September 3, 2009

Law of Conservation of Mass (picture on the right)



a) an atom of carbon and an atom of oxygen can combine chemically to form a molecule of carbon monoxide; the mass of the carbon monoxide molecule is equal to the mass of the oxygen atom plus the mass of the carbon atom


b) the reverse holds true as well











Law of definite proportions and Law of multiple proportions (picture on left)
a) carbon monoxide molecules are always composed of one carbon atom and one oxygen atom
b) carbon dioxide molecules are always composed of one carbon atom and two oxygen atoms; note that a molecule of carbon dioxide contains twice as many oxygen atoms as does a molecule of carbon monoxide

Dalton's Atomic Theory

• all matter is composed of extremely small particles called atoms
• atoms of a given element are identical in size, mass, and other properties
• actually atoms of a given element have the same number of protons not mass
• atoms of different elements differ in size, mass, and other properties
• atoms cannot be subdivided (chemically, they can in a nuclear reaction however), 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

Modern Atomical Theory

• atoms are divisible into smaller particles such as protons, neutrons, electrons, etc.
• a given element can have atoms with different masses (isotopes)
• all matter is composed of atoms

Structure of the atom

• atom is the smalles particle fo an element that retains that element's chemical properties
• nucleus is the very small region located at the center of the atoms
• the nucleus is made up of at least one charged particle called a proton and usually of one or more neutral particles called neutrons
• surrounding the nucleus are negatively charged particles called electrons which are found in electron clouds
• all these are called subatomic particles

Discovery of electron

cathode rays and electrons-- experiments in late 1800s shoewd that cathode rays were composed of negatively charged particles called electrons

charge and mass of electron: JJ Thomson's cathode ray tube experiments measured the charge to mass ratio of an electron

Robert A. Milikan's oil drop experiment measured the charge of an electron

later scientists were able to combine the findings of these two experiments to figure out the mass of an electron