Download Structure of the Nuclear Atom

Structure of the Nuclear Atom
Electrons
Section 5.2
Electrons
Remember Dalton?
Most of his atomic theory is still accepted.
Most !!
Atoms are known to be divisible.
Atoms are composed of smaller particles. These are the subatomic particles. Like…
Electrons
An electron is a negatively charged subatomic particle.
An English physicist J.J. Thomson “discovered” the electron in 1897.
Electrons
Electrons
He concluded that the rays must have a property. There must be a part of an atom that is
negatively charged. He called them electrons.
Thompson went on to determine(in 1900) that the mass of an electron is 1/2000 of the
mass of a hydrogen atom.
Protons and Neutrons
So, cathode rays are flowing electrons that are removed from atoms. What is left of the
atom?
Points to ponder.
Atoms have no net charge. Huh?
Electric charges are carried by particles of matter(Thompson.)
Electrical charges are whole number quantities. Huh?
An equal number of (+) and (-) charges result in no charge.
So, if you remove one electron(one unit of (-) charge) you should have one excess unit of
(+) charge left.

Protons and Neutrons
Proton
One can now assume that there is a particle which carries one unit of positive charge.
E. Goldstein noticed a “backward” flowing stream under the cathode ray. It was being
attracted to the cathode(-) side of the apparatus. He called them canal rays. Made up of
protons.
Protons and Neutrons
Neutrons
Summary
Atoms are made up of protons, neutrons and electrons.
Protons have a positive charge of 1.
Neutrons have no charge.
Electrons have a negative charge of -1 and are 1/1840 times smaller than protons and
neutrons.
Table 5.1 page 111

The Atomic Nucleus
How are these subatomic particles put together?
Democritus: hard spherical
Dalton: like Democritus
J.J. Thompson: “Plum pudding” electrons stuck to the nucleus
Ernest Rutherford: nucleus surrounded by electrons.
Protons and Neutrons
Rutherford
The results were well…

So now, the atom has a nucleus. The protons and neutrons make up the nucleus. Most
of the atom is made up of empty space. The chain link fence analogy.
Section review page 112 #’s 4,5,6
