Download nuclear force

Nuclear Chemistry
Nuclear Forces
•
THE POSITIVE PROTONS TEND TO REPEL EACH
OTHER THROUGH ELECTROSTATIC REPULSION.
•
BUT THE PRESENCE OF THE NEUTRONS PROVIDES A
‘NUCLEAR FORCE’, OR STRONG FORCE WHICH
HOLDS THE NUCLEUS TOGETHER AND STABILIZES
IT.
Neutron
Proton
nucleons
Nuclear Forces
 The
nuclear force is only strong when the
nucleons are close together.
 As
the number of protons increases, the
electrostatic repulsion increases and the
nuclear force weakens.
 So,
even more neutrons are needed to
stabilize the nucleus.
Nuclear Forces
A stable neutron - proton ratio is 1:1.
As the number of protons
increases, so does the number of
neutrons. This keeps the nucleus
stable. But, this increases the
neutron – proton ratio.
Nuclear Forces
A neutron-proton
ratio of 1.5 : 1 is at
the limit of stability.
Nuclear Forces
•Atoms with more than 83 protons
cannot reach stability even with
their larger numbers of neutrons.
•All elements beyond bismuth on the
periodic table are unstable
and undergo some sort
of ‘decay’ in order to
become stable.
RADIOACTIVE
DECAY
Radioactive Decay
 Unstable
nuclei are those with a high
neutron:proton ratio; this will result in
decay or a change in the nucleus in order
to become stable.
 Transmutation:
a change in the identity of
an element as a result
of a change in the
number of protons.
Radioactive Decay
 Atoms
will decay by ejecting nucleons, or altering
the nucleons into different particles by releasing
one or more of the following:
 Alpha
 Beta
rays
rays
 Gamma
rays
 Positron
Emission
 Electron
Capture
Alpha Particles
Helium Nucleus
2+ charge

Alpha particles consist of two protons
and two neutrons, and are emitted
during some kinds of radioactive decay.

Remember that protons determine the
identity of the element : if an alpha
particle is emitted, the identity of the
element changes.
The atomic number will be reduced
by two, and the mass number
reduced by four.
Alpha Particles
 Alpha
rays are streams of alpha particles given
off during nuclear decay.
rays are relatively slow and easy to stop –
a piece of paper will stop alpha particles.
 Alpha
 They
will travel only a few centimeters before
stopping even if they do not encounter any matter.
 The
positive charge of the alpha particle attracts
electrons nearby and the particle becomes a
harmless helium atom.
Beta Particles

A Beta particle is an electron created and emitted when a
neutron is transformed* into a proton and an electron during
radioactive decay.

This action adds a proton and thus changes the identity of the
atom.

The mass number stays the same.
*The proton and electron are not
‘inside’ the neutron. They are
created at the time of the action.
Beta Particles
 Beta
rays travel faster than alpha rays and
can penetrate paper, but are generally
stopped by thin sheets of metal such as
aluminum.
 Their negative charge causes them to
interact with other atoms which slows their
speed.
-
Gamma Rays
 Gamma
rays are photons of
electromagnetic radiation with
high frequency and energy.
 Gamma
rays are given off when
the nucleons undergo an abrupt
energy difference.
rays have no mass – they
are pure energy.
 Gamma
Gamma Rays
 Because
they have no charge and are
high energy, gamma rays travel far
and penetrate further than alpha or
beta rays.
 Thick concrete or lead is
needed to stop gamma rays.
Gamma Rays
 The
release of gamma rays alone do not affect
the identity of the atom since they have no mass
and no charge.
 But, gamma radiation may be released along
with release of an alpha or beta particle.
Beta Particle
Gamma Ray
Positron Emission
• A positron is a particle that has the same mass as
that of an electron, but has a positive charge.
• A positron is emitted from the nucleus as a proton
is converted into a neutron.
• The atomic number decreases by one but the mass
number stays the same.
1
p
1
1
n
0
0
B
+1
Positron
Electron Capture
• The nucleus can ‘capture’ one of its own inner-orbital
electrons if the atom is unstable due to too many
protons.
• The electron will combine with a proton in the nucleus
and form a neutron.
• The atomic number decreases by one but the mass
number stays the same.
0
-1
e
1
1
p
1
0
n