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Internal Structure of
Atoms
Chapter 3-Section 2
1
Subatomic Particles
 Electrons,
protons, and neutrons
 Electricity – J.J.Thomson –
Electrons
 Metal plates – electrodes
–Cathode –Anode +
2
Cathode Ray Tube
3
Electrons
The
wheel in the cathode ray
tube-mass
Originate at negatively
charged side – negatively
charged electrons
Magnet
4
Mass
 Further
experiments found the
mass of an electron to be
5.485799 x 10-4 amu
 2000 times lighter than an atom
 The charge was found to be
-1.602189 x 10-19 C
 C = Coulombs – SI unit
5
Charges
An
atom is not the simplest
form of matter
Since electrons (e-) are
negative, how is an atom
neutral?
6
The Nucleus
Thompson’s
plum pudding
model
This is no
longer a valid
model
7
Rutherford’s Nucleus
Gold
foil experiment
–Alpha particles
–Some passed through and
some were repelled away
from the foil
8
Rutherford’s Nucleus
2
years later…
 Must be a concentrated tiny
space with a mass larger than
the α particle
The rest of the gold foil would
be empty space.
9
The modern nucleus
The
nucleus is a dense central
portion of the atom that is
positively charged.
The nucleus has most of the
mass of the atom, but is very
small compared to the entire
atom.
Marble and football
10
Protons and Neutrons
Protons
=+
-19
Charge = 1.602189 x 10
C
Mass = 1.0073 amu (2000
times the mass of an electron)
11
Protons and Neutrons
A
neutral atom has equal
numbers of protons and electrons
Masses
of all atoms (except H)
were greater than the mass of
e- and p+ combined so where
does this mass come from?
12
Neutrons
 Joliot-Curie
and Chadwick
experiment with beryllium
–Alpha particles
–Beam of power not deflected by
electric or magnetic fields
These particles must be neutral
Mass of 1.0087 amu
13
Coulomb’s Law
States
that the force between
two charged particles is
inversely proportional to the
square of the distance
between them.
14
Coulomb’s Law
Force = 0.58 N Distance = 0.020 pm
Force = 2.32 N
Distance = 0.010 pm
15
Strong Force
The
force that holds the protons
and neutrons in a nucleus.
It is greater than the repulsive
forces at close distances.
Must be neutrons in nucleus if
there is more than one proton.
16
Atomic Number
Atomic
number – the number of
protons in the nucleus of an atom
# of protons =#of electrons
Always whole numbers
Same for all atoms of the same
element.
17
Mass Number
The
total number of protons
and neutrons in the nucleus of
an atom.
#
of Neutrons=Mass# - Atomic#
Can
vary for atoms of the
same element.
18
Boron
The
atomic number of boron is 5.
The mass number of boron is 11.
The number of protons and
electrons are equal to the atomic
number.
19
Atomic
Number
Element
Symbol
Boron
Mass
Number
20
Boron
Electrons
=5
Protons = 5
Neutrons = 11 – 5 = 6
21
Copper
How
many protons, neutrons,
and electrons are present in
copper which has an atomic
number of 29 and a mass
number of 64?
22
Copper
Electrons
= 29
Protons = 29
Neutrons = 35
23
Atomic Structure
Atomic
and
mass
numbers can
be written
beside the
elements
symbol
Mass Number
9 Be
4
Atomic Number
24
Concept Check
What
three subatomic particles
are important to chemistry and
what are their electrical
charges?
25
Isotopes
Isotope
– An atom that has
the same # of protons as
other atoms of the same
element but has a diff. # of
neutrons
Many elements exist as
mixtures of isotopes.
26
Lead Isotopes
Name
# of
Mass
Amt
Neutrons (amu) (%)
Lead - 122
203.973 1.4
204
-206
124
205.974 24.1
-207
125
206.976 22.1
-208
126
207.977 52.4
27
Radioisotopes
Some
elements have only one
stable isotope and will degrade
and are called radioisotopes.
Unstable nuclear
configurations.
Unstable atoms that undergo
radioactive decay.
28
Unstable in Nature
Some
elements have no stable
isotopes.
Technetium has no stable
nucleus.
Was the first element to be
made artificially in a
laboratory.
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Section Review
Complete
numbers 1-6 in
the section review on page
89.
30