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
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)
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!
• Hydrogen “burning” = the main energy
source of stars over long, long time (e.g., for
Sun, 10 billion years).
• Later stages of Nuclear fusions in Stars
Cosmic abundance
Not much Li (H+He), Be, B (Be+H or
He+Li)…
Cosmic Abundance
• Big Bang + nuclearsynthesis by Stars created the current distribution of
chemical elements.
o Hydrogen is the most abundant element in the Universe
o Helium is the next abundant, but it is inert
o 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
• 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 Abundance
Hydrogen
Helium
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Argon
After many, many recycles
Noble gases (he, Ne, Ar, etc.) do
not participate in chemical reactions.
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  Carbonbased life!
Any chemical compounds contain C
 “organic compound”
Why Carbon-based?
Any other base element?
C60 (Bucky-balls)
Requirements for the base element:
o Abundant element
o Combine easily with itself and others
• Advantages of Carbon!
Let’s consider all different ways of combining with
Hydrogen…
1. Oxygen : H2O and H2O2
2. Nitrogen : NH3 (ammonia) and N2H2
(hydrazine)
3. Carbon : so many different ways…
E.g., C90H84 or C167H336, …
# of possible isomers for C167H336 = 9×1083
Entirely made of Carbon.
(superconductor material, etc.)
? Can this may be due to the difference in the
maximum number of bondings for C (No=4),
O (No=2), N (No=3)?
Why not Silicon?
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!
• Si has four “bonds” similar to Carbon.
• Then why not Si-based life?
For example, CH4 (methane), SiH4 (silane).
o Si-Si bond strength is ½ of the C-C bond
o 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.)
o 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.)
Is Water (H2O) the only choice of
fluid for life in the Universe?
Solvents
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
• Requirements for a good solvent:
1. A solvent must be abundant!
2. Remain liquid for a wide range of Temp!
3. Dissolve a wide variety of chemical
compounds!
• Two other possibilities
1.
2.
Ammonia (NH3)
Methyl alcohol (CH3OH)
Solvent
T range for liquid
Total range of T
water
0 to 100 C
100 C
ammonia
-78 to -33 C
45 C
methyl alcohol
-94 to +65 C
159 C
methane
-182 to -164 C
18 C
ethane
-183 to -89 C
94 C
At low temperature, chemical reactions are slower hence slower metabolism.
Unique characteristics of Water
• Heat capacity and heat of vaporization  help to regulate temperature of life.
Water
Ammonia
M. Alcohol
Heat
Capacity
1
1.23
0.6
Heat of
Vapor.
595
300
290
• 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!
• Non-water based life forms do not expand upon freezing 
space travel in hibernation!
Requirements for Life
• Liquid Water as the best solvent for life
1. A wider, higher range of temperature for being
liquid
2. Solid water floats over liquid water
3. Polar molecule  dissolve certain types
(polar) of molecules only : cell membrane
4. Water is the most common liquid (H is the
most abundant element and Oxygen is the 3rd
most common element in the Universe)
5. Heat capacity  regulate temperature
Environmental Requirements for Life
1.
2.
3.
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?
Conservatively…
A habitable world = only if it has a liquid water!
Source of Energy and
Carbon
• Classification of metabolic sources
o Carbon
 Heterotroph : by eating…
 Autotroph : self-producing…
o Energy
 Photo- : from sunlight
 Chemo- : either from food or inorganic
material
Classification
Carbon
source
Energy source
photoautotroph
CO2
sunlight
chemoautotroph
CO2
inorganic chemical
photoheterotroph
food
sunlight
chemoheterotroph
food
food
Examples
plants
in extreme environ.
bacteria/archaea
animals
In summary…
Important Concepts
Important Terms
• 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
• Cosmic abundance (nuclear
synthesis)
Chapter/sections covered in this lecture : 5.3