ATOMIC THEORY PROPOSALS Bayana Crane 6th 1/12/14 DEMOCRITUS • Democritus’s model stated that matter consists of invisible particles called atoms and a void (empty space). He stated that atoms are indestructible and unchangeable. Also that they are homogenous, meaning they have no internal structure. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them. 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’s atomic theory also stated that all compounds were composed of combinations of these atoms in defined ratios. BOHR • In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. • The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump. THOMSON • Thomson's discovery of the electron completely changed the way people viewed atoms. Up until the end of the 19th century, atoms were thought to be tiny solid spheres. In 1903, Thomson proposed a model of the atom consisting of positive and negative charges, present in equal amounts so that an atom would be electrically neutral. He proposed the atom was a sphere, but the positive and negative charges were embedded within it. • Thomson's model came to be called the "plum pudding model" or "chocolate chip cookie model". Modern scientists understand atoms consist of a nucleus of positively-charged protons and neutral neutrons, with negatively-charged electrons orbiting the nucleus. Yet, Thomson's model is important because it introduced the notion that an atom consisted of charged particles. RUTHERFORD • Rutherford atomic model, also called nuclear atom or planetary model of the atom, description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford. The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun ELECTRON CLOUD • The electron cloud model says that we can not know exactly where an electron is at any given time, but the electrons are more likely to be in specific areas. It is theoretically possible, for an electron to be a nearly infinite distance away from the atomic nucleus it is orbiting, although the probability of an electron decreases dramatically the further away from the nucleus you search. This is the most modern and accepted way to describe the situation.