Atomic Theory

Foundations of the Atomic Theory Foundations of the Atomic Theory Original idea Ancient Greece (400 B.C.) Democritus and Leucippus- Greek philosophers, not scientists. Democritus named the “atom’; called it nature’s basic particle.

Atomic Theory Aristotle did not agree with Democritus; he didn’t believe in ‘atoms’; matter was continuous. All matter made of 4 elements Fire - Hot Air - light Earth - cool, heavy Water - wet

Atomic Theory Late 1700’s - John Dalton- England. Teacher- summarized results of his experiments and those of others. Elements substances that can’t be broken down Dalton’s Atomic Theory Combined idea of elements with that of atoms.

Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of different atoms are different. Atoms of different elements combine in whole number ratios to form compounds. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Atomic Theory Dalton’s Atomic Theory lead to discovery of some basic laws: Law of conservation of mass – mass is neither created nor destroyed in ordinary chemical reactions. Law of definite proportions - each compound has a specific ratio of elements by mass.

Atomic Theory J. J. Thomson - English physicist Made a piece of equipment called a cathode ray tube. (see pages 70 – 71) Found the electron. Said model of the atom was called the plum pudding model.

Atomic Theory 1909 – Robert A. Millikan, an American physicist; 1909 – Robert A. Millikan, an American physicist; Oil Drop Experiment Oil Drop Experiment Experiment confirmed the tiny mass of the electron and its negative electric charge

Atomic Theory 1910 – Ernest Rutherford English physicist. Believed the plum pudding model of the atom was correct. Wanted to see how big atoms are.

Atomic Theory Used radioactivity. 1911; Shot Alpha particles (positively charged pieces given off by uranium) at gold foil Gold Foil Experiment conducted with Geiger and Marsden. Gold Foil Experiment conducted with Geiger and Marsden.

Atomic Theory Geiger –1945; ; best known as the co-inventor of the Geiger counter and for the Hans Geiger- (Ernest) Marsden experiment (called the gold foil experiment) which discovered the atomic nucleusGeiger counterGeiger- (Ernest) Marsden experiment atomic nucleus

Atomic Theory Niels Bohr – 1962; 1913; best known for the investigations of atomic structure and also for work on radiation

Atomic Theory Henry Moseley – 1887 – 1915; 1914; developed the application of X- ray spectra to study atomic structure; Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbersPeriodic Table

Atomic Theory Albert Einstein – 1955; 1915; Known for theory of relativity; photon theory, and law of photoelectric effect

Atomic Theory Chadwich – 1974; In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical charge.

Modern View Discovered that the nucleus was small compared to the total size of the atom. (see p. 73) atom is mostly empty space Two regions. Nucleus- protons and neutrons. Electron cloud region where you might find an electron.

Atomic Structure Proton - positively charged, heavier than the electron. Neutron - no charge but the same mass as a proton. Electron – negatively charged particle

Atomic Structure Mass Number Mass Number protons + neutrons protons + neutrons always a whole number always a whole number NOT on the Periodic Table! NOT on the Periodic Table! © Addison-Wesley Publishing Company, Inc.

Atomic Structure Isotopes Atoms of the same element with different mass numbers  Nuclear/Isotopic symbol: Mass # Atomic # Hyphen notation: carbon-12

Atomic Structure Isotopes © Addison-Wesley Publishing Company, Inc.

Atomic Structure Chlorine-37 atomic #: 17 mass #: 37 # of protons: 17 # of electrons: 17 # of neutrons: 20 Isotopes

Atomic Structure 12 C atom = × g 12 C atom = × g atomic mass unit (amu) atomic mass unit (amu) 1 amu = 1 / 12 the mass of a 12 C atom 1 amu = 1 / 12 the mass of a 12 C atom 1 p= amu 1 n= amu 1 e - = amu 1 p= amu 1 n= amu 1 e - = amu Relative Atomic Mass © Addison-Wesley Publishing Company, Inc.

Atomic Structure Average Atomic Mass weighted average of all naturally occurring isotopes weighted average of all naturally occurring isotopes Decimal number on the Periodic Table Decimal number on the Periodic Table Round to 2 decimal places Round to 2 decimal places Avg. Atomic Mass

Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. Avg. Atomic Mass amu

Atomic Structure EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Avg. Atomic Mass amu

Atomic Structure Calculate the atomic mass of copper if copper has two isotopes % has a mass of amu and the rest has a mass of amu.