Ch. 14 Concepts to know
The building blocks of matter are atoms
and molecules. (They make up
everything)
Charge is the fundamental property of
matter that can be either positive or
negative. (one of the two forces that holds
atoms together comes from electric
charge)
Forces between electric charges can be
either attractive or repulsive. Opposites
attract and likes repel.
+> < - Attract
<+ +> Repel
<- -> Repel
The letter e represents the elementary
charge, the smallest unit of electric
charge. The charge only appears in whole
units, not fractions or decimals.
British physicist J.J Thomson was the first
to identify the electron. (first particle
discovered smaller than an atom)
When Ernest Rutherford directed helium
ions at gold foil, some of the helium ions
bounced back in the direction they came
from rather than passing through. He
explained this unexpected result was due
to the mass of the gold atom being
concentrated in a tiny, dense core.
The nucleus is the tiny core at the center
of an atom containing most of the atoms'
mass and all of its positive charge.
Protons and neutrons are found grouped
together in the nucleus.
The proton carries a positive charge, +1e.
The neutron has no charge.
Compared to protons, electrons have a
much smaller mass and opposite charge.
The mass of an electron is about 1800
times lighter than a proton.
The mass of an atom is determined
mainly by the mass of the neutrons and
protons.
Electrons are bound to the nucleus by an
electromagnetic force. (between e's and
p's)
Protons and neutrons in the nucleus are
held together by a strong nuclear force.
(all the p's repel each other and this holds
the nucleus together)
Atomic number is the number of protons
in an atom. (each element has it's own
# of p's in the nucleus)
All atoms of an element contain the same
number of protons.
The charge on an atom is equal to zero.
(because # of p's and e's are the same)
Atoms of the same element may contain
the same number of protons but a
different number of neutrons. (these are
called isotopes)
Atoms with the same atomic number but
different atomic mass are called isotopes.
 The number of neutrons may be
different between two isotopes.
 One isotope might be radioactive
while the other isotope might be
stable
 One isotope might have a larger
mass than the other isotope
Ex. A common isotope of Aluminum has
an atomic number of 13 and a mass
number of 27. How many neutrons are in
an atom of this isotope? 14
Ex. A common isotope of Krypton has an
atomic number of 36 and a mass number
of 84. The total number of subatomic
particles in the nucleus is? 84
When an unstable isotope undergoes
alpha decay, it gives off two protons and
two neutrons. (the nucleus ejects a
helium-4 nucleus)
If Atom A has 9 protons, 9 neutrons, and 9
electrons ; Atom B 10 protons, 10
neutrons, and 10 electrons; Atom C 9
protons, 10 neutrons, and 9 electrons. Do
any of these atoms represent the same
element? Yes, atom A and atom C are the
same element (because they have the
same # of p's)
See diagrams on p.320:
 Know about Rutherford's experiment
with helium and that it involves alpha
decay.
 Beta decay, the nucleus converts a
neutron to a proton and an electron,
ejecting the electron, but the mass
number remains unchanged.
 Gamma decay is how the nucleus gets
rid of excess energy. It emits gamma
radiation, which is a form of pure
energy. The atomic number is
unchanged.