Transcript Atoms

Atoms
400 b.c. Greeks
• Greeks philosophers ponder the nature of
matter: what is it made of?
• Democritus: basic particle of matter =
“atom” which means “indivisble”.
Envisions these to be “hard spheres”
• Aristotle: does not believe in atoms
Dalton’s Atomic Theory
1803
1) All matter is
composed of
extremely small
particles called atoms
Dalton’s Atomic Theory
1803
2) Atoms of one
element are all alike
(size, mass)
Dalton’s Atomic Theory
1803
3) Atoms can’t be
created, destroyed
or subdivided
Dalton’s Atomic Theory
1803
4) Atoms of different elements combine in
simple whole number ratios to form
compounds
Dalton’s Atomic Theory
1803
5) In a chemical
reaction, atoms are
combined,
separated, or
rearranged
Conservation of mass
in chemical reactions
Conservation of mass
in chemical reactions
1897 J.J. Thompson Cathode
Ray Experiments
The electron is discovered….
“PLUM PUDDING” model
Rutherford’s Gold Foil Experiment
1909
Ernst Rutherford bombards a piece of gold
foil with positively charged alpha particles
(the nucleus of a helium atom)
Some alpha particles
bounce back
Rutherford’s Gold Foil Experiment
Observations:
1. Most of the alpha particles passed
straight through the gold foil as if it
wasn’t even there!
2. The nucleus of the atom is small, dense,
and positively charged
Rutherford’s Gold Foil Experiment
Interpretation (conclusions):
1. Atoms are mostly empty space and
therefore MATTER is mostly empty
space
2. Atoms have a small, dense, positively
charged nucleus
+
+
+
+
+
+
This alpha particle
passes through the atom’s
empty space
Alpha particles have + charge
two + charges will repel each other
Subatomic Particles
(Table O)
Proton
Electron
Neutron
Protons
• Located in the atom’s nucleus
• Positive charge
+1
• Mass = 1 amu (Atomic Mass Unit)
Neutrons
• Located in the atom’s nucleus
• zero charge
(neutral)
• Mass = 1 amu (Atomic Mass Unit)
Electrons
• Located in the electron cloud outside the
nucleus
• Negative charge
-1
• Mass = 1/1836 amu (negligible)
What are the subatomic particles in
lithium and carbon?
Models of the atom
A model of the atom to scale
• If YOU are the nucleus of an atom, your
closest electron would be….
• ……..in downtown Rhinebeck!
Actual image of 2 atoms