The History and Anatomy of the Atom Unit Four The History and Anatomy of the Atom

History of the Atom Democritus 460-370 B.C. He was a pre-Socratic philosopher known as “the laughing philosopher.” Democritus was extremely wealthy and went around Greece purchasing writings of other philosophers. He argued that matter consisted of tiny particles called atoms-derived from atomos meaning “indivisible.”

History of the Atom Aristotle 384-322 B.C. He was a Greek philosopher under the tutelage of Plato who eventually taught Alexander the Great. Contrary to Democritus, he believed that matter consisted of five elements: Earth, Air, Fire, Water, and Aether.

History of the Atom

History of the Atom John Dalton 1766-1844 He was born into a Quaker family. His father worked as a weaver. Dalton became a teacher at the ripe age of 15 with his older brother Jonathan. In 1793, he began teaching at the New College in Manchester.

History of the Atom Beginning in 1787, John Dalton began keeping a meteorological journal of observations. He continued this practice for 57 years; recording over 200,000 observations. In 1794, Dalton presented his first paper on color-blindness. Dalton, in 1800, began developing his Atomic Theory based on the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

Dalton’s Atomic Theory 1808 All matter is made of atoms. Atoms of different elements have different appearance and properties. Atoms can be rearranged but they cannot be subdivided, created, or destroyed. Atoms can combine in whole-number ratios to form new compounds.

Revision to Dalton’s Theory Dalton’s model of the atom consisted of electrons embedded within a positive fluid. Due to later discoveries, Dalton’s view that atoms could not be divided had to be revised.

The Structure of the Atom: The Electron Sir Joseph John Thomson 1856-1940 In the late 1800’s, Thomson was conducting experiments with vacuum tubes called Cathode Ray Tube (CRT’s). He modified his tube to contain a cathode (-) and an anode (+). Later, he added a paddle wheel and a phosphorus plate.

The Structure of the Atom: The Electron Thomson noticed two things which led to a hypothesis of the electron: The production of shadows on a plate. The spinning the wheel within the CRT.

The Structure of the Atom: The Mass of the Electron Robert Millikan 1868-1953. He was a Nobel Laureate in physics because of his discovery of the mass of an electron. Millikan was the president of Caltech for 24 years.

The Structure of the Atom: The Mass of the Electron In 1909, Millikan conducted an experiment to determine the mass of the electron. He placed oil drops between an electrical field and determined the precise mass of an electron at 1/1837 the mass of a hydrogen atom or simply zero.

The Structure of the Atom: The Charge/Mass of a Proton Ernest Rutherford 1871-1937. He was a New Zealand born physicist who became known as the father of nuclear physics. Proposed the orbital view of the atom.

The Structure of the Atom: The Charge/Mass of the Proton Gold Foil Experiment: Alpha particles were aimed at gold foil. Flashes of light were viewed on a detecting screen at random locations.

The Structure of the Atom: The Charge/Mass of the Proton Since the charge of the alpha particles were positive, Rutherford correctly speculated that the nucleus must contain a positive particle. He referred to this positive particle as a proton and later found the mass to be 1.007276 amu or simply 1.

The Structure of the Atom: The Charge/Mass of the Neutron James Chadwick 1891-1974. An English physicist who won the Nobel prize for discovering the neutron using a similar technique as Rutherford. Later used to split nuclei or fission. The neutron is neutral and has a mass of 1.008665 amu or simply 1.

Nuclear Forces or Strong Forces The force which holds atoms together is called nuclear or strong forces. These forces are extremely powerful!

Summary of Subatomic Particles J.J. Thomson-Electron and negative charge. Millikan-Mass of electron; zero. Rutherford-Proton in the nucleus with a charge of +1 and a mass of 1. Chadwick-Neutron in the nucleus with a neutral charge and a mass of 1.

Models of the Atom John Dalton J.J. Thomson Rutherford Bohr Electron Cloud Billiard Ball Model Plum-Pudding Model + Nucleus with – electrons zipping around Electrons orbit around nucleus Electrons exist in probable locations around the nucleus

Atomic Number Super easy to remember: ATOMIC # = # OF PROTONS * Found on the periodic table just above the elemental symbol.

Mass Number MASS # = # OF PROTONS + # OF NEUTRONS Can be found on the periodic table under the elemental symbol. You must round to the nearest whole number. Ex. Carbon 12.011 is rounded to 12.

Isotopes Isotopes are atoms of the same element that have different numbers of neutrons and thus different mass numbers. Example: Hydrogen has 3 isotopes; protium, deuterium, and tritium. ***Remember: The number of protons never changes! If you change the number of protons, you change the element.

Average Atomic Mass To calculate the average atomic mass, each exact atomic mass is multiplied by its percent abundance. Then, add the results together and round. See example: Nitrogen Mass # Exact Mass % Abund. 14 14.003074 99.63 15 15.000108 0.37

Calculating Avg. At. Mass (14.003074) (0.9963) + (15.000108) (0.0037) = 14.007 Notice that this number is equal to the number on the periodic table!!!