Download Fusion and the Beginning of the Universe The Big Bang

Fusion and the Beginning of the Universe
All matter was formed in the explosion, the big bang, and the universe has been expanding since that
time. In this lesson, we'll explore the formation of the first hydrogen nuclei after the explosion and the
formation of elements up to iron by fusion reactions within stars.
This is an overview lesson. Don't worry! You are not expected to master all of this content in one day. We
will be covering sections of it in the next several lectures. Today, it is important for you to see the overall,
big ideas that tie together energy and the atom.
Outline:
The Big Bang
Energy and Matter
Fusion in Stars
Homework
If you have Zumdahl's "Chemistry”, you can read more about these topics in the following sections.
Atomic nuclei: 2.5; Energy: 6.1
The Big Bang
How do we know anything about the beginning of the universe? A good part of our understanding results
from the observations of astrophysicists on the ever expanding universe and the processes that currently
occur in stars and supernovas. Theoreticians develop mathematical models to explain these observations
and nuclear physicist test the models in supercolliders.
Just Physics: 0 - 3 Minutes
In the beginning, the universe was extremely small, incredibly hot, and homogenous. Then it exploded.
Immediately after the explosion, there were only sub-atomic particles and electromagnetic radiation. It
took 3 minutes for the universe to cool down enough to make the protons, electrons, and neutrons that
form atoms.
Neutrons, Protons, and Electrons
The nucleus, or central part, of all atoms is made up of protons and neutrons. Electrons are outside the
nucleus.
A neutron is an uncharged subatomic particle with a mass of 1.675 x 10-30 grams. A proton is almost equal
in mass to a neutron (1.673 x 10-30 grams) but it is a +1 charged particle.
There are two other common charged particles, electrons with a -1 charge and positrons with a +1 charge.
These have a much smaller mass (~9.1 x 10-34 grams) than neutrons or protons.
Under very high energy conditions, a neutron can convert to a proton and an electron. A proton can
convert to a neutron and a positron. An electron and a positron annihilate when they meet and produce
electromagnetic radiation.
The mass of an atom is the sum of the mass of all subatomic particles that make it up. The electrons have
negligible mass so we don't include this in the total. To 3 significant figures, neutrons and protons have
the same mass. We give the name atomic mass unit (amu) to the mass of one of these nuclear particles.
Chemistry begins: 3 - 20 Minutes
During the next 17 minutes, the universe was still dense but cool enough so that protons and neutrons
could combine to make the nuclei of hydrogen and helium atoms. It was still too hot to allow negativelycharged electrons to combine with the positively-charged nuclei.
A single proton is the nucleus of a hydrogen atom. Another kind of hydrogen, deuterium, has a proton and
a neutron in its nucleus.
Both kinds of hydrogen are represented by the symbol H. Hydrogen and deuterium are isotopes.
That is, they are the same element with the same number of protons in the nucleus but they have different
atomic mass.
The symbols at left show the number of protons in the nucleus or the atomic
number below the atomic mass number in atomic mass units.
The most common isotope of hydrogen, also indicated as 1H or H-1, has only 1 proton in the nucleus and
so has a total mass of 1 amu. Deuterium, 2H or H-2, also has 1 proton in its nucleus but has an atomic
mass of 2 amu because its nucleus also has 1 neutron.
When two deuterium nuclei collided during this period, a helium nucleus was produced.
At T= 20 minutes, all the matter in the universe was a mixture of about 75% hydrogen and 25% helium,
along with a trace of lithium. It took thousands of years for the universe to cool enough so that electrons
could combine with the positively charged nuclei to make neutral H, He, and Li atoms.
Click on the picture below to see how hydrogen nuclei could fuse to make helium.
Rob Scharein, Astronomy, Universtiy of British Columbia
Begin the simulation and answer the questions.
1. What happens when you increase the number of hydrogen atoms per unit volume (gas density)?
2. What is the effect of changing temperature?
3. How did gas density and temperature change in the early minutes of the universe?
4. What isotopes of hydrogen and helium do you see?
Hydrogen and Helium Atoms
As you've seen above, each elementary particle has a mass of
about 1 atomic mass unit.
The exact masses of 3 hydrogen isotopes and 2 helium isotopes
are shown at right. It includes an unstable isotope of hydrogen
called tritium.
Do the nuclei have masses that are the sum of the masses of their
nuclear particles?
Once the universe cooled enough, electrons combined with nuclei
to make neutral hydrogen and helium atoms. For every proton in
the nucleus, there will be an electron surrounding that nucleus.
The nucleus of an atom is very small relative to the surrounding electrons(s). Because an electron is so
large relative to its mass, we use analogies to talk about it. Sometimes the electron is characterized as a
cloud. We can also relate an electron to a standing wave in water.
The first picture below represents the electron density around a neutral hydrogen atom (with 1 electron) or
a neutral helium atom (with 2 electrons). You can see that the electron density is highest in the center and
decreases with distance from the center. When we draw a shape for electron density, it means that 90% of
the electron density is within the shape. This shape is called an electron orbital. The electron density for
hydrogen and helium atoms is represented by a sphere.
Hydrogen and Helium Atoms
from Wikipedia
http://en.wikipedia.org/wiki/Hydrogen_atom
from the full wiki
http://www.thefullwiki.org/Helium
Atoms and nuclei are small. Distances on that scale are measured in angstroms (1.0
= 1.0 x 10-10
-15
meters) or femptometers (1.0 fm = 1.0 x 10 meters). The electron orbital of either a hydrogen or helium
atom has a diameter that is about 100,000 times greater than the diameter of the nucleus.
Energy and Matter
Chemistry is a science that concerns matter and the changes of matter that result from the interaction of
matter and energy. The terms matter and energy are familiar to all of us but their definitions, at least
within chemistry, are not simple.
What is Energy?
We talk about energy all the time. You could be feeling particularly energetic (or not) as you read this
page. We also talk about energy contained in things: energy in a battery, in the sun, or in gasoline.
A good definition is that energy is the ability to do work or give off heat.
The definition leads to more questions than it answers. What is this ability? How do we measure it? First,
let's go over some of the common types of energy.
work
Work, one of the 2 types mentioned in the definition, is force on an object applied over a distance.
Throwing a ball is work because you apply force from your arm to the ball to overcome forces of gravity
and resistance of the air as the ball travels some distance.
joule, J
work done by accelerating 1 g at 1 cm/sec2 for 1 m
heat
Heat, the other of the 2 types mentioned in the definition, is related to the movement of atoms or
molecules within a substance, its kinetic energy. As heat energy is added to a gas, the molecules of the gas
increase their velocity. Individual gas molecules have different velocities, though. The average kinetic
energy determines the temperature. Temperature is not the same as heat.
calorie, cal
heat needed to raise the temperature of 1 g of H2O by 1 deg C
1 cal = 4.184 J
electrical energy
We typically measure electricity in watts. This the rate at which electrical energy is delivered. A watt is 1
joule per second. The energy must include a time unit so we typically use kilowatt hours (1 kilowatt or 1
kW = 1000 W).
kilowatt hours, kWh
1 kWh = 3.6 x 106 J
light
Light is one form of electromagnetic radiation. It has the properties of a wave and can be characterized by
its wavelength. The energy of electromagnetic radiation is inversely proportional to its wavelength.
reciprocal centimeter, cm-1
1 cm-1 = 1.196 x 10-2 J
Potential energy is energy that is due to the position or composition of an object. Water behind the dam
of a mountain stream has potential energy. As it is released through the dam, the potential energy is
converted into kinetic energy. Kinetic energy is the energy of motion.
We can express all of the different types of energy in units of joules.
Matter and Mass
Matter has mass and occupies space. Mass, but not weight, is a fundamental property of the matter. We
typically measure mass in grams, g, or in some multiple of that. Weight is a force, the force of gravity
acting on the matter.
A collection of 6.0 x 1023 atoms of hydrogen has a mass of 1.0 gram. On Earth, the weight of that
collection of atoms would be 2.3 x 10-3 lbs. On the moon, with 1/6 Earth's gravity, the collection of atoms
would still have a mass of 1.0 g but its weight would be 3.7 x 10-4 lbs.
That number of atoms, more
precisely 6.02214 x 1023, is called a
mole. A collection of that number of
atoms has a mass in grams equal to
the average atomic mass in atomic
mass units (AMU) of the atoms.
Interconverting Mass and
Energy
Alfred Einstein gave us an equation
showing that energy and mass can interconvert.
A small amount of mass is the same as a very large amount of energy. The
proportionality constant is the square of the speed of light, (3.0 x 108 m/s)2 or 9.0 x 1016 m2/s2.
Only in nuclear reactions can mass and energy interconvert. Both energy and mass are separately
conserved in ordinary chemical reactions.
Fusion in Stars
Swirls of hydrogen and helium gas condensed into huge clouds. The gravitational force at the core
brought the matter closer and closer together until some of the nuclei coalesced. This produced energy,
the heat and light of the stars.
What is Fusion?
Fusion a type of nuclear reaction where two nuclei come together to form the nucleus of a different
element. Each element has a particular number of protons in the nucleus. Isotopes of an element all have
the same number of protons but different numbers of neutrons.
In the core of a star, gravity produces high density and high temperature. The density of gas in the core of
our sun is 160 g/cm3, much higher than the densest metal, and the temperature is 15,000,000 K (27
million degrees Fahrenheit). At this temperature, the hydrogen and helium gases become a plasma. That
is, the electrons separate from the nuclei to give a mix of positively charged ions and electrons.
Under these conditions protons (H-1) react with other protons to make deuterium nuclei (H-2) and
positrons. The deuterium nuclei can merge to form a helium nuclei (He-4), or they can interact with other
protons to make another isotope of helium (He-3). Two He-3 nuclei can fuse to make a nucleus of an
unstable beryllium nucleus (Be-6) that breaks apart to give He-4 and two protons. Energy is released at
each step.
The fusion of hydrogen nuclei uses up hydrogen to produce helium and energy. Hydrogen is the fuel for
the process. As the hydrogen is used up, the core of the star condenses and heats up even more. This
promotes the fusion of heavier and heavier elements, ultimately forming all the elements up to iron.
Energy from Hydrogen Fusion
In going from hydrogen to iron, energy is released as nuclei fuse to make bigger ones. Why?
The protons and neutrons are held together through a type of energy called nuclear binding energy. The
nuclear binding energy for H-1, a proton, is zero because there is only one particle in the nucleus.
As the number of particles in the nucleus increases, energy is released. This is the same amount of energy
that is required to break them apart.
Energy in this table is reported in units of MeV or mega-electron volts. A MeV is equal to 1.602 x 10-13
joules.
From Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html
Another way to think of the energy released by fusion is to look at the change in mass. The total mass of
the helium nucleus is less than the sum of the mass of the 4 particles that make it up.
Why stop at Iron?
Adding additional protons and neutrons to iron doesn't release energy
because the binding energy peaks at this element. For element heavier
than iron, fusion requires energy.
How did the heavier elements form?
It was from the energy of other explosions. A large, exploding star or
supernova releases the energy needed to fuse all of the heavier
elements.
The Earth and all of the material on it were formed from stardust!