Transcript notes ch 39 1st half Atomic Nucleus and Radioactivity

Atomic Nucleus and
Radioactivity
Chapter 39 (1st half)
39.1 Atomic Nucleus
• It would take 30,000 carbon nuclei to
stretch across a single carbon atom.
• The nucleus is so VERY VERY tiny
compared to the rest of the atom.
• It’s like a cookie crumb on the 50 yard line
of a NFL football stadium.
39.1 Atomic Nucleus cont.
• The nucleus contains nucleons, which
when positive are protons, and when
neutral, neutrons.
• Neutrons and protons have roughly the
same mass.
• Electrons are so very very light compared
to nucleons, so the mass of an atom is
really only based on the nucleus.
39.1 Atomic Nucleus cont.
• The number of protons equal the number of
electrons in an atom (unless it is an ion).
• The protons keep the electrons moving around
the nucleus (positive charge attracts negative
charge).
• The neutrons do not have to equal the protons or
electrons, and as such, they act a little like a
nuclear cement holding the nucleus together.
• The force holding protons and neutrons together
is called the strong force.
39.1 Atomic Nucleus cont.
• The strong force is only strong enough to hold
things together at very small distances.
• When the number of protons increases in a
nucleus, then the number of neutrons needed to
hold the nucleus together must get larger too.
• At small atomic numbers, the number of neutrons
may be equal or slightly more than the number of
protons, but at higher atomic numbers, the
number of neutrons must be A LOT more than
the protons.
39.2 Radioactive Decay
• The neutron is unstable.
• A lone neutron will spontaneously decay into a
proton plus an electron (and also an antineutrino,
a tiny particle that will not be discussed).
• Out of a bunch of lone neutrons, about half of
them will decay in 11 minutes.
• Particles that decay like this are said to be
radioactive.
39.2 Radioactive Decay cont.
• Particles only decay when their combined
products weigh less than what they started
out as.
• The mass of a neutron is greater than a
decayed neutron (a proton, electron and
antineutrino), so there is less mass after
decay.
39.2 Radioactive Decay cont.
• All the elements heavier than Bismuth
(atomic number 83) decay in one way or
another.
• There are three types of radioactive decay:
Alpha, Beta, and Gamma.
39.2 and 39.3 Radioactive Decay
cont.
• Alpha rays are streams of particles that are made
of two protons and two neutrons, and are
identical to the nuclei of a helium atom.
• These particles are called alpha particles.
• They can be stopped by a few sheets of thin
paper or a sheet of heavy paper.
• They quickly grab some random loose electrons
in the air and become harmless Helium atoms.
39.2 and 39.3 Radioactive Decay
cont.
• A beta ray is a stream of electrons.
• An electron is ejected from the nucleus
when a neutron is transformed into a
proton.
• Beta particles go through paper, but are
stopped by several sheets of aluminum foil.
• They lose energy by collisions with other
atomic electrons.
39.2 and 39.3 Radioactive Decay
cont.
• Gamma rays are mass-less energy. They are like
photons of light, but of much higher frequency
and energy.
• Visible light is given off when electrons jump
from one orbit to another of lower energy, and
gamma rays are emitted when nucleons do a
similar sort of thing inside the nucleus.
• Gamma rays require lead or other heavy
shielding to stop them.
39.4 Radioactive Isotopes
• An isotope is an atom that has the same number
of protons as another atom of the same element,
but it has a different amount of neutrons.
• Example: Carbon-14 is an isotope of regular
carbon, Carbon-12.
• All elements have isotopes.
• Some isotopes are radioactive. ALL isotopes of
elements above atomic number 83 are
radioactive.
Assignment:
• Questions 1, 3, 5, 6, 7, 9, 27, 29, 30……..
on page 627.
• Google search: What type of radiation is
used to treat cancer? Gamma, Beta, Alpha
or other? Do they pick certain kinds of
radiation based on the type of cancer they
are treating? Explain what you find out
and list the website you used.