Download 4.1 Studying Atoms

4.1 Studying Atoms
Ancient Greek Models
Democritus (2500 yrs ago) he said everything is made of small particles
He called them atoms from atomos “uncut” or “indivisible” (undevideable)
He thought there were different types of atoms with different properties
Aristotle thought there was no limit to the number of times atoms could be divided.
Though wrong, this was the thinking for a long time.
Dalton’s Atomic Theory 1800’s
Worked with gases in the air
Noted that compounds have fixed composition
All matter is made of small particles called atoms that cannot be divided
1. all elements made of atoms
2. all elements of same element have the same mass
3. compounds contain atoms of more than one element
4. atoms of different elements combine in same way in a particular compound
Dalton DEPICTED ATOMS AS SOLID SPHERES
Later, his theory was revised as new evidence was found.
Thomson’s Model 1856-1940
Used electric current to study atoms (Review transparencies of experiment)
Stated the charged particles from his experiment came from within atoms
This was the first evidence that atoms were made of smaller particles.
Model stated atoms are neutral, but both positive and negative charges are present.
Positively charged mass and negative charges flying around
Charges and mass evenly spread out (like chocolate chip ice cream).
Rutherford’s Atomic Theory 1899
Discovered “alpha particles” - fast moving particles with positive charge.
His “Gold Foil experiment” found the path of an alpha particle passing through a piece of
gold foil
Most went through foil undeflected, or deflected only a tiny bit
A few were deflected a lot
About one in 20,000 was deflected more than 900
A very tiny number bounced straight back like they had struck something
He thought that since the alpha particles were positively charged, they must be being deflected by
a positive charge within the atom. He theorized that:
All atoms have positive charge concentrated in nucleus