Early Greek Theories of Matter Greek philosophers first proposed an atomic theory of matter in the 5th century B.C. They believed that all substances were composed of small, indivisible particles called atoms They were thought to be different sizes, have regular shapes and be in constant motion. Empty space was through to exist between atoms. Aristotle severely criticized this theory arguing that atoms in continuous motions in a void is illogical (his opinion might have been colored by the fact that belief in a void at the time was considered to be atheistic)

Aristotle’s Theory Aristotle developed a theory of matter based on the idea that all matter is made up of four basic substances – earth, air, fire and water. He believed that each basic substance had different combinations of four specific qualities – dry, hot, cold and moist. His theory of the structure of matter was the prevailing model for almost 2000 years, including the period of alchemy in the middle ages.

Fire Hot Dry Air Earth Wet Cold Water

Dalton’s Atomic Theory Dalton introduced and atomic theory of matter in 1803, which came to replace Aristotle’s model. His theory states: All matter is composed of tiny, indivisible particles called atoms Atoms of an element have identical properties Atoms of different elements have different properties Atoms of two or more elements can combine in constant ratios to form new substances

Dalton’s Atomic Theory Dalton’s theory was successful in explaining the laws of conservation of mass, definite composition and multiple proportions. Since atoms are indivisible, and are rearranged only when compounds are formed, you must end up with the same number and kinds of atoms after a chemical reaction.

Development of Atomic Theory In the late 1800’s, J.J. Thompson found evidence for the existence of negatively charged particles that could be removed from atoms. In 1987, he postulated the existence of electrons, subatomic particles possessing a negative charge. With the new idea, Thompson developed a model of the atom that has electrons evenly distributed inside the spherical positive part of the atom. In 1904, H. Nagaoka represented the atom as a large, positively charge sphere surrounded by a ring of negative electrons.

Development of Atomic Theory It was Ernest Rutherford who did work (at McGill University) to test the current atomic models. He shot alpha particles through very thin pieces of gold foil. He predicted that all the particles would travel through the foil and although most did, a small percentage were deflected. He deduced that an atom must contain a tiny, positively charge core (the nucleus) which is surrounded yb a mostly empty space containing negative electrons.

Development of Atomic Theory In 1914, Rutherford coined the word “proton” for the smallest unit of positive charge. It was one of his students (H.G.J Moseley) who showed that the positive charge in the nucleus of atoms increases by one unit in progressing from each element to the next in the periodic table. This discovery led Moseley to the concept of atomic number, defined as the number of protons in the nucleus of an atom. Originally the elements were listed and numbered in order of increasing atomic mass, this sequence now had an explanation, the periodic table is a list of elements in order of the number of protons in the nucleus

Development of Atomic Theory The estimated mass of a proton and the number of protons could not account for all the atomic mass. Rutherford predicted the existence of a neutral particle similar in mass to a proton. In 1932 James Chadwick demonstrated that atomic nuclei must contain heavy neutral particles as well as positive particle in order to account for the atom’s mass. These were called neutrons. An atom, therefore is composed of a nucleus containing protons and neutrons, and a number of electrons equal to the number of protons. An atom is electrically neutral

Isotopes Frederick Soddy, a colleague of Rutherford’s at McGill, was the first to propose that the number of neutrons can vary from atom to atom, even in atoms of the same element. An isotope is a form of an element in which the atoms have the same number of protons as all other form of that element, but a different number of neutrons.

Carbon All carbon atoms (atomic number 6) have 6 protons in the nucleus. The most common form of carbon, carbon-12, also has 6 neutrons Carbon-14, however, is an isotope of carbon because it has 6 protons and 8 neutrons in the nucleus. .

Isotopes Different isotopes of the same element have the same chemical properties but different masses. All elements exist naturally as a mixture of isotopes. The mass number is based on the most common isotopes.

Summary of the Rutherford Model An atom contains equal numbers of negatively charged electrons and positively charged protons Most of the mass of the atom and all of its positive charge is contained in a tiny core called the nucleus The nucleus contains protons and neutrons that have approximately the same mass each The number of protons is the same as the atomic number The total number of protons and neutrons is the same as the mass number