1.3 Atomic Theory

(c) McGraw Hill Ryerson 2007 Early ideas about matter Greek philosophers thought matter was made of atomos which would be the smallest pieces of matter. Aristotle believed matter was made of different combinations of earth, air, fire, and water. See pages 28 - 29 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Alchemists tried to turn common metals into gold. (c) McGraw Hill Ryerson 2007

Development of Atomic Theory I John Dalton (1766 - 1844) He studied gases that make up Earth’s atmosphere. He suggested that matter is made of small, hard spheres This is the basis for Dalton’s Atomic Theory. See page 29 (c) McGraw Hill Ryerson 2007

Dalton’s Atomic Theory: All matter is made of small particles called atoms. Atoms cannot be created, destroyed, or divided into smaller particles. All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements. Compounds are created when atoms of different elements link together in definite proportions. See page 30 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Atomic Theory II J. J. Thomson (1856 - 1940) Studied electric currents in gas discharge tubes Determined that the currents were streams of negatively charged particles Hypothesized that atoms are made of smaller particles. Proposed the “raisin bun” model of the atom: - positively charged bun with negatively charged particles See page 30 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Atomic Theory III Ernest Rutherford (1871 - 1937) Found that some particles were deflected in directions not originally predicted. Suggested that the deflection of the charged particles was because the atom had a tiny dense centre called a nucleus, & electrons moved around the nucleus. See page 31 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Atomic Theory IV Niels Bohr (1885 - 1962) Studied gaseous samples of atoms, which were made to glow by passing an electric current through them. Based on his observations, Bohr proposed that electrons surround the nucleus in specific “energy levels” or “shells.” See page 31 - 32 (c) McGraw Hill Ryerson 2007

(c) McGraw Hill Ryerson 2007 Inside the Atom Atom = smallest particle of an element that retains the properties of the element made up of three kinds of particles called subatomic particles: See pages 32 - 33 (c) McGraw Hill Ryerson 2007

LecturePLUS Timberlake Subatomic Particles Particle Symbol Charge Relative Mass Electron e- 1- 0 Proton p+ 1+ 1 Neutron n 0 1 LecturePLUS Timberlake

Atomic number = number of protons Atomic mass = mass of an atom Expressed in amu (atomic mass units) Equal to the number of protons + neutrons In a neutral atom, # of protons = # of electrons

For example: 1. 2. 3.