The Development of the Atomic Theory

Democritus Greek philosopher First to use atomos (not being able to divide) Believed indivisible, indestructible, unchangeable Atoms- different shapes and sizes Determined physical properties of material Ex: atoms of liquids were thought to be smooth, which would allow the atoms to slide over each other. held that an atom was indivisible, indestructible, and unchangeable. These Greeks believed that atoms had different sizes and shapes, and the size and shape determined the physical properties of the material. For example, the atoms of a liquid were thought to be smooth, which would allow the atoms to slide over each other

Aristotle Disagreed with Democritus Believed you never end up with a single particle EVER! Four Elements: earth, water, air, fire So who was right?! The giant of Greek scientific and philosophical thought, Aristotle, fiercely argued against atomism. His arguments held sway for around 1,500 years, as he influenced the Roman Catholic theologians who dominated Medieval scient ific thinking, so atomic philosophy fell by the wayside. The seeds of atomism remained, however, contained in the Roman Lucretius’ De Rerum Natura (On the Nature of Things), which was written in the first century, CE. Lucretius was among those who caught scholars’attention after the 15th century CE as Renaissance humanists began to unearth, translate and discuss Greek and Roman texts.

John Dalton Late 1700s it was known: Elements combine in certain proportions based on mass to form compounds. Dalton experiment with different gases to show why. Experiments provided results! John Dalton stood upon Proust’s shoulders to develop the law of multiple proportions. Dalton’s efforts to explain the formation of nitrogen dioxide instead of Virginia Journal of Science Education Volume 1, Number 1 10 dinitrogen trioxide when he doubled the amount of oxygen used in the combination, led to the development of his theory of atoms. Dalton proposed that an element is composed of atoms of a single, unique type, and that although their shape and structure is immutable, atoms of different elements could combine only in small whole- number ratios to form more complex structures, and atoms of different elements had different weights. Dalton’s method for actually determining atomic weight was the distinguishing feature of his work. He was also the first to propose standard symbols for the elements.

John Dalton 1803 Five main points of Dalton's atomic theory: All elements are made of atoms. Atoms of a given element are identical in size and other properties; atoms of different elements differ in all these properties. Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole- number ratios to form chemical compounds. In chemical reactions, atoms are combined separated, or rearranged. What do we know to be wrong with this theory today?

J.J. Thomson 1897 Discovered small particles inside the atom: ELECTRONS! (negatively charged particles) Experimented with a cathode-ray tube. Showed streams of negatively charged particles Model was known as the “plum pudding” model Electrons located throughout an atom.

Ernest Rutherford 1909 Student of Thomson Created an experiment to study the parts of an atom Aimed a beam of positively charged particles at a thin sheet of gold that had a special coating behind it. Coating glowed when positively charged particles hit it.

Ernest Rutherford 1911 Revision of the atomic theory. Atoms mostly dead space, with a tiny part of highly dense matter. Center of the atom contained a positively charged part (proton) and neutrons (neutral charged particles) NUCLEUS (atom’s central region made up of protons and neutrons) 100,000 times smaller than the diameter of the atom Electrons surround the nucleus at a distance.

Niels Bohr 1913 Worked with Rutherford Studied atoms reaction to light Proposed electrons move around nucleus in certain paths, AKA energy levels. Electrons could jump from a path in one level to another level.

Modern Atomic Theory Electrons do not travel in definite paths Can not be predicted! Regions inside the atom where electrons are likely to be found – the electron cloud! Nucleus contains both protons (+) and neutrons (neutral charge)