Democritus460 BC-created the 1st Atomic Theory of Matter (not
modern atomic theory however)
HIS THEORYMatter is composed of empty space through which
atoms moveAtoms are solid, homogeneous, indestructible, and
indivisibleDifferent kinds of atoms have different sizes and shapesThe differing properties of matter are due to the size,
shape, and movement of atomsChanges in matter result from changes in groupings of
atoms and NOT from changes in the atoms themselves
Father of the Modern Atomic Theory5 parts
Each element is composed of tiny, indestructible particles called atoms
All atoms for a given element have the same mass and properties
Atoms cannot be created, divided into smaller particles, or destroyed
Atoms combine in whole number ratios to form compounds
In a reaction, atoms are separated, combined, and rearranged
John Dalton
J.J. ThomsonHe used a cathode ray tube experiment to
discover electrons. He measured both magnetic and electric fields
on the cathode rays and observed that there was a teeny tiny particle, much smaller than the Hydrogen atom Electron
Plum Pudding ModelNegatively charged electrons are distributed
throughout a uniform positive charge
Robert MillikanDiscovered the charge (-1) of an electron by
doing the oil drop experimenthttp://www.suite101.com/content/millikan-oil-drop-
experiment-a124624
He was interested in studying how alpha particles (+ charge ) interact with atoms
Rutherford thought that, in his experiments, only a few positively charged alpha particles would slightly deflect and the rest would go straight through.
Using the Gold Foil Experiment he discovered that some of those particles deflected at large angles and even bounced straight back at him.
Ernest Rutherford
That means that the plumb pudding model was wrong and there was a small dense center to an atom called a NUCLEUS!
Which means that the electrons floated around in empty space….so he also discovered the orbital theory of electrons, in other words, electrons orbit the nucleus (small charge) in circular paths
Mendeleev’s Periodic Table (Side Note)Mendeleev was the first to put all the known
elements into some sort of arrangement. His periodic table is NOT the periodic table we know today. He arranged the table in order of atomic mass,
NOT number.
Arranged the periodic table according to atomic number NOT atomic mass (Mendeleev)This is our periodic table that we use today
Periodic Law-Elements close to one another have similar properties.
Henry Moseley
Niels BohrBohr’s Model is based on the light emission
spectrum of Hydrogen…He proposed that a hydrogen atom has only
certain allowable energy levels. (energy is quantized)Lowest energy level is ground stateWhen an electron is excited it moves up to an
excited state
He proposed that electrons can only orbit in certain circular paths (like planets)Each path is a different energy level
When an electron falls from an excited state to the ground state, light is emitted….
His model explained Hydrogen’s spectrum, but not any other element
Louis went one step further than Bohr…Theorized that all moving particles have wave
like characteristics, even electrons!
Louis De Broglie
Went further with the wave/particle theoryDiscovered electron clouds (s, p, d, f)Explains that electrons move like waves and
not in a fixed patterns
Erwin Schrödinger
Essentially discovered neutrons (no charge) and how it pairs with protons in a mass called a nucleus
James Chadwick