hey guys its professor Dave, let's learn about nuclear reactions
the electromagnetic force is responsible for the entirety of chemical phenomena. that
positively charged protons and negatively charged electrons are
attracted to one another is the reason that atoms form, the reason that chemical
bonds and therefore molecules form, the reason that those chemical bonds have
differing polarities which lead to differing reactivities as well as a
method for enzyme recognition of substrate and DNA base pairing which
leads to life itself. but there are three other fundamental forces in the universe
the four forces are as follows:
there's gravity which is the weakest but it operates on astronomical distances so
when you're looking at planets and stars and galaxies this is the only force that
matters
electromagnetism as we mentioned is what makes chemistry happen and it operates
on the scale of atoms and molecules but the other two forces operate on a scale
even smaller than that, the scale of the atomic nucleus. these are called the
strong nuclear force and the weak nuclear force. the strong nuclear force
is what keeps the nucleus together preventing positively charged protons
from pushing each other away and splitting up the nucleus, and the weak
nuclear force is what facilitates nuclear decay. these are instances in
which the nucleus of an atom changes in some way. they're called nuclear
processes and though there are more in the realm of physics they are important
for chemistry because they often cause transmutation of elements. that's when an
atom will change from one element to another so we need to know a bit about
them. to reiterate in a chemical reaction only the valence electrons in atoms are
rearranging when chemical bonds break and form, the identity of each individual
atom remains unchanged
in a nuclear reaction changes occur to the fundamental particles in the nucleus
of an individual atom which means it will change from one element to another
these reactions are some kind of nuclear decay where something in the nucleus
disintegrates giving off some form of radiation in the process
radioactivity was first discovered by Henri Becquerel in 1896. he noticed that
photographic plates had bright spots when they were exposed to uranium
minerals. this radiation was found to be composed of three types when exposed to
a magnetic field since they were deflected in different manners. at the
time we didn't know about the subatomic particles the radiation was comprised of
so we just name them with Greek letters and discovered their identities later. an
alpha particle is essentially a helium nucleus, two protons and neutrons.
emitting an alpha particle will result in transmutation as seen here. a beta
particle is an electron. a positron is the anti-matter particle of the electron
which has the same mass as the electron but a positive charge, and a gamma
particle is a photon of light
this is the same as the electromagnetic radiation from the Bohr model just no
longer associated with the transition of an electron. let's quickly learn the ways
that we can notate these particles so that we can write nuclear reactions
remember that when we write nuclide symbols the lower number is atomic
number, or number of protons and the upper number is atomic mass, or the
protons + neutrons. so a proton and neutron will be signified this way with
a p and an n each with mass one but only the proton has atomic number one. an
alpha particle can be represented as helium or with alpha symbol, an electron
can be either an e or beta symbol and the atomic number will be negative one
while the mass though technically not zero is negligible so we call it zero
a positron will be the same but with a positive one here and a photon will be
gamma, both numbers zero as it legitimately has zero mass. so if we want
to describe the emission of say an alpha particle we would write it this way
show the first particle the arrow signifies the reaction and then show the
alpha particle. to figure out the resulting nucleus after a mission we
just do some arithmetic. because the atomic numbers and mass numbers have to
add up to the same number on both sides
kind of like how we need the same number of atoms on both sides of a chemical
equation, if the atomic number is 86 on the left and there's a two on the right
we need 84 for it to work out. again with mass 222 on the left, four on the right
we need the other particle to be 218. any atom with eighty four protons is
polonium so the resulting nuclide symbol must be this. again the numbers on each
side of the arrow add up to the same thing. now that we are aware of these
processes, why do they occur? it's always due to some instability in the nucleus
one reason might be that the nucleus is just too large. you see the strong
nuclear force which is mediated by particles called mesons is very strong
and it keeps the protons and neutrons fused together with a hundred times
greater force than the electromagnetic propulsion that wants to push the
protons apart. but the strong nuclear force drops off with distance more
quickly than the electromagnetic so if the nucleus gets too big
all of a sudden the strong nuclear force is too weak over the diameter of the
nucleus to keep it all together and the protons will push apart. so for atoms
larger than bismuth the nucleus is just too big to be stable. nuclei like these
will often rapidly emit an alpha particle to try to get a little smaller and a
little more stable but even if the nucleus isn't too large it may have a
number of protons or
neutrons that isn't ideal. the shell model of the nucleus describes nucleons
as existing in levels or shells kind of the way electrons do. as it happens there
are so-called magic numbers for each nucleon that correspond to a special
type of stability kind of like a full valence shell of electrons. these are the
magic numbers. in addition most stable isotopes of a given element have even
numbers of both protons and neutrons and most of the rest have at least one of
them in even numbers. and lastly an atom will tend to want a certain neutron to
proton ratio in the nucleus. for smaller atoms this is roughly one-to-one which
is represented by this line but for larger atoms this becomes closer to
1.5 to 1. more neutrons than protons. this ratio gives a nucleus
balance so if an atom veers too much in one way or the other nuclear decay will
allow it to adjust the ratio. so as we said if a nucleus is too large it will
emit an alpha particle or it can undergo spontaneous fission where it breaks into
multiple lighter nuclei and usually a few neutrons. in beta emission the reason
an electron is emitted is because a neutron spontaneously transforms into a
proton. remember before how we said that protons and neutrons both weigh one atomic
mass unit even though technically the neutron is slightly heavier? well it is
heavier by exactly the mass of the electron which is why a neutron is
neutral, it's a proton and electron combined. so when the neutron becomes a
proton it will emit an electron in the process
a nucleus would do this if the neutron to proton ratio is too high
meaning too many neutrons, as this would adjust that ratio favorably
positron emission is just the opposite. when a proton becomes a neutron this
will emit a positron because this is the process that is the opposite of beta
emission and the positron is the opposite
of an electron. this would happen if the neutron to proton ratio was too low
meaning too many protons. with both of these types of decay there is also a
tiny particle called a neutrino that is emitted. in beta and mission it's an
anti neutrino and with positron emission it's a regular neutrino. more on that in
particle physics. another process that can occur is electron capture, this is
similar to positron emission in that a proton becomes a neutron but it does
so by absorbing an electron rather than emitting a positron. once again a proton
plus an electron makes a neutron. the electron in question will tend to come
from one of the inner orbitals of the atom and the electron will be a reactant
since it is absorbed by the proton to give a new particle. and lastly if a
nucleus is in an excited state it can emit a high-energy gamma photon. in this
process there is no transmutation because we don't change around any
protons and neutrons. so we can predict what kind of nuclear decay a nucleus
might undergo by looking at its condition. is it too large?
what's the proton neutron ratio? each decay process has its typical cause. too
many protons: positron emission or electron capture. too many neutrons: beta
emission. too big: alpha emission or a spontaneous fission. excited: gamma
emission. an unstable atom can undergo a radioactive decay series, over multiple
emissions generate a stable nucleus like uranium 238 which eventually
becomes lead 206. so now we know what radiation is. its unstable nuclei
emitting high energy particles. the reason this is bad for biological
organisms is that these high-energy particles can tear through our cells and
if one strikes a DNA molecule it can potentially change the genetic code at
that location
resulting in a potentially harmful mutation. most elements have naturally
occurring isotopes that do decay over time, even carbon, hydrogen, and oxygen so
there are radioactive
nuclei decaying all over your body every second. luckily we have enzymes that
repair DNA damage as it happens. but if we were to be in the presence of
highly radioactive substance that is emitting particles at a high frequency
this would do much more damage than our bodies can repair so we have ways of
detecting radiation like geiger counters. we will often talk about radioactive
material in terms of a half-life. this is the time it takes for a material to be
depleted to half the original amount. after two half lives there would be one
fourth the original amount. after three half-lives 1/8, etcetera. half-life is
given by the following formula where k is a constant specific to the material
by learning about nuclear processes we can harness the massive energy that
are contained. with nuclear processes matter is converted directly into energy as is
given by Einstein's famous equation e=mc^2 squared. here c is the speed
of light which is very big, so with this equation says is that matter is simply
extremely dense energy, the densest form there is. we have harnessed this power
already with atomic bombs much to the dismay of mankind. these worked by
bombarding unstable uranium nuclei with neutrons which caused them to split
generating more neutrons which collide with other uranium nuclei causing a
chain reaction that releases tremendous energy. an even more powerful process is
nuclear fusion. this is where small nuclei combine to give larger ones, a
process that involves a loss of mass which is converted into even more energy
efficient nuclear reactors that harness the power of fusion promise to solve all
kinds of problems regarding renewable energy sources and could be the key to
the next step in societal advancement. maybe you can aid in the advancement of
this or other important technologies. let's check comprehension
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