Transcript Symbols of Elements

Chapter 3 Atoms and Elements
3.4
The Atom
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Dalton’s Atomic Theory
John Dalton (1766-1844)
In Dalton’s Atomic Theory, atoms
• are tiny particles of matter.
• of an element are similar and
different from other elements.
• of two or more different elements
combine to form compounds.
• are rearranged to form new
combinations in a chemical
reaction.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
2
Subatomic Particles
Atoms contains subatomic particles,
• protons have a positive (+) charge.
• electrons have a negative (-) charge.
• like charges repel and unlike charges
•
attract.
neutrons are neutral.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
3
Rutherford’s Gold-Foil Experiment
In Rutherford’s gold-foil experiment, positively
charged particles
•
were aimed at atoms of gold.
•
mostly went straight through the atoms.
•
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively
charged nucleus in the atom that deflects
positive particles that come close.
4
Rutherford’s Gold-Foil Experiment
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
5
Separation of alpha, beta, and
gamma particles by applying
an electric field.
Ernest Rutherford Gold foil
experiment. A beam of
positively charged alpha
particles hits the gold foil.
Most particles passed
straight, some slightly
deflected and some deflected
back. The reason for
deflection were the positions
of the nucleus.
6
Ernest Rutherford (1871-1937)
Rutherford’s interpretation of the gold foil experiment done by
Geiger and Marsden.
7
Modern View of the Atom
For an atom, which always has no
net electrical charge, the number
of negatively charged electrons
around the nucleus equals the
number of positively charged
protons in the nucleus.
8
Structure of the Atom
An atom consists
• of a nucleus that
contains protons
and neutrons.
• of electrons in a
large empty
space around the
nucleus.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
9
Atomic Mass Scale
On the atomic mass scale for subatomic particles,
• 1 atomic mass unit (amu) has a mass equal to 1/12 of
the mass of the carbon-12 atom.
• a proton has a mass of about 1 (1.007) amu.
• a neutron has a mass of about 1 (1.008) amu.
• an electron has a very small mass, 0.000549 amu.
10
Particles in the Atom
11
Scientists have been able
to obtain computerenhanced images of the
outer surface of atoms
using the scanning
tunneling microscope
(STM) and the atomic
force microscope (AFM).
12
Learning Check
Identify each statement as describing a
1) proton
2) neutron 3) electron
A. found outside the nucleus
B. has a positive charge
C. is neutral
D. found in the nucleus
13
Solution
Identify each statement as describing a
1) proton
2) neutron 3) electron
A. 3
B. 1
C. 2
D. 1, 2
found outside the nucleus
has a positive charge
is neutral
found in the nucleus
14