This is the periodic table of the chemical elements.
Beautiful, isn’t it?
I bet you have already seen it hundred times.
There are 118 chemical elements in this table.
The elements are ordered according to their atomic number, which is the number of protons
in their nucleus.
The lightest elements (hydrogen, helium) are those with lowest atomic number, which means
lowest number of protons; and as we move along the rows and the columns of the table, the
atomic number increases.
But… have you ever asked yourself, what is the origin of all these elements?
How are they produced?
Do they form all in the same way, or through different mechanisms?
Watch this video and you will find out the answer!
Hydrogen, helium, carbon, oxygen… all these names are very familiar to us.
These are some of the most common chemical elements that we can find on Earth, and also
in the Universe.
But have you ever wondered how did they form?
Did they all exist from the Big Bang, or did they form through some specific processes?
Which of them are naturally occurring and which ones are not?
To answer this question, we need to analyse the periodic table starting from the lightest
elements: hydrogen and helium.
Hydrogen is the lightest element in the Universe: one atom of this element contains only one
proton and one electron.
There are also two slightly different “versions” of this element, known as “isotopes”:
the deuterium, in which the nucleus has one proton and one neutron, and the tritium, where
the nucleus has one proton and two neutrons (all of them have one electron as well).
Helium is the second lightest element: an atom of helium contains two protons and two
electrons (and a few neutrons too, depending on the isotope).
These two are the simplest chemical elements, and also the most abundant ones: in fact,
together they account nearly all ordinary matter in the Universe.
And they are also the oldest!
In fact, the origin of these two elements dates back to the very beginning of the Universe,
about 14 billion years ago: the Big Bang.
Immediately after the Big Bang, the Universe was very hot and dense.
Too much, indeed: in these conditions, chemical elements were not able to form yet, because
protons, neutrons and electrons were moving too fast and they were not able to recombine
together and to form atoms.
As the early Universe kept expanding, it also kept cooling down.
As the temperature decreased, particles started moving slower and slower, and the temperature
became low enough for protons and neutrons to form hydrogen and helium nuclei: this process
is called nucleosynthesis, and it occurred in the first 3 minutes of life of the Universe.
At this point, however, electrons were still moving too fast, and the nuclei were not able
to capture them to form atoms.
For this process to occur, we need to wait approximately 380,000 years after the Big
Bang.
At that time, in fact, the temperature became low enough to allow nuclei to capture electrons
and finally form the very first atoms of hydrogen and helium: this moment is known as “recombination”.
Very tiny amounts of lithium (the third lightest element, having 3 protons and 3 electrons)
were also produced at this time.
So, this is how hydrogen and helium were formed.
The next three elements in the periodic table (lithium, beryllium and boron, having respectively
3, 4 and 5 protons in their nuclei), have a totally different origin.
In fact, these three elements are mostly produced in a process known as “cosmic ray spallation”.
What is it?
You may already know that the Earth is constantly bombarded by high-energy particles (mainly,
but not only, protons) coming from space, known as “cosmic rays”.
When these cosmic rays reach our atmosphere, they interact with the atoms and molecules
present there, breaking them apart and leading to the formation of new atoms and particles
through chain reactions.
Most of the lithium, beryllium and boron that we find on Earth are produced exactly through
this mechanism.
That’s also why scientists observe a “shortage” of these three elements when they analyse
the composition of the Universe: it’s because they are almost only produced through these
collisions between cosmic rays and other particles, so they are relatively less abundant than
other elements, in the Universe.
Ok, we have seen how the five lightest elements formed.
What about the others?
To understand the origin of the heavier elements, we have to look at what happens inside stars.
A star can be thought as a giant “power plant”, producing enormous quantities of
energy all the time.
The process responsible for the creation of such huge amounts of energy is the nuclear
fusion, which occurs in the core of the star.
In the simplest nuclear fusion reaction, two nuclei of hydrogen (so, two protons) fuse
together into a nucleus of helium.
However, the mass of the final nucleus of helium is smaller than the sum of the initial
masses of the two hydrogen nuclei: this means that part of the mass has been converted into
energy, according to Einstein’s famous equation , where is the mass that has been converted,
is the speed of light and is the energy released.
If you plug in numbers, you will find out that even for a very tiny amount of mass converted,
the amount of energy released is huge!
With this nuclear fusion process, therefore, helium nuclei are produced.
This continues for billion years, until the core of the star runs out of hydrogen.
What next?
Well, the star can then “change” type of fuel… and it starts burning helium instead!
In fact, when the conditions of temperature and pressure become right, helium fusion occurs:
nuclei of helium fuse together forming nuclei of heavier elements, such as carbon (6 protons)
and oxygen (8 protons), and releasing energy again.
So, with this new type of nuclear fusion, the star is able to continue to produce energy
and live for a while.
And what happens when the star runs out of helium as well?
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So, what happens when a star runs out of helium?
For low-mass stars, such as our Sun, this means the end of their life: nuclear fusion
stops, and as there is no more “fuel” to be burnt, the star is no longer able to
produce energy.
As a consequence, the outer layers of the star are expelled into space, forming a “planetary
nebula”, while the core remains as a white dwarf.
For high-mass stars, however, something different happens.
In fact, in the collapsing core of these enormous stars, very extreme conditions of temperature
and pressure are reached.
And in these extreme conditions, it becomes possible for nuclei of elements heavier than
helium to undergo nuclear fusion.
So, in a sort of cascade process, the star can burn in sequence carbon, oxygen, nitrogen,
and so on.
As each of these elements undergo nuclear fusion, nuclei of heavier elements are produced.
However, this process does not continue forever.
In fact, the last element that is produced by this mechanism is iron: a nucleus of iron
has 26 protons.
Iron is quite a special element.
In fact, the nuclear fusion of iron is not energetically convenient.
What does it mean?
We say previously that in the fusion of hydrogen into helium, energy is released, because the
mass of the final product is less than the sum of the masses of the initial nuclei.
The same is true for the nuclear fusion of helium, carbon, oxygen, and so on.
However, for iron, this is not true.
In fact, when two nuclei of iron fuse together… they absorb energy, instead of releasing it!
This means that when the core is full of iron, the star is no longer able to produce energy…
because the nuclear fusion of iron does not produce energy.
So, this is the beginning of the end, for the star: the core collapses once more, and
it eventually explodes in what is known as “supernova event”.
So, nuclear reactions in the core of stars are able to produce all elements up to iron,
which has an atomic number of 26.
Then, what about all the heavier elements?
How are they produced?
Elements heavier than iron cannot be produced through nuclear fusion, because as we have
seen, this process is not energetically convenient.
In order to be produced, these elements require conditions even more extreme than those found
inside the core of stars.
And these conditions are found when supernova explosions happen.
In a supernova explosion, the material in the outer layers of the star is violently
expelled to the outer space.
During this powerful explosion, the nuclei of the elements previously produced in the
nuclear reactions inside the core continuously collide with different particles, such as
neutrons (also present in the star).
Therefore, thanks to the extreme conditions, the nuclei of these elements are able to “capture”
some of these neutrons.
When this occurs, these nuclei transform into heavier isotopes of the same element (remember:
isotopes are nuclei having same number of protons, but different number of neutrons).
As these nuclei accumulate neutrons, they become more and more unstable, and eventually
decay in a process known as beta-decay, in which neutrons turn into protons.
As a result, some of the nuclei undergoing this process will eventually end up having
more than 26 protons (so, heavier than iron!).
This phenomenon is known as “r-process”, or “rapid neutron-capture process”.
There is also a variation of this process, known as “s-process”, or “slow neutron-capture
process”, which is essentially the same but occurs much slower, and so the nuclei
decay before being able to capture too many neutrons.
During a supernova explosion, there is one more process that contributes to the production
of elements heavier than iron.
This is known as “p-process”, in which the nuclei expelled during the supernova event
capture protons instead of neutrons, producing therefore nuclei of elements heavier than
iron.
So, summarizing: hydrogen and helium were produced after the Big Bang.
Lithium, beryllium and boron are produced through cosmic ray spallation.
Elements up to iron are produced in the nuclear fusion reactions inside the core of high-mass
stars, while heavier elements are produced during supernova explosions.
So, did we explained the origin of all the elements in the periodic table?
Not yet.
In fact, the heaviest element that can be produced during a supernova explosion is plutonium.
A nucleus of plutonium has 94 protons.
However, if we look again at the periodic table, we see that there are a few elements
heavier than plutonium.
So the question is, how are those elements produced?
The answer is: not naturally…
In fact, they are produced by humans!
Yes, all the elements with more than 94 protons do not exist in nature.
All of them are synthetic, which means that they have been created in laboratory.
Producing atoms of these heavy elements can only be done exploiting difficult and expensive
processes.
For instance, americium (atomic number 95) is produced by bombarding uranium and plutonium
with neutrons in nuclear reactors, while nobelium (atomic number 102) can only be produced in
particle accelerators.
To give you an idea of how precious these elements are, think about this: the price
of one gram of californium (the element having 98 protons in its nucleus) is about 60 million
dollars!
So… we have finally seen how all chemical elements of the periodic table formed.
It’s really amazing to see how different their origin was… and we can say that it’s
because of these differences between them that the Universe look so diverse!
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