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Transcript
The Atom
Mrs. DeMedio
Chemistry Unit 4
Objectives



You will be able to:
Evaluate the factors that
influence the deflection
of a charged particle.
Compare and contrast
the atomic models of
Dalton and Thomson.
Identify the characteristics of
alpha, beta, and gamma radiation.


Analyze how Rutherford’s atomic
model explains the results of his
gold foil experiment.
Part I-
The
Atom
Democritus
Fifth century B.C.
Greek philosopher
proposed that all matter was
composed of indivisible particles
called “atomus” (Greek for
indivisible).
Everyone
thought
he was
CRAZY!!!!
Democritus
is
CRAZY!!!!
Nothing happened
with atoms until…
John Dalton
A. Experiment
1. Dalton experimented with
gases and water evaporating
into the atmosphere.
2. His question was why do
gases not act as solids or
liquids?
B. Results
Gases in the atmosphere were
mixed together and not layered.
1.
2.Water vapor is absorbed into
the atmosphere.
C. Inferences from experiments.
1. Dalton’s Atomic Theory
All matter consists of tiny
particles called atoms.
All atoms of the same element are
chemically identical. Atoms of
different elements are chemically
different.
Chemical reactions occur when
atoms are joined, separated or
rearranged.
Compounds are formed from
atoms of the constituent
elements.
D. Dalton’s Atomic Model
1. Dalton viewed the atom as a
hollow sphere.
Indivisible
Neutral charge
Smallest particle
Dalton’s Atomic Model
Part II:
The
Electron
Thomson
A.Experiment
1. J. J. Thomson wanted to
test Dalton’s model of the
atom.

2. Thomson used an evacuated
Cathode Ray Tube.
CATHODE RAY TUBE
3. An element was stimulated
with electricity.
4. Raytraveled from the
positive to the negative
electrode.
J.J Thomson
5.

He placed a positive magnet on
one side of the ray and a
negative magnet on the other.
6. Ray deflected from the
negative plate to the positive
plate
7. Inferred ray must contain
negative particles.
B. Results
1. Joseph John Thomson (1897)
a. Ray has constant charge to
mass ratio-> all same particle
b. Cathode rays are composed
of identical negatively charged
particles (electrons)
C- Inferences from the
properties of electrons
1. Atoms are neutral, so there
must be positive charges to
balance the negatives
2. Electrons have little mass, so
atoms must contain other
particles that account for most
of the mass.
D.Atomic Model
1. Thomson’s Plum Pudding Model
a. His results led him to see the
atom as a positively charged base
with – particles interspersedmuch like plum pudding.
Thomson’s Plum Pudding Model
Part III:
electron
neutron
proton
The Nucleus
Rutherford
A. Experiment
1. Ernest Rutherford (1911)
a. Alpha particles (helium nuclei)
fired at a thin sheet of gold
Rutherford’s Experiment
B- Results
Very few particles were greatly
deflected back from the gold
What does that
sheet
mean about the
1.
atom?
D-Rutherford’s Atomic Model
1. The atom has a dense center,
called the nucleus, which is
positively charged.
2. The nucleus is surrounded by
mostly empty space with small
electrons floating around in the
space.
C- Inferences
Assumed that the positively
charged particles were bounced
back if they approached a
positively charged atomic nucleus
head-on (Like charges repel one
another)
1.
2. The atom had a nucleus.
a. nucleus is very small, dense
and positively charged
b. most of the atom is empty
space