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The Atom
Physical Science
Chapter 6.1
Fundamental Particles and Forces C6.1
 Atoms are the smallest
elemental particle
 Made of protons neutrons
and electrons
 Electrical charge is a
fundamental property of
matter, positive and negative
 Opposites attract, likes repel
 Elementary charge,
(electrical) use symbol e
 Units of e, +2, +2e, -e, -3, no
partial e’s
Static Electricity
 Neutral -- electrical charge of
 Charged means the electrical
charge is not zero, it is
positive or negative
 To find a charage, add all the
positive charges and subtract
all the negative charges
 Static electricity is a tiny
imbalance of positive or
negative charge. Friction
pulls a charge off of one
object and temporarily puts it
on another object
Static Electricity Demo
 Volunteer with long hair
 A balloon
 Hair has a positive charge
 Balloon has a negative charge
History of Atomic Discovery
 450 BC. Greece, Democritus -
Matter is discontinuous and can
not be infinitely divide. At some
point an fundamental indivisible
particle would emerge. Aristotle
did not believe him. Aristotle won
the debate.
 1803 England, John Dalton –
Matter is discontinuous. Boyle
(1620) had proven that gas was
made of small particles. Dalton
proposed that everything is. Also
cited Lavoisier (law of
conservation of matter) and
Proust (law of definite
composition – two volumes of the
same stuff have the same mass)
History of Atomic Discovery
 1897, England, J. J
.Thompson discovered
small negatively charged
particle, electrons (-)
Thought it was stuck into
atom like plum pudding.
 1911, England (originally
New Zealand), Rutherford,
shot helium ions at gold
foil -- lots of holes
(expected, he knew atom
was mostly space), some
solid parts, protons (+)
 1832 Chadwick, discovers
neutron, no charge
Your Turn
 Write
 down
the name of
two elements – remember,
an element is a pure
substance that contains only
one kind of atoms
Three subatomic particles
 Electron, outside the nucleus
in the electron cloud,
negative charge, mass 9.09 X
 Proton, inside nucleus,
positive charge, mass 1.637 X
 Neutron, inside the nucleus,
no charge, mass 1.675 X 10-24
 Electron:proton
 1:1000
 softball: football field
Forces Inside Atoms
 Electromagnetic, neutron &
protons, balanced by kinetic
 Strong nuclear force, keeps
protons together, stronger
than repulsion but for short,
nucleus distance.
 Weak force, holds proton and
electron together to make a
 Gravity, weaker than weak
force, but not distance limit.
Combines well.
How do atoms differ?
 Number of protons,
defines the atom, atomic
number -> 1
 Each element has a unique
atomic number (whole
 Complete atom has same
number of protons,
neutrons, and electrons.
Electrically neutral.
 Atomic mass (weighted
average) -> 1.00197 (see
next slide)
Your Turn
 Find out how many protons and electrons are in a balanced
atom of your element.
 Isotopes are atoms that
have the same number
of protons but a
different number of
 Mass number is the
number of protons plus
the number of neutrons.
Isotopes have the same
atomic number but
different mass numbers.
 Radioactive atoms have an
unstable nucleus. Atom will try
to become stable by:
 Alpha decay -- nucleus
ejects two protons and two
neutrons (helium nucleus)
 Beta decay -- Neutron splits
into a proton and an
 Gamma decay -- nucleus
gets rid of excess energy,
number of protons and
neutrons stay the same.