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The Atom and Atomic Structure
1. Historical Ideas
2. The Structure of the Atom
3. The Size of Atoms
4. Periodic Table
©2013 Robert Chuckrow
1. Historical Ideas
Ancient-Greek philosophers reasoned that if
an element such as gold were cut in half
repeatedly, at a certain point, something
indivisible would be reached. In 1805, John
Dalton called these particles atoms, from the
Greek word atmos, meaning indivisible.
2. The Structure of the Atom
Scientists now accept as a fact that ordinary matter
consists of atoms. We know that atoms consist of
protons, neutrons, and electrons plus other
subatomic, elementary particles (positrons and
mesons), which will not be discussed here. Protons
are positively charged, electrons are negatively
charged, and neutrons are neutral. The mass of a
proton is approximately equal to that of a neutron and
about 1,845 times that of an electron.
Rutherford’s Experiment in 1911
Before about 100 years ago, it was not known how
protons, electron and neutrons are arranged within
the atom. Rutherford’s experiment (next slide)
showed that atoms contain very small, heavy atomic
nuclei. He used a radioactive source that emitted
alpha particles (helium nucleii consisting of two
neutrons and two protons). He then measured how
much the alpha particles were deflected after passing
through a very thin gold foil.
Rutherford’s Apparatus
From Linus Pauling, College Chemistry, W.H. Freeman and Co., San
Fancisco, CA, 1954, p. 36.
Conclusions Drawn from Rutherford’s
Experiment
The fact that most of the alpha particles went straight
through the foil without being deflected showed that
the atoms consisted mostly of empty space. That a
small percentage of alpha particles were deflected
through large angles showed that the nuclei were
very dense. In fact, the atomic nuclei occupy only one
million-millionth of that of the whole atom. If an atom
were the size of a bushel basket, the nucleus would
be only about a thousandth of an inch in diameter.
Review Questions
1.
2.
3.
What is an alpha particle?
Why did Rutherford use gold foil?
If the thickness of the foil were doubled, what effect
would that have on the results?
Answers
1.
2.
2.
An alpha particle consists of two neutrons and two
protons and is a helium nucleus.
Gold foil can be rolled into extremely thin sheets.
The thinnest foil Rutherford used was only 1,000
atoms thick.
Doubling the thickness of the foil resulted in a
doubling of the fraction of alpha particles being
deflected into large angles.
4. The Size of Atoms
Atoms are very small. Guess how many atoms there
are in an ounce of lead:
A) eighty million
B) eighty billion
C) eighty-thousand billion
D) eighty-thousand million-million
Answer
The correct answer is (D). There are eighty-thousand
million-million atoms in an ounce of lead.
Model of a Carbon Atom
The Periodic Table
It began to be realized that the properties of chemical
elements are not arbitrary but depend on the
structure of the atom and vary systematically. After
much experimentation, it was possible to create a
systematic arrangement of the elements into a table
called the periodic table.
Current Periodic Table
http://www.ptable.com