers, Democritus and Leucippus in the 5th century BC. The word atom comes from the Greek word “atomos” which means not “cut-able” or divisible. J. Dalton Dalton viewed the atom as a small solid ball that cannot be broken. However, in the late 19th century, scientists began to obtain experimental evidences which showed that atoms are not like solid balls, and they could be broken. J. J. Thomson English physicist Discovered the electron, a negatively-charged particle. In 1903 Thomson proposed a new model of atom. The atom was described as positively-charged sphere embedded with electrons E. Rutherford In 1911, he discovered the proton, a positively- charged particle. He proved that atom has nucleus. He described the new model as tiny, dense, positively-charged electrons. N. Bohr In 1913, Bohr solved this problem by proposing that the electrons could move in shells around the nucleus with particular energies. Bohr’s Atomic Model J. Chadwick In 1932, James Chadwick proved the existence of neutrons, the neutral particles in the nucleus. He discovered that nucleus is surrounded by electrons. Neutrons contribute approximately of the half of the mass of an atom. This model is known as the nuclear atom and includes the three subatomic particles. The atom has a nucleus at its center. The nucleus has protons and neutrons. The nucleus has positive charges because of the protons. A “cloud” of electrons surrounds the nucleus. The electrons move at nearly the speed of light. Most of the atom is empty.