Scientific Laws and Theories
Law vs. Theory
Law• Everyday language
– Rules
• Science– What happens– Measurable pattern
Theory• Everyday language
– A judgment, guess, or opinion
• Science– Why it happens– An explanation of data
Early Laws
• Law of Conservation of mass: – Mass of reactants = mass of products
– Baking Soda Reacts with Vinegar in a closed container
Trial Baking Soda Vinegar Products & Excess
#1 5.7 g 8.2 g 13.9 g
#2 2.3 g 1.7 g 4.0 g
Early Laws
• Law of Definite Proportions: – The mass ratios of elements in a given compound
are always equal.– Reaction of Mg with O2 gas data:
Trials Trial #1 Trial #2 Trial #3
Mg reacted (g) 12.15 4.86 339
O2 reacted (g) 16 6.4 441
Atomic Theories
• Dalton’s Atomic Theory
• Based on Laws of Cons of Mass, Law of Definite Proportions, and other Laws …
• 1) Each element composed of indivisible particles (atoms)• 2) All atoms of same element identical; of different element
are different.• 3) Atoms can not change, only rearranged into different
combinations (compounds) during chemical reaction.
Atomic Theories
• Thomson’s Atomic Theory (Plum Pudding Model)
• Based on Experiments with a Cathode Ray Tube
• Determined Rays must have charge.• Determined Atom can be broken into charged
particles, but overall charge of atom is neutral.
Atomic Theories
• Rutherford’s Atomic Theory (Nuclear Model)
• Based on Experiments with Gold Foil
• Determined most of the atom is empty space• Tiny Dense nucleus
Atomic Theories
• Bohr’s Atomic Theory (Planetary Model)
• Based on Line Spectra of Elements
• Believed that different electrons rotate around the nucleus in different energy orbits
Atomic Theories
• Modern Atomic Theory (Quantum Model)
• Based on Quantum Mechanics
• Electrons move randomly in “orbitals”
• They do not revolve around the nucleus like planets revolve around the sun.
Atomic Theories
• Isotopes
• Mass Spectrometer
• Not all atoms of the same element are identical.
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