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Chapter 3
Brief History of Electricity
1. Ancient Greeks discovered that if a piece of amber were
rubbed with fur- pieces attracted each other. Later attributed to
opposite charges (+ and -) – called static electricity
2. Benjamin Franklin – mid 1700’s – showed that lightning
was a form of static electricity
Alessandro Volta – 1800 – First battery
G. Johnstone Stoney – 1874 – coined the word
“electron” for the basic unit of electricity
By the end of the 19th Century scientists had discovered that
the atom was not solid and indivisible as Dalton had
proposed. Several important experiments led to these
discoveries. Let’s look at 2 of them:
1. J.J. Thompson and the Crookes Tube Experiment
A) Led to discovery of electrons as basic unit of negative
charge, found randomly inside atom.
Cathode Rays
are electrons
B) Plum-Pudding Model of the atom
2. Ernest Rutherford and discovery of the nuclear atom
A) Natural radioactivity – 3 types – alpha particles (),
beta particles () and gamma radiation ()
B) Gold foil Experiment
C) Nuclear Model of the atom
atom
C) Nuclear Model of the atom
First model
After discovery of neutrons
Summary of particles inside atom:
Particle
Symbol Location
Relative Charge Relative Mass
(Mass #)
Electron
e-
Outside nucleus
-1
0.005 ≈ 0
Proton
Neutron
p+
n0
Inside nucleus
Inside nucleus
+1
0
1.000
1.000
Atomic Number (Z) – The number of protons in an atom.
Also the number of electrons in a neutral atom. Unique
for each element. Identifies the element.
•Mass Number – The number of protons + the number of
neutrons in an atom. For example:
An atom with 5 protons and 7 neutrons
•Mass # = 5p+ + 7n0 = 12
•This number is not unique for each element. All but one
element have atoms with different numbers of neutrons and
therefore different mass numbers.
•For example, there are 3 different atoms of C found in
Nature, all having 6 protons, but one having 6 neutrons, one
7 neutrons and one with 8 neutrons. What are their Mass
Numbers?
•These 3 atoms are called isotopes of C. Isotopes are
atoms of the same element with different Mass Numbers.
Mass #
12
Atomic #
Isotopic Symbols - 6 C
•Atomic Weight (AW) or better Atomic Mass – The relative
weight of an average atom of one element compared to that
of another atom chosen to be the standard.
•The modern standard is the C-12 isotope (arbitrarily
assigned a value of 12.0000 amu (atomic mass unit).
Actually the AW of an element is the average (taking into
account the relative amount of each) mass of all the naturally
found isotopes.
•This is the non-whole # found under each element in the
Periodic Table.
Niels Bohr – Danish - 1912
Bohr Model – Also called Solar System Model. Nucleus
with electrons revolving around in orbits of specific
energies (called Energy levels)
•Each successive orbit has a maximum # of electrons that
can exist (2, 8, 18, 32, 50 etc) We will only worry about the
first 20 elements. For these the maximum # of electrons is 2
for the first and 8 for the rest (actually 18 in the third, but the
last 10 are not important for us in this course).
•For every atom the electrons fill up the orbits in order
beginning with the lowest energy. The closer to the nucleus,
the lower the energy. In other words Li has an atomic # of 3,
therefore 3 electrons (2 in the first level and 1 in the next).
•See the following table. The electrons in the outermost energy
level are called Valence electrons and that level is called the
Valence Shell.
Dimitri Mendeleev and the Periodic Table
Modern Version
•
Names of Families
•
Metals, Non-metals and metalloids
and Noble Gases
•
Representative elements and transition metals
Modern Periodic Table:
1. Metals – Left side a) Properties – Shiny, malleable, ductile, conduct
electricity & heat
2. Non-Metals – Right Side
b) Properties – Brittle as solids, non-conductors
3. Metalloids – In between
c) Properties – Some of both metals & non-metals
4. Noble Gases – Far Right Column
d) Properties – All are gases even at very low
temperatures, non-conductors, very little tendency to
chemically react
Representative Elements – Those in columns headed by
an A (IA, IIA etc). We will spend most of our time dealing
with these elements.
Transition Metals – Those in columns headed by a B.
Inner Transition Elements – Bottom 2 rows