Name: ___________________________
Block: ____________
Science 9 – Chemistry
7.1 Development of a Modern Atomic Theory
Our theory about the atom has changed over time as new studies are done. Even though no one has
ever seen an atom up close we are still able to make new discoveries – just like we have made new
discoveries about dinosaurs.
What does the Greek word atomos mean?
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What is an Atom?
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CARBON ATOM
The First Scientific Atomic Theory: John Dalton 1776-1844
• By the late 1700s Scientists agreed that an element was a type of matter that could not be broken
down into simpler substances. Then a British chemist and schoolteacher John Dalton brings back
Democritus’s idea of the atom.
• He performed many experiments to study how elements join together to form new substances
• He found that they combine in specific ratios (remember the electrolysis of water) and he
supposed it was because the elements are made of atoms.
What 3 new ideas did John Dalton propose about the atom?
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2. __________________________________________________________________.
3. __________________________________________________________________.
In the 1830s, Michael Faraday showed that atoms could gain electric charges. Dalton’s theory did
not include charged atoms (Figure 5), and so was modified to include the following ideas:
Revisions to Dalton’s Theory
• Matter must contain positive and negative charges.
• Opposite charges attract, and like charges repel.
• Atoms combine to form the particles of a compound because of the electrical attraction
between charged atoms.
J.J. Thomson 1856-1940
What particle did Thomson discover?
• Thomson discovered that atoms are made of ________________________________ called
___________________.
• Thomson’s discovery was the result of doing experiments with “cathode ray tubes”
Thomson’s Revision to the Atomic Theory: Electrons
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An Interactive Model of Rutherford’s Gold Foil Experiment
read pg. 206-207
Rutherford’s Revised Atomic Theory (1911)
Result: Most of the positively charged particles went straight through the gold foil.
Atomic Theory: Most of the matter of the atom is found in a _____________ part of the atom. This
is called the ________ of the atom. It is very tiny and extremely ___________.
Result: Some of the positively charged particles were deflected or even
bounced back.
Atomic Theory: Like charges repel so the nucleus must have a _______________. If electrons have
a negative charge they could not be in a positively charged _____________Electrons
must ____________________________________.
Result: The diameter of the nucleus is 100,000 times smaller than the diameter of the entire gold
atom.
Atomic Theory: Atoms are mostly ______________ with a tiny, _________________ at the center.
Rutherford’s Revisions to the Atomic Theory: the Nucleus
His _________________ showed that the atoms have a small dense core of _________________________
__________________ by empty space. ......In other words, most of the mass concentrated in a small space
at the ___________________........Electrons travel around ______________ in well-defined ____________.
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The nucleus contains all of the positive charge and most of the mass of the atom.
The nucleus contains positively charged protons and uncharged neutrons.
Neutrons have the same mass as protons.
The nucleus is very small, compared with the size of the atom.
The electrons orbit the nucleus, like satellites around a planet.
The mass of an electron is 1/1800 the mass of a proton.
The size of the atom is determined by the size of the orbit of the electrons.
There is only empty space between the electrons and the nucleus.
Question for next class: What did Bohr learn about electron movement?