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? 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” = the main energy source of stars over long, long time (e.g., for Sun, 10 billion years). 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 4 Hydrogens  Helium + energy US yearly energy consumption: 1.107×10 20 Joules = can light Sanford stadium for 100 billion years! = can be produced from a fusion of mere 80lb of Hydrogen!

Cosmic abundance Later stages of Nuclear fusions in Stars Later stages of Nuclear fusions in Stars 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. 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 3 rd, Carbon is the 4 th, Neon is the 5 th, and Nitrogen is the 5 th 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! Elements want to have electrons in certain numbers! 2, 10, 18, etc. 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 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 Ionic bond : Example of a chemical bonding that makes up salt covalent bond : sharing an electron (s)

Examples of Good pairs are H + F  Hydrogen Floride Na + Cl  Sodium Chloride (salt) or 2H + O  H 2 O

Cosmic Abundance After many, many recycles HydrogenHeliumCarbonNitrogenOxygenFluorineNeonSodiumMagnesiumAluminumSiliconPhosphorusSulfurArgon 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. 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 H 2 O 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? Requirements for the base element: o Abundant element o Combine easily with itself and others Advantages of Carbon! Advantages of Carbon! Let’s consider all different ways of combining with Hydrogen… 1.Oxygen : H 2 O and H 2 O 2 2.Nitrogen : NH 3 (ammonia) and N 2 H 2 (hydrazine) 3.Carbon : so many different ways… E.g., C 90 H 84 or C 167 H 336, … # of possible isomers for C 167 H 336 = 9×10 83 ? Can this may be due to the difference in the maximum number of bondings for C (No=4), O (No=2), N (No=3)? C 60 (Bucky-balls) Entirely made of Carbon. (superconductor material, etc.)

Why not Silicon? Si has four “bonds” similar to Carbon. Si has four “bonds” similar to Carbon. Then why not Si-based life? Then why not Si-based life? For example, CH 4 (methane), SiH 4 (silane). For example, CH 4 (methane), SiH 4 (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 SiO 2 (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.) 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!

Is Water (H 2 O) the only choice of fluid for life in the Universe?

Solvents Requirements for a good solvent: 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 Two other possibilities 1.Ammonia (NH 3 ) 2.Methyl alcohol (CH 3 OH) Is water (H 2 O) 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 Solvent T range for liquid Total range of T water0 to 100 C100 C ammonia-78 to -33 C45 C methyl alcohol-94 to +65 C159 C methane-182 to -164 C18 C ethane-183 to -89 C94 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. 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. 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! 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! Non-water based life forms do not expand upon freezing  space travel in hibernation! WaterAmmonia M. Alcohol Heat Capacity Heat of Vapor

Requirements for Life Liquid Water as the best solvent 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 3 rd most common element in the Universe) 5.Heat capacity  regulate temperature

Environmental Requirements for Life 1.Must have a source of molecules 2.Must have a source of energy 3.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 Classification of metabolic sources o Carbon  Heterotroph : by eating…  Autotroph : self-producing… o Energy  Photo- : from sunlight  Chemo- : either from food or inorganic material

In summary… Important Concepts 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 Important Terms Cosmic abundance (nuclear synthesis) Chapter/sections covered in this lecture : 5.3

How can protons be compacted into a so tiny space? Coulomb repulsive force Coulomb repulsive force

Four Fundamental Forces in Nature