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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Case Studies > Case Study: The Natural Abundance of Elements

Case Study: The Natural Abundance of Elements

Many of the elements that comprise not only the Earth, but also humans and other living organisms have their origin in the beginning of the universe. Thus, the very hydrogen atoms in water molecules that we drink are ancient in age! The origin of the universe is inextricably connected to the elements that we find on Earth and in the universe because the particles that formed during the origin are related to the elements of Earth and to those in the universe. The first elements to come into existence at the universe's establishment were hydrogen and a lesser amount of helium.

Introduction - The Big Bang

The big bang was an explosive event that started it all nearly 13.7 billion years ago according to recent estimates (Michael Seeds, 2007). Without this event, atoms would not have existed. According to astronomers and the big bang theory, the contents of the universe were initially packaged into a very small point that  exploded and rapidly expanded to create the universe. Of course, there is evidence to support the big bang theory, one of which is the Doppler effect. Consider a police vehicle with an active siren speeding towards an individual. As the vehicle gradually approaches the sound waves emitted from the siren are compressed causing the wavelength to decrease and the frequency to increase. The siren's sound waves moves closer to you each time a new wave is emitted so one can imagine that these waves are very close to each other. As a result, one would hear a higher pitched siren. On the other hand, if the police vehicle is speeding away from the listener, the sound waves are no longer compressed and one would hear a lower pitched siren because the sound waves gradually move further apart from the listener as the vehicle moves away.

cartoon of sound waves from moving fire truck

                                                                               Courtesy:NASA.gov 2007 Official: Phil Newman 

 

Fortunately, the Doppler effect applies to light waves as well. One can see different colors depending on the frequency of the light source. Again, consider a light source that is speeding towards an observer. Utilizing the same concept explained in the police vehicle siren, one would expect the light waves to be close to each other. Since the color one sees depends on the frequency this short wavelength, high frequency wave would be a high frequency color, blue. Conversely, one would see red light if the light source is speeding away. These observations are conveniently called blue shifts and red shifts, respectively. In the early 1900's astronomers began studying the wavelength of light emitted by distant objects in space; remarkably the astronomers and among them Edwin Hubble, found that galaxies produced red shifts. Furthermore, Hubble found that all light from distant galaxies irrespective of orientation or direction displayed red shifts (Stephen Marshak, 2008). One can logically support the expanding character of the big bang theory through the Doppler effect.

receding waves redshifted, approaching waves 
blueshifted

                                                        Courtesy: NASA.gov 2007 Official: Phil Newman

 

After the big bang explosion, the universe was extremely hot and compact. Soon after, the universe had cooled enough to allow the smallest atoms to form. These were hydrogen atoms. According to estimates, once the universe reached an age of 3 minutes, its temperature had cooled and its diameter had grown to around 100 billion km (Marshak, 2008). More importantly, the cooling allowed new atoms to form. No longer jostling and agitated at the initial high temperature, the nuclei of the hydrogen atoms could collide and stay stuck together to create another small atom, helium. It is important to note that only small atoms were produced because once the universe reached an age of 5 minutes, it had expanded to such a degree that the atoms were too far distanced to collide frequently (Marshak, 2008). Over time, the universe consisted merely of electrons arbitrarily floating in space while its temperature continued to drop until molecules like H2 could form (the cooler temperature allowed atoms to bind and form molecules). As the temperature continued to fall, atoms and molecules proceeded to slow down and began to form amorphous groups of gas called nebulae. It is no surprise that the nebulae is made of small atoms like hydrogen and helium. 

Courtesy of University of California, Berkeley. Department of Astronomy. Professor: Martin White 

 (Since Helium is not seen below an abundance of 23%, this suggests the Universe was extremely hot in its early stages.)

The Universe, Stars and Planets

The aforementioned nebulae forms the stars and thus a solar system. Since the nebulae is a patchy cloud of gas it exerts gravity and it continued to pull in gas from the surrounding region. According to Stephen Marshak, this is a prime example of " 'the rich getting richer.' " In other words, as the nebulae pulled in more gases and increased its density, it was able to pull in larger amounts of gas. Over time, the motion of gas pushed the nebulae into a rotation that increases in speed. A great way to understand this concept is an ice skater who tucks in the arms to increase her speed in a spin. Eventually, the rotating dense center of gas became an accretion disk that caused an inward collapse of gas into the very center creating a hot ball of gas which ultimately becomes the star. The remaining gases surrounding the newly formed sun developed into concentric disks that will ultimately form a planet. The material in these rings condense and accrete  (smaller chunks collide and remain stuck to each other forming a larger piece) to form a body of sufficient mass to be labeled a planetesimal. Usually planetesimals are interstellar bodies that have a diameter greater than 1km (Seeds, 2008). Just as the nebulae grew in size by pulling in more gas, a planetesimal can transition into a protoplanet by pulling in more material around itself. The following image illustrates the central protostar and concentric rings forming around it as premature orbits. Indeed, this process called the solar nebula theory has validity because we are able to see distantly forming solar systems with a dim central protostar surrounded by gas. The following images represent what we think an accretionary disk looks like while the second image is an actual image of other protostars and planets.

 

  

Courtesy of NASA. 2007 Updated: Silvia Stoyanova. 

Courtesy of the Hong Kong Space Museum, Deparment of Physics. 2010.

Why is the central protostar hot?

The kinetic energy of the inward collapsing gases were instantly changed into heat energy. Moreover, the increasing density caused by the inward collapse of gases and the proceeding addition of these gases contributed to the high temperature. The newly formed, hot protostar continues to grow by continuing to pull in more gas from the surrounding space thus increasing the temperature until nuclear fusion reactions - the combination of hydrogen nuclei to form helium nuclei - occur. Thus, the protostar has ignited and a star is formed. 

Stars Produce Elements - Nucleosynthesis

Now, you know that the first stars produced mainly hydrogen and helium, however the present universe contains many more elements. The origin of these elements we are so familiar with today like carbon or silicon were formed during the life of stars. Specific reactions within the star like fusion reactions help fuse small nuclei to make larger nuclei. Moreover, the type of reactions that occur in a star depend on the mass of the star. With greater mass, the star will be more dense and hold more heat. With higher temperatures particle velocity greatly increases to allow larger nuclei to collide and form even larger nuclei. For example, the sun in our solar system is low in mass relative to other known stars and produces elements up to atomic number 26 (Marshak, 2005). Sometimes, large atoms all for cataclysmic circumstances to form like supernova explosions. These explosions actually cause neutrons to collide to other atoms and decay into a proton adding to the atom's mass which forms a new element. This process of combining lighter nuclei to form heavier nuclei is called nucleosynthesis.

 

Diagram of nucleosynthesis in stars

Courtesy of NASA 2010. Example of nucleosynthesis within a star.

 

Over time, as stars live and form elements they eventually die and release gas or undergo a supernova explosion which hurdles matter out into space. In their wake, these atoms in space will form new nebulae and mix with other surrounding atoms. As a result, nebulae increase in diversity with each succeeding generation of stars. Once a star lives and dies, it contributes unique elements to the succeeding generation of stars. In turn, these stars will live and die to contribute elements to the next generation and so on.

Elemental Abundance in the Universe

Stars make elements throughout their life and contribute them to the interstellar space at death. Since we know that the first atoms made by the earliest stars were hydrogen and helium, one can predict that hydrogen and helium are the most abundant elements in the universe. Indeed, hydrogen dominates our solar system comprising around 75% of the matter. Helium also contributes a significant amount. Together, hydrogen and helium make up 98% all the matter in our solar system. Other abundant elements like carbon and oxygen were finally produced in later generations of stars. The carbon, nitrogen, oxygen, and even calcium atoms were all created by the star factories.Notice the dominance of hydrogen in the solar system in the following chart. Most of the elements that comprise our bodies are "impurities" in the solar system when compared to hydrogen and helium.

                              Courtesy of NASA.gov 2010. 

Different elements in different areas - Condensation Sequence

Although elements like hydrogen and helium are the most abundant in the universe, one will find different types of matter. For example, in our solar system there are two types of planets terrestrial and Jovian planets. Different materials make up the composition of these planets thus giving our solar system diversity in elemental abundance. Planets like Mars and the Earth - those that are closer to the sun - are composed mainly of metals and rock. This is a direct result of higher temperatures from proximity to the sun causing only metals and rocky silicates to condense. Farther out in the solar system are the Jovian planets like Jupiter and Saturn that are composed primarily of gases and ices because the farther the distance from the sun the colder it becomes in space. As a result, only ices of elements like methane, nitrogen, and ammonia could condense. Moreover, since we know that hydrogen and helium are the most abundant elements these Jovian planets took advantage of this surplus to grow to enormous sizes. This sequence of heavier elements near the sun versus low-density ices far from the sun is called the condensation sequence. This distance from the sun is critical in determining the composition of planets and the characteristics that may dominate that planet. 

   Condensation Sequence and visualization of the "Frost Line".

  © 2010 National Earth Science Teachers Association

 

Elements on Earth

The Earth is a geologically dynamic planet. It is composed of heavy elements in contrast to the lighter, low-density elements found in the Jovian planets. Earth is composed of iron (35%), oxygen (30%), silicon (15%), and magnesium (10%) mainly. The other 10% is composed of the other 88 naturally occurring elements (Marshak, 2008). The most common materials in the Earth are silicate minerals which is simply silicon mixed with oxygen; the mixture is also mixed with other elements like iron, magnesium, aluminum, or potassium. This silicate mineral is the main constituent of Earth's silicate rocks.

      

            ELEMENTAL ABUNDANCE IN THE EARTH         

 

ELEMENT ABUNDANCE: % BY MASS
iron 35
oxygen 30
silicon 15
magnesium 13
nickel 2.4
sulfur 1.9
calcium 1.1
aluminum 1.1
other <1

  Charts Provided by NASA and Associated Partners, 2010

Where Can I Find These Elements?

We have explained that hydrogen and helium dominate our solar system. We have also described the elemental composition of the Earth relative to the elemental abundance in the Universe. But where exactly is most of this hydrogen and helium? Since hydrogen and helium dominate and that star formation originates from a nebulae, it is only logical to find that the composition of the sun, which is the largest body in our solar system is mainly hydrogen and helium with very small traces of heavier elements, around 2% (Seeds, 2008). So, these elements in the sun must have been the original nebular gases. Iron is also abundant in the universe and we find much of it in the Earth. However, the Jovian planets also have metallic cores, albeit smaller cores, since a planet's structure is dictated by the density of the materials; denser materials differentiate toward the center like iron or nickel. We also find iron in the core of meteorites in the solar system. Since it's in the center, it supports the differentiation of materials during planetary formation. Iron is also very abundant! If one would like to know where to find ices of methane or ammonia, one would use the condensation sequence to narrow down which planets to study. For instance, if you were to search for a rocky planet it would be best to look for a planet closer to the sun and is not beyond the "frost line." On the other hand, gas giant planets filled with ices and gas are normally found far from the sun and beyond the frost line like Uranus and Neptune. 

There are also some peculiar cases like on the moons of Jupiter and Saturn. For instance Io, one of Jupiter's moons exhibits active volcanoes on its surface and continues to blast out ash. These volcanoes release massive amounts of oxygen gas, which is something common on the Earth. Io's atmosphere also smells like sulfur. Another unique case is the moon system of Saturn.One of these moons, Titan has giant lakes of liquid methane and also spews out large amounts of methane ice. On Enceladus, another of Saturn's moons actually spews out icy particles and organic materials. There is also strong evidence for powerful heat sources in the interior. Combining heat, organic materials, and possibly liquid water, Enceladus has the potential to harbor life. How is all this possible so far away from the sun, which is a primary source of heat? 

Tvashtar Catena, Io

                 Active eruption on Io taken by NASA's Galileo spacecraft. Courtesy of NASA 2010.

These moons represent separate condensation sequences and the mini solar system character of the Jovian planets. As these planets were forming long ago, they also had their own concentric rings of material surrounding the dense, central protoplanet. The powerful gravitational pull of Jupiter and Saturn provides a constant heat source to drive geological processes on these moons.

Saturn's Moons

                         Courtesy of the Jet Propulsion Lab in California. NASA 2010.

References

  1. Cox, P.A. "The Elements on Earth: Inorganic Chemistry in the Environment." New York: Oxford University Press, 1995 
  2. Seaborg, Glenn Theodore. "Elements of the Universe." New York: Dutton, 1958   
  3. Seeds, Michael. "The Solar System." Boston: Brooks/Cole, Cengage Learning, 2011. 
  4. Marshak, Stephen. "The Earth: Portrait of a Planet." New York: W.W. Norton & Company, Inc., 2005.

Practice Problems

1. What two elements is the universe mostly comprised of?

     A. Carbon and Hydrogen

     B. Hydrogen and Helium

     C. Oxygen and Hydrogen

     D. Phosphorous and Oxygen

2. Can we predict the types of planets of other solar systems? If so, how?

3. What is evidence for the Big Bang? How does the Big Bang figure into our lives?

4. Explain the difference between blue shifts and red shifts and what they represent.

5. Where did the material making up your body and the Earth for that matter originate?

6. Is it possible to form newer elements in the future? If so, how would these elements be formed?

7. How are Jovian Planets similar to a star?

8. Can we conclude that other solar systems were formed the same way ours was? Support your answer with evidence and logical explanations.

Contributors

  • Lea Porteous (UCD)
  • Charlie Wang (UCD)

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Last Modified
20:14, 27 Jun 2014

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