carbon dating problems
Carbon and Organic Chemistry
The phrase "carbon-based life forms," is often used in science-fiction books and movies by aliens to describe the creatures of Earth, the only planet known to support life. Not only all living things contain carbon but also carbon is in plenty of things that were once living and now are the source for living, which makes it useful for dating the remains of past settlements on Earth. Of even greater usefulness is petroleum, a substance containing carbon-based forms that died long ago, became fossilized, and ultimately changed chemically into fuels. Then again, not all materials containing carbon were once living creatures; yet because carbon is a common denominator to all living things on Earth, the branch of study known as organic chemistry is devoted to the study of compounds containing carbon. Though among the most important organic compounds are the many carboxylic acids that are vital to life, carbon is also present in numerous important inorganic compounds, most notably its small but unavoidable compounds such as carbon dioxide, carbon monoxide and carbonates.
Carbon's name comes from the Latin word carbo, or charcoal which, indeed, is almost pure carbon. Its chemical symbol is C is generally crowned with its atomic number of 6, meaning that there are six protons in its nucleus. Its two stable isotopes are 12C, which constitutes 98.9% of all carbon found in nature, and 13C, which accounts for the other 1.1%. The mass of the 12C atom is the basis for the atomic mass unit (amu), by which mass figures for all other elements are measured: the amu is defined as exactly 1/12 the mass of a single 12C atom. The difference in mass between 12C and 13C, which is heavier because of its extra neutron, account for the fact that the atomic mass of carbon is 12.01 amu: were it not for the small quantities of 13C present in a sample of carbon, the mass would be exactly 12.00 amu.
Carbon makes up only a small portion of the known elemental mass in Earth's crust, oceans, and atmosphere (just 0.08%, or 1/1250 of the whole) yet it is the fourteenth most abundant element on the planet. In the human body, carbon is second only to oxygen in abundance, and accounts for 18% of the body's mass. Thus if a person weighs 100 lb (45.3 kg), he/she is carrying around 18 lb (8.2 kg) of carbon, interestingly the same material from which the cherished and much valued diamonds are made. Present in the inorganic rocks of the ground and in the living creatures above it, carbon is everywhere. Combined with other elements, it forms carbonates, most notably calcium carbonate (CaCO3), which appears in the form of limestone, marble, and chalk. In combination with hydrogen, it creates hydrocarbons, present in deposits of fossil fuels: natural gas, petroleum, and coal. In the environment, carbon, in the form of carbon dioxide (CO2), is taken in by plants, which undergo the process of photosynthesis and release oxygen for sustenance of animals that breathe in oxygen and release carbon dioxide to the atmosphere.
We know that carbon forms tetravalent bonds, and makes multiple bonds with a single atom. In addition, we have mentioned the fact that carbon forms long chains of atoms and varieties of shapes. But how does it do these things, and why? These are so good questions that an entire branch of chemistry, organic chemistry, is devoted to answering these theoretical questions, as well as to determining solutions to a host of other, more related and practical problems. Organic chemistry is the study of carbon, its compounds, and their properties. At one time, chemists thought that “organics” were synonymous with "living," and even as recently as the early nineteenth century, they believed that organic substances contained a supernatural "life force." Then, in 1828, German chemist Friedrich Wöhler cracked the code that distinguished the living from the nonliving species, and the organic from the inorganic.
Wöhler took a sample of ammonium cyanate (NH4OCN), and by heating it, converted it into urea (H2N-CO-NH2), a waste product in the urine of mammals. In other words, he had turned an inorganic material into an organic one, and he did so, as he observed, "without benefit of a kidney, a bladder, or a dog." It was almost as though he had created life. In fact, what Wöhler had glimpsed—and what other scientists who followed came to understand, was this: ‘what separates the organic from the inorganic is the manner in which the carbon chains are arranged’.
Ammonium cyanate and urea have exactly the same numbers and proportions of atoms, yet they are different compounds. They are thus isomers: substances which have the same formula, but are different chemically. In urea, the carbon forms an organic chain, and in ammonium cyanate, it does not. Thus, to reduce the specifics of organic chemistry even further, it can be said that this area of the field constitutes the study of carbon chains, and ways to rearrange them in order to create new substances.
Rubber, vitamins, cloth, and paper are all organically based compounds we encounter in our daily lives. In each case, the material comes from something that once was living, but what truly make these substances organic in nature is the common denominator of carbon, as well as the specific arrangements of the atoms. We have organic chemistry to thank for any number of things: aspirins and all manner of other drugs; preservatives that keep food from spoiling; perfumes and toiletries; dyes and flavorings, and so on.
It may not be out of context to mention that radiocarbon dating is used to date the age of charcoal, wood, and other biological materials as when an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the amount of the radioisotope (that is, radioactive isotope) carbon-14 that it receives from the atmosphere. As soon as the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 will begin to change as the carbon-14 decays to form nitrogen-14. Carbon-14 has a half-life of 5,730 years, meaning that it takes that long for half the isotopes in a sample to decay to nitrogen-14. Therefore a scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to guess the age of an organic sample. The problem with radiocarbon dating, however, is that there is a good likelihood the sample can become contaminated by additional carbon from the soil. Furthermore, it cannot be said with certainty that the ratio of carbon-12 to carbon-14 in the atmosphere has been constant throughout time.
About the Author
Dr. Badruddin khan teaches Chemistry in the University of kashmir, Srinagar, India.
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