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.