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Chemistry of Life (Origin of the Elements). ASTR 1420 Lecture #3 Section 3.2 & 5.3. There are 103 elements known to exist. Yet, all known life forms are mainly based on C, H, O, & N, and most advanced organisms are using H 2 O! Why?

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chemistry of life origin of the elements

Chemistry of Life (Origin of the Elements)

ASTR 1420

Lecture #3

Section 3.2 & 5.3


There are 103 elements known to exist.

Yet, all known life forms are mainly based on C, H, O, & N, and most advanced organisms are using H2O! Why?

Why shouldn’t there be life forms with iron skeleton and using methanol?

origin of the elements
Origin of the Elements

After Big Bang, lightest elements (H, He, Li, Be) were created, but only for a limited time period (3 to 20 minutes).

92% H + 8% He and almost no other elements.

creation of the elements nuclear synthesis
Creation of the Elements(Nuclear Synthesis)
  • Hydrogen “burning” = the main energy source of stars over long, long time (e.g., for Sun, 10 billion years).

Hydrogen burning :

4 Hydrogens Helium + energy

US yearly energy consumption:

1.107×1020 Joules

= can light Sanford stadium for 100 billion years!

= can be produced from a fusion of mere 80lb of Hydrogen!

cosmic abundance
Cosmic abundance
  • Later stages of Nuclear fusions in Stars

Not much Li (H+He), Be, B (Be+H or He+Li)…

cosmic abundance1
Cosmic Abundance
  • Big Bang + nuclearsynthesis by Stars created the current distribution of chemical elements.
    • Hydrogen is the most abundant element in the Universe
    • Helium is the next abundant, but it is inert
    • Oxygen is the 3rd, Carbon is the 4th, Neon is the 5th, and Nitrogen is the 5th most abundant elements. Neon is inert.

 Therefore, if one wants to make something out of the current mix of elements in the Universe, the most efficient outcome will be a compound of H, O, C, N.

chemical bonding
Chemical bonding
  • Elements want to have electrons in certain numbers!

2, 10, 18, etc.

  • Thus an element with 11 electrons wants to get rid of one electron while an element with 7 electrons wants to accept one more electron from somewhere

 Chemical reactions

covalent bond : sharing an electron (s)

Ionic bond : Example of a chemical bonding that makes up salt


Examples of Good pairs are

H + F  Hydrogen Floride

Na + Cl Sodium Chloride (salt)

or 2H + O  H2O

cosmic abundance2
Cosmic Abundance

After many, many recycles















Noble gases (he, Ne, Ar, etc.) do

not participate in chemical reactions.

earth life is carbon based
Earth Life is Carbon-Based
  • Life on Earth is made from more than 20 chemical elements, but four elements make up about 96% of the mass.

(right figure) chemical composition of the human body by weight.

Oxygen  mostly in H2O

Cell structures and their functions are due to Carbon Carbon-based life!

Any chemical compounds contain C  “organic compound”

why carbon based any other base element
Why Carbon-based? Any other base element?

Requirements for the base element:

    • Abundant element
    • Combine easily with itself and others
  • Advantages of Carbon!

Let’s consider all different ways of combining with Hydrogen…

    • Oxygen : H2O and H2O2
    • Nitrogen : NH3 (ammonia) and N2H2 (hydrazine)
    • Carbon : so many different ways…

E.g., C90H84 or C167H336, …

# of possible isomers for C167H336 = 9×1083

? Can this may be due to the difference in the maximum number of bondings for C (No=4), O (No=2), N (No=3)?

C60 (Bucky-balls)

Entirely made of Carbon. (superconductor material, etc.)

why not silicon
Why not Silicon?
  • Si has four “bonds” similar to Carbon.
  • Then why not Si-based life?

For example, CH4 (methane), SiH4 (silane).

    • Si-Si bond strength is ½ of the C-C bond
    • C-C, C-H, and C-O bond strengths are ~same, but Si-H and S-O bonds are stronger than Si-Si  hard to make a large complex structures (chain, rings, etc.)
    • At low temperature, nearly all Si atoms will form SiO2 (quartz!) and this rock-forming material are very difficult to dissociate!
  • Astronomical Evidence = Astronomers found no silicones or silanes in meteorites while many complex carbon-based molecules are seen in everywhere (meteorites, comets, interstellar clouds, etc.)

Although life based on silicon may be possible, it will be extremely uncommon at best!

 Life based on carbon seems favored as the dominant kind of life in the Universe!

  • Requirements for a good solvent:
  • A solvent must be abundant!
  • Remain liquid for a wide range of Temp!
  • Dissolve a wide variety of chemical compounds!
  • Two other possibilities
    • Ammonia (NH3)
    • Methyl alcohol (CH3OH)

Is water (H2O) inevitable?

Solvent : dissolve other chemical compounds (to transport nutrients and wastes in the cells)  other advanced alien life forms are very likely use solvent in their metebolism

At low temperature, chemical reactions are slower hence slower metabolism.

unique characteristics of water
Non-water based life forms do not expand upon freezing  space travel in hibernation!Unique characteristics of Water
  • Heat capacity and heat of vaporization  help to regulate temperature of life.
  • Surface tension : tendency of liquid to form a droplet! Water has the highest surface tension among all known liquids.Before cells evolved, surface tension would force some compounds together and would preserve the boundaries between inside/outside.
  • Unique properties of Water: larger volume when frozen!
  • Bursting pipes in winter by frozen water
  • Frozen ponds only on the surface!
requirements for life
Requirements for Life
  • Liquid Water as the best solvent for life
    • A wider, higher range of temperature for being liquid
    • Solid water floats over liquid water
    • Polar molecule  dissolve certain types (polar) of molecules only : cell membrane
    • Water is the most common liquid (H is the most abundant element and Oxygen is the 3rd most common element in the Universe)
    • Heat capacity  regulate temperature
environmental requirements for life
Environmental Requirements for Life
  • Must have a source of molecules
  • Must have a source of energy
  • Must have a liquid medium

Any worlds meet the #3 requirement, likely meet the first two requirements also.  Why?


A habitable world = only if it has a liquid water!

source of energy and carbon
Source of Energy and Carbon
  • Classification of metabolic sources
    • Carbon
      • Heterotroph : by eating…
      • Autotroph : self-producing…
    • Energy
      • Photo- : from sunlight
      • Chemo- : either from food or inorganic material
in summary
In summary…

Important Concepts

Important Terms

Cosmic abundance (nuclear synthesis)

  • Life on Earth relies on carbon chemistry and water solvent  not by accident but because of their inherent superiorities!
  • Cosmic abundance
  • Superiority of Carbon as the base element
  • Superiority of Water as solvent
  • Source of energy and carbon
  • Chapter/sections covered in this lecture : 5.3