What Is The Origin Of The Elements?

0:00This is the periodic table of the chemical elements.

0:02Beautiful, isn’t it?

0:04I bet you have already seen it hundred times.

0:07There are 118 chemical elements in this table.

0:11The elements are ordered according to their atomic number, which is the number of protons

0:16in their nucleus.

0:17The lightest elements (hydrogen, helium) are those with lowest atomic number, which means

0:24lowest number of protons; and as we move along the rows and the columns of the table, the

0:29atomic number increases.

0:30But… have you ever asked yourself, what is the origin of all these elements?

0:35How are they produced?

0:36Do they form all in the same way, or through different mechanisms?

0:40Watch this video and you will find out the answer!

0:44Hydrogen, helium, carbon, oxygen… all these names are very familiar to us.

0:57These are some of the most common chemical elements that we can find on Earth, and also

1:02in the Universe.

1:03But have you ever wondered how did they form?

1:06Did they all exist from the Big Bang, or did they form through some specific processes?

1:11Which of them are naturally occurring and which ones are not?

1:15To answer this question, we need to analyse the periodic table starting from the lightest

1:20elements: hydrogen and helium.

1:23Hydrogen is the lightest element in the Universe: one atom of this element contains only one

1:27proton and one electron.

1:28There are also two slightly different “versions” of this element, known as “isotopes”:

1:33the deuterium, in which the nucleus has one proton and one neutron, and the tritium, where

1:38the nucleus has one proton and two neutrons (all of them have one electron as well).

1:45Helium is the second lightest element: an atom of helium contains two protons and two

1:49electrons (and a few neutrons too, depending on the isotope).

1:53These two are the simplest chemical elements, and also the most abundant ones: in fact,

1:58together they account nearly all ordinary matter in the Universe.

2:02And they are also the oldest!

2:03In fact, the origin of these two elements dates back to the very beginning of the Universe,

2:08about 14 billion years ago: the Big Bang.

2:11Immediately after the Big Bang, the Universe was very hot and dense.

2:15Too much, indeed: in these conditions, chemical elements were not able to form yet, because

2:20protons, neutrons and electrons were moving too fast and they were not able to recombine

2:25together and to form atoms.

2:27As the early Universe kept expanding, it also kept cooling down.

2:31As the temperature decreased, particles started moving slower and slower, and the temperature

2:36became low enough for protons and neutrons to form hydrogen and helium nuclei: this process

2:42is called nucleosynthesis, and it occurred in the first 3 minutes of life of the Universe.

2:47At this point, however, electrons were still moving too fast, and the nuclei were not able

2:53to capture them to form atoms.

2:55For this process to occur, we need to wait approximately 380,000 years after the Big

3:00Bang.

3:01At that time, in fact, the temperature became low enough to allow nuclei to capture electrons

3:06and finally form the very first atoms of hydrogen and helium: this moment is known as “recombination”.

3:13Very tiny amounts of lithium (the third lightest element, having 3 protons and 3 electrons)

3:19were also produced at this time.

3:22So, this is how hydrogen and helium were formed.

3:25The next three elements in the periodic table (lithium, beryllium and boron, having respectively

3:303, 4 and 5 protons in their nuclei), have a totally different origin.

3:34In fact, these three elements are mostly produced in a process known as “cosmic ray spallation”.

3:41What is it?

3:42You may already know that the Earth is constantly bombarded by high-energy particles (mainly,

3:47but not only, protons) coming from space, known as “cosmic rays”.

3:52When these cosmic rays reach our atmosphere, they interact with the atoms and molecules

3:56present there, breaking them apart and leading to the formation of new atoms and particles

4:01through chain reactions.

4:03Most of the lithium, beryllium and boron that we find on Earth are produced exactly through

4:07this mechanism.

4:08That’s also why scientists observe a “shortage” of these three elements when they analyse

4:13the composition of the Universe: it’s because they are almost only produced through these

4:18collisions between cosmic rays and other particles, so they are relatively less abundant than

4:24other elements, in the Universe.

4:25Ok, we have seen how the five lightest elements formed.

4:29What about the others?

4:31To understand the origin of the heavier elements, we have to look at what happens inside stars.

4:36A star can be thought as a giant “power plant”, producing enormous quantities of

4:41energy all the time.

4:43The process responsible for the creation of such huge amounts of energy is the nuclear

4:48fusion, which occurs in the core of the star.

4:51In the simplest nuclear fusion reaction, two nuclei of hydrogen (so, two protons) fuse

4:56together into a nucleus of helium.

4:59However, the mass of the final nucleus of helium is smaller than the sum of the initial

5:04masses of the two hydrogen nuclei: this means that part of the mass has been converted into

5:10energy, according to Einstein’s famous equation , where is the mass that has been converted,

5:18is the speed of light and is the energy released.

5:22If you plug in numbers, you will find out that even for a very tiny amount of mass converted,

5:27the amount of energy released is huge!

5:30With this nuclear fusion process, therefore, helium nuclei are produced.

5:34This continues for billion years, until the core of the star runs out of hydrogen.

5:39What next?

5:40Well, the star can then “change” type of fuel… and it starts burning helium instead!

5:45In fact, when the conditions of temperature and pressure become right, helium fusion occurs:

5:51nuclei of helium fuse together forming nuclei of heavier elements, such as carbon (6 protons)

5:57and oxygen (8 protons), and releasing energy again.

6:01So, with this new type of nuclear fusion, the star is able to continue to produce energy

6:06and live for a while.

6:08And what happens when the star runs out of helium as well?

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6:26So, what happens when a star runs out of helium?

6:30For low-mass stars, such as our Sun, this means the end of their life: nuclear fusion

6:35stops, and as there is no more “fuel” to be burnt, the star is no longer able to

6:40produce energy.

6:41As a consequence, the outer layers of the star are expelled into space, forming a “planetary

6:47nebula”, while the core remains as a white dwarf.

6:50For high-mass stars, however, something different happens.

6:53In fact, in the collapsing core of these enormous stars, very extreme conditions of temperature

6:58and pressure are reached.

7:00And in these extreme conditions, it becomes possible for nuclei of elements heavier than

7:04helium to undergo nuclear fusion.

7:07So, in a sort of cascade process, the star can burn in sequence carbon, oxygen, nitrogen,

7:13and so on.

7:15As each of these elements undergo nuclear fusion, nuclei of heavier elements are produced.

7:20However, this process does not continue forever.

7:23In fact, the last element that is produced by this mechanism is iron: a nucleus of iron

7:28has 26 protons.

7:31Iron is quite a special element.

7:32In fact, the nuclear fusion of iron is not energetically convenient.

7:37What does it mean?

7:38We say previously that in the fusion of hydrogen into helium, energy is released, because the

7:43mass of the final product is less than the sum of the masses of the initial nuclei.

7:48The same is true for the nuclear fusion of helium, carbon, oxygen, and so on.

7:53However, for iron, this is not true.

7:55In fact, when two nuclei of iron fuse together… they absorb energy, instead of releasing it!

8:02This means that when the core is full of iron, the star is no longer able to produce energy…

8:08because the nuclear fusion of iron does not produce energy.

8:11So, this is the beginning of the end, for the star: the core collapses once more, and

8:16it eventually explodes in what is known as “supernova event”.

8:20So, nuclear reactions in the core of stars are able to produce all elements up to iron,

8:26which has an atomic number of 26.

8:28Then, what about all the heavier elements?

8:31How are they produced?

8:33Elements heavier than iron cannot be produced through nuclear fusion, because as we have

8:37seen, this process is not energetically convenient.

8:41In order to be produced, these elements require conditions even more extreme than those found

8:46inside the core of stars.

8:48And these conditions are found when supernova explosions happen.

8:52In a supernova explosion, the material in the outer layers of the star is violently

8:56expelled to the outer space.

8:59During this powerful explosion, the nuclei of the elements previously produced in the

9:04nuclear reactions inside the core continuously collide with different particles, such as

9:09neutrons (also present in the star).

9:12Therefore, thanks to the extreme conditions, the nuclei of these elements are able to “capture”

9:17some of these neutrons.

9:19When this occurs, these nuclei transform into heavier isotopes of the same element (remember:

9:25isotopes are nuclei having same number of protons, but different number of neutrons).

9:31As these nuclei accumulate neutrons, they become more and more unstable, and eventually

9:37decay in a process known as beta-decay, in which neutrons turn into protons.

9:42As a result, some of the nuclei undergoing this process will eventually end up having

9:47more than 26 protons (so, heavier than iron!).

9:50This phenomenon is known as “r-process”, or “rapid neutron-capture process”.

9:57There is also a variation of this process, known as “s-process”, or “slow neutron-capture

10:02process”, which is essentially the same but occurs much slower, and so the nuclei

10:07decay before being able to capture too many neutrons.

10:11During a supernova explosion, there is one more process that contributes to the production

10:15of elements heavier than iron.

10:17This is known as “p-process”, in which the nuclei expelled during the supernova event

10:23capture protons instead of neutrons, producing therefore nuclei of elements heavier than

10:29iron.

10:30So, summarizing: hydrogen and helium were produced after the Big Bang.

10:34Lithium, beryllium and boron are produced through cosmic ray spallation.

10:39Elements up to iron are produced in the nuclear fusion reactions inside the core of high-mass

10:44stars, while heavier elements are produced during supernova explosions.

10:48So, did we explained the origin of all the elements in the periodic table?

10:53Not yet.

10:54In fact, the heaviest element that can be produced during a supernova explosion is plutonium.

10:59A nucleus of plutonium has 94 protons.

11:02However, if we look again at the periodic table, we see that there are a few elements

11:07heavier than plutonium.

11:08So the question is, how are those elements produced?

11:12The answer is: not naturally…

11:13In fact, they are produced by humans!

11:16Yes, all the elements with more than 94 protons do not exist in nature.

11:20All of them are synthetic, which means that they have been created in laboratory.

11:26Producing atoms of these heavy elements can only be done exploiting difficult and expensive

11:30processes.

11:31For instance, americium (atomic number 95) is produced by bombarding uranium and plutonium

11:37with neutrons in nuclear reactors, while nobelium (atomic number 102) can only be produced in

11:43particle accelerators.

11:45To give you an idea of how precious these elements are, think about this: the price

11:50of one gram of californium (the element having 98 protons in its nucleus) is about 60 million

11:57dollars!

11:58So… we have finally seen how all chemical elements of the periodic table formed.

12:02It’s really amazing to see how different their origin was… and we can say that it’s

12:07because of these differences between them that the Universe look so diverse!

12:12"This video ends here!

12:14Thanks for watching everyone!

12:15Did you find the origin of chemical elements interesting?

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