Formation of the Elements and
Nuclear Reactions
Elements are Formed in Different Ways in our
Universe
Nucleosynthesis
• Nucleosynthesis is the process of element (nuclei) formation.
• Three types: Big Bang nucleosynthesis
Stellar (star) nucleosynthesis
Supernova nucleosynthesis
• Today, only stellar and supernova nucleosynthesis are
occurring in our universe.
• Element formation in our universe relies on nuclear fusion
reactions.
(fusion = come together)
Nuclear Fusion
• In nuclear fusion, smaller nuclei collide together
to make larger nuclei, and energy is released in
the form of electromagnetic radiation.
• Requires extremely high temperatures and
pressures beyond those found on or within
Earth. However, these temperatures and
pressures are found inside stars and did occur
during the initial formation of our universe
(during the Big Bang event).
• Fusion involves only the nuclei of atoms. At the
temperatures at which fusion can occur, matter
exists as a plasma. This is the state of matter
where the electrons have been stripped off of
the atoms. Plasma is basically a super high
energy, electrically charged gas.
• When nuclei collide, some of the mass of the
nuclei is converted to energy
by Einstein’s
2
famous equation, E=mc . Nuclear fusion
releases a lot of energy per gram of material;
much more energy than is released by burning a
comparable amount of wood, coal, oil, or
gasoline!
The Big Bang
• The Big Bang Theory is the most widely
accepted scientific theory about the origin of
the universe. It is supported by multiple lines
of evidence.
• The “Big Bang” was a phenomenally energetic
explosion that initiated the expansion of the
universe.
• At the moment prior to the Big Bang explosion,
all matter and energy were compressed at a
single point (a singularity – a point of infinite
density).
• We do not know what was before…..?
• The universe has been expanding ever since,
with galaxies moving farther and farther apart.
• Using the rates of expansion measured in the
universe and astronomical distances, the age
of the universe can be calculated back to the
time of the Big Bang. The age of the universe
is calculated at about 13.7 billion years old. By
contrast, our Sun and its surrounding planets
(i.e. our Solar System) is 4.65 billion years old.
Big Bang Nucleosynthesis
• All Hydrogen and most Helium in the universe was produced
during the Big Bang Event, starting ~100 seconds after the
explosion. A small amount of Lithium was also produced.
• Big Bang nucleosynthesis ceased within a few minutes after
the Big Bang because the universe had expanded and cooled
sufficiently by then such that the temperatures and pressures
were too low to support additional nuclear fusion reactions.
Stellar Nucleosynthesis
• A star is a very hot ball of gas (plasma). Stars create elements by combining lighter nuclei into
heavier nuclei via nuclear fusion reactions in their cores and releasing energy in the process.
They are natural nuclear reactors!
• Enormous temperatures (15,000,000 K), pressures, and densities of matter are needed to
initiate the fusion (thermonuclear) reactions which squeeze nuclei together and release energy.
• The basic nuclear reaction in the Sun converts hydrogen to helium and releases energy in the
form of electromagnetic radiation (see the basic fusion reaction below). This is why our Sun
shines!
• Our Sun is only large enough to fuse hydrogen into helium within its core.
Stellar Nucleosynthesis
• Stars much larger than our Sun can
fuse heavier elements from lighter
elements.
• These giant stars have an “onion
layer” structure.
• As you proceed deeper into the
star, temperatures and pressures
increase, and heavier and heavier
nuclei are fused together.
• The heaviest element that can be
made in a star is iron. Elements
heavier than iron have fusion
reactions with temperature and
pressure requirements greater than
those that can occur within the
core of a giant star.
• Note: In the adjacent diagrams, the
term “burning” really means
nuclear fusion!
Nuclear Fusion Requirements
(in stars)
Fusion
Fusion By-product
Minimum Core
Temperature
Minimum Core
Density
Minimum Stellar
Mass*
Hydrogen
He
13 million K
100 gm/cc
0.08 solar masses
Helium
C, O
100 million K
100,000 gm/cc
0.5 solar masses
Carbon
O, Ne, Mg, Na
500 million K
200,000 gm/cc
4 solar masses
O, Mg
1.2 billion K
4 million gm/cc
about 8 solar masses
Oxygen
Mg, Si, S, P
1.5 billion K
10 million gm/cc
about 8 solar masses
Silicon
Si, S, Ar, Ca, Ti, Cr, Fe,
Ni
around 3 billion K
30 million gm/cc
about 8 solar masses
Neon
gm/cc = grams per cubic centimeter (units of density)
https://sites.uni.edu/morgans/astro/course/Notes/section2/fusion.html
Supernova Nucleosynthesis
• Elements heavier than Iron (Z = 26) are
made primarily when giant stars explode
in supernovae.
• Even the largest stars do not have core
temperatures and pressures high enough
to fuse iron into heavier elements.
Therefore, when a star runs out of
nuclear fuel (lighter nuclei) and can no
longer undergo fusion reactions, gravity
causes the star to collapse. The
gravitational collapse triggers a
phenomenally large explosion called a
supernova. The explosion of the star
momentarily generates high enough
temperatures and pressures to cause
nuclear fusion reactions that make
elements with atomic numbers 27-92
(Cobalt to Uranium).
• Since only the largest stars can explode in
supernovae events, elements with
atomic numbers 27-92 are rarer than
elements with atomic numbers 1-26
(see abundance diagram to right)
An exploded star
(supernova)
Relative Abundance of the Elements in our Universe
A summary…
(You are made of stardust from exploded stars)
Nuclear Fission
• We have learned that elements form in the universe by nuclear fusion reactions which assemble larger
nuclei by forcing smaller nuclei together under tremendous temperatures and pressures.
• However, elements can also form when a large, unstable nucleus breaks apart in an attempt to achieve
a more stable, lower energy state.
• The splitting of a nucleus to form two or more smaller, more stable nuclei is called nuclear fission.
(fission = split)
• Fission may occur spontaneously (without energy being added) or it may be prompted by firing a
nuclear bullet (like a proton or neutron) at an unstable nucleus, as seen in the example below.
• Like fusion, fission also releases energy stored in the nucleus of an atom. However, not as much energy
is released from fission as from fusion. Still, the energy released per gram of material by fission is
considerably more than the energy released by burning a comparable amount of wood, oil, gasoline,
etc. Fission of uranium-235 atoms is used in nuclear power plants to produce energy.
• Fission also occurs naturally within the layers of the earth as radioactive elements in rocks
spontaneously decay to more stable elements, creating a natural source of heat within the earth. You
also contain a small proportion of radioactive isotopes within your body. These isotopes decay
naturally, releasing radiation. Therefore, you are slightly radioactive too! So is the banana you ate for
breakfast!
Nuclear
bullet
Radioactivity
• Radioactivity is the release of energy,
in the form of energetic particles and
waves, from the nuclei of unstable
(radioactive) isotopes. Radioactive
atoms undergo fission-type reactions
in order to try to become more stable
nuclei with lower energies.
Radioactive atoms are called
radioisotopes.
• The nuclei of unstable, radioactive
isotopes have the wrong ratio of
neutrons to protons (n/p). Generally,
it is too high. When n/p of an isotope
falls between 1 to 1.5, the nucleus is
stable (within the “Band of Stability”
on a n0 vs. p+ plot). Outside of that
range, nuclei tend to be unstable and
break apart over time. This “breaking
apart” of unstable nuclei over time
and the accompanying release of
nuclear particles and energy is called
radioactive decay.
Types of Radioactive Decay – Alpha Decay
In alpha decay, an unstable
nucleus releases two
neutrons and two protons.
This is called an alpha ()
particle. It is equivalent to a
4 He nuclei. Energy is also
2
released in the process.
As a result, the mass
number of the remaining
nucleus decreases by 4 and
the atomic number
decreases by 2. A new
element is formed in the
process!
Credit: Khan Academy
Types of Radioactive Decay – Beta Decay
In beta decay of an unstable
nucleus, a neutron suddenly
changes to a proton, releasing an
electron, a ghostly, low mass
particle called a neutrino (not
pictured), and energy!
As a result, the atomic number of
the remaining nucleus increases
by 1 but the mass number does
not change. A new element is
formed!
Note: The released electron did
not come from outside the
nucleus. It came from inside the
nucleus. It is called a beta ()
particle.
Credit: Khan Academy
Types of Radioactive Decay – Gamma Decay
In gamma decay, an unstable
nucleus releases a high energy form
of electromagnetic radiation (light)
called a gamma () particle or a
gamma ray. This particle of light is
also known as a photon.
The energy is released as the
protons and neutrons in the
unstable nucleus reposition
themselves in an attempt to find a
lower energy arrangement.
Since no protons or neutrons are
released, the mass number and
atomic number of the nucleus
remain unchanged, and no new
element is formed. Gamma decay
usually accompanies alpha and beta
decay.
Credit: Khan Academy
Nuclear Reactions can be Represented by
Nuclear
Equations
• Fusion
Making a
larger nucleus
from two or
more smaller
nuclei
• Fission
Making two or
more smaller
nuclei from a
larger nucleus
Important Symbols Used in Nuclear Equations
• To write a nuclear
reaction, you
must remember
how to read and
use isotope
symbol notation
Particle
Proton
How written in a nuclear reaction
1
1
p
or
H
1
1
Neutron
1
n
0
Electron
(Beta particle)
• You must know
the symbols used
for various
subatomic
particles like
protons,
neutrons, etc.
Alpha Particle
(Helium nuclei)
0
e
or
β
-1
-1
4
4
He
2
Gamma Particle or
Ray
0

or
2

(a massless packet of pure
electromagnetic radiation, a form of
energy)
Balancing Nuclear Reactions
238 U
92
32 P
15
10 B
5
Check the math on
these examples of
nuclear equations to
see if the sums of the
mass numbers and
the atomic numbers
are the same on each
side of the
equations.
Can you figure out
which equations are
fission and which are
fusion?
Transmutation
Transmutation is a general term for the changing of chemical element
or isotope to another by changing the number of protons and/or
neutrons. Fusion and fission reactions both qualify as transmutations.
The bombardment of a nucleus by a nuclear bullet in order to change it
into another element also counts as transmutation.
Synthetic Elements
• Elements with atomic numbers Z ≥ 93
are synthetic (man-made)
• These elements have been made in
particle accelerators, either by
smashing smaller nuclei together or
else by shooting nuclear bullets at
large nuclei.
• These elements are all radioactive.
They decay over time to more stable
elements, releasing radiation (particles
and energy) from their nuclei. Some
have very short half-lives and have
only existed for fractions of a second.
• Some synthetic elements have uses for
mankind. Americium (Am) is used in
smoke detectors. Others have no
current use but were made during
basic research to better understand
atomic nuclei and the forces that hold
them together. The heaviest synthetic
element has an atomic number of 118.
It has no uses at present.