John Dalton
- English Chemist
- Developed an atomic theory which used scientific method
- Elements are made of invisible particles called atoms (not invisible=not true)
- Atoms of the same element are identical. Atoms from 1 element= dif from other element
- Atoms from diff atoms form mixtures/ compounds
- Chem. reactions arrange atoms in dif combos, but atoms can’t change from one atom to another.
Democritus’ Atomic Theory
- Greek Philosopher
- Suggested the existence of atoms
- Believed atoms were indivisible and indestructible
- Theory flawed cuz: didn’t explain chem. behavior
- Not based scientific method
- Not supported by experiment data
How did John Dalton observe atoms?
- Observed when atoms mix, they maintain their own identity unless they combine in a chem. reaction
- Studied ration of these combinations
Evidence from Daltons experiments
-Every drop of mercury has the same chemical properties
J.J. Thomson
- Stated electrons are negatively charged particles
- Proved by filling with gas and put electrodes at each end connected to source of electricity
- Neg electrode: cathode
- Positive electrode: anode
- When electricity passed through tube, stream of light appears (cathode ray)
Explanation of J.J. Thomson’s experiment
- Anode attracts cathode ray
- Cathode repels the cathode ray
- Cathode ray=stream of tiny neg charged particles (fast moving)
- Names particles corpuscles (electrons)
- type of gas in tube does not effect electrons
Robert A Millikan
- Carried out experiment to measure an electrons charge
- Oil-droplet experiment: suspended neg charged oil droplets between two charged plates
- Changed the voltage on the plates to see how this affected the droplet’s fall
Robert A Millikan charge of electron
-Found the charge of each oil droplet to be a multiple of 1.6 x 10^-19 coulomb ( the charge of an electron)
Eugen Goldstein
- Observed the cathode ray tube: found rays traveling in the opposite direction of the cathode rays
- concluded rays were subatomic positive particles (protons)
James Chadwick
-Confirmed the existence of the neutrons
Plum Pudding Model
-JJ Thomson thought electrons were evenly distributed throughout a postitively charged atoms
Plum pudding model vs. Atomic Model
- PP no nucleus
- PP spread through out the atom
- PP Protons have no definite shape and appear as dough in a ball of cookie dough
- PP no neutrons
- PP no empty space
- PP no motion
Rutherford’s Gold Foil experiment
- Shot alpha beams (Helium atom wo/ electrons) through gold foil
- If plum pudding model was correct, beams would pass through (atoms are soft)
- What happened: most passed through, some went in diff direction
- Concluded: Existence of Nucleus, nucleus is dense w/ positive charge, most of atom is empty space
Rutherford’s atomic model
- Protons and neutrons located in positive charged center(nucleus)
- electron orbits the nucleus
- Nucleus as small as marble in football stadium
Atomic number
- Number of protons in element
Mass Number
- The number of protons and neutrons in an element
How do you find the number of neutrons in an elements
- (Mass number - Atomic Number)= number of neutrons
What are the two ways to show an elements mass number?
- Shorthand: atomic number is subscript, mass number super script, left of element
- Element with dash then mass number ( gold-197 or Au-197)
Isotopes
- Atoms that have the same number of protons but different number of neutrons
- Diff number of neutrons= different mass number
- Isotopes of carbon (black gunk and diamond)
What is used to find the mass of an atom?
- spectrometer
- Even the largest atoms are incredibly small
Atomic Mass Unit
- Defined as one-twelfth of the mass of a carbon-12 atom
- Neutrons and protons= 1 amu
arithmetic mean
- the average of 2 or more pieces of data
Atomic mass
- a weighted average mass of the atoms in a naturally occurring sample of the element
- Below element symbol on a periodic table
Nucleus and Electron Mass and volume
- Nucleus takes up small volume, 99% of mass
- Electrons take up a lot of volume, very little mass
