Download atoms.

Chapter 4
Atoms
4.1
The Development of the
Atomic Theory
Atoms are everywhere!
• Aluminum containers are lightweight
because of the properties of the Al atoms
that make them up.
• Nanotechnology (making products that are
atom-size) is being used to make microsubmarines which will eventually be able to
travel in our bodies to detect health
problems.
U decide…Fact or Fiction
• An atom cannot be broken down into
smaller parts.
• An atom has the same charges throughout.
• Atoms are made up of several different
particles.
Democritus
• Beginning of “The Atomic
Theory” (~2000 years ago)
• Believed all matter consisted of
extremely small particles that
could not be divided. Called
these particles atoms.
• Problem  no evidence / data
to support his theory
Dalton
Dalton’s Atomic Theory
• Like Democritus, proposed atoms could not
be divided.
• All atoms of a particular element are exactly
alike, and atoms of different elements join
to form compounds.
• Substances join together in consistent ways
(Law of Definite Proportions).
– Based his theory on experimental evidence.
• Example: 100 g of Mg combines with 65.8 g of
O. 10 g of Mg combines with 6.58 g of O.
Theories can change…
• Dalton made some significant
contributions to the Atomic Theory, BUT
the theory did change over time as more
experiments were done and additional
data collected.
Thomson’s Model
Thomson’s Model
• First to suggest that atoms were not
indivisible.
– Based on an electricity experiment with a
cathode ray tube.
• Particles in the beam must be negative b/c
they were attracted to the positive plate.
• In a nutshell, Thomson discovered ELECTRONS!
Thomson’s Plum Pudding Model
• Said that negative
charges
(electrons) are
evenly scattered
throughout
atoms.
• To bring this into
2009, call it the
blueberry muffin
model or the
chocolate chip ice
cream model.
Rutherford’s Atomic Theory
• Worked with a student
to find out what
happens to positively
charged alpha particles
when they pass through
a thin sheet of gold foil.
• He believed the
particles would travel
straight from their
source to the screen that
lit up when struck.
What actually happened.
The Results of Rutherford’s
Experiment
• The results of Rutherford’s experiment did
not match with his predictions.
– More particles were deflected than he expected.
Some of the particles even acted as if they
struck an object and bounced straight back.
• SO, Rutherford concluded that the positive charge of
an atom is NOT evenly spread throughout. The
positive charge is concentrated in a small, central
area he called the nucleus.
More about Rutherford…
• So, in essence, Rutherford discovered
PROTONS and the nucleus!
• He also proposed that electrons orbited the
nucleus of an atom like planets orbit the
sun.
– Today we know that the nucleus not only
contains protons, but also neutrons (which have
no charge).
FUN FACT
• IF AN ATOM was the same size as the
Cleveland Browns football stadium,
its nucleus would be the size of a
marble.
– Most of an atom’s volume is the space around
the nucleus, not the nucleus itself.
Review Questions
• 1. Democritus coined what word for a tiny
piece of matter that cannot be divided?
• Answer – Atom
• 2. If 2 g of element X combine with 4 g of
element Y to form compound XY, how many
grams of element Y would combine with 12
grams of X to form the same compound?
• Answer – 24 grams
Review Questions
• 3. In Rutherford’s experiment, what
caused some of the alpha particles to
bounce straight back from the gold foil?
• -Answer – The positively charged nucleus