MODELS OF THE ATOM
Dalton, Thomson and Crookes
John Dalton (1766-1844)
• Dalton was the first to define an
element
• Element = a pure substance
made up of one type of particle
or atom. Each element has its
own distinct properties and
cannot be broken down into
simpler substances by means of
chemical change
• Dalton’s vision of the atom is
also called the “Billiard Ball”
model because he thought that
the atom was a solid sphere of
matter
Dalton’s Atomic Theory
• all matter is made up of small particles called ATOMS
• atoms cannot be created, destroyed or divided into smaller
particles
• all atoms of the same element are identical in mass and
size, but they are different in mass and size from the atoms
of other elements
• compounds are created when atoms of different elements
link together in definite proportions
William Crookes (1832-1919)
• Using a gas discharge tube, he
noticed that a positively
charged pinwheel (“cathode”)
would rotate when an electric
charge was passed through the
tube
• Crookes believed that this
electric charge (called cathode
rays) must have mass as well as
motion
J.J. Thomson (1856-1940)
• Thomson took Crookes’ work
and created a new experiment
• he designed an experiment to
investigate how these charged
corpuscles would move in an
electric field
• found that the corpuscles were
attracted towards positively
charged plates - therefore the
corpuscles must be negatively
charged
• called them ELECTRONS
Protons
• Thomson conducted similar experiments, this time with
anode rays and negatively charged plates
• anode rays were attracted to the negatively charged plates therefore they must contain positively charged particles
• Thomson called these particles PROTONS
Thomson’s inferences...
• All atoms contain both protons and electrons
• all protons are identical, all electrons are identical.
Electrons differ from protons
• electrons have a negative charge, protons have a positive
charge
• an electron has the same amount of charge as a proton,
even though the charges are opposite in kind.
• A proton has much more mass than an electron
Thomson’s model of the atom
• Thomson pictured the atom as a
combination of protons and
electrons
• he thought that the electrons
were embedded in a sphere or
positively charged protons
• looks like a blueberry muffin
• called the “Blueberry muffin”
model