Building blocks and glue

Carbon-based molecules are constructed from atoms held together by the sharing of electron pairs these are arranged in space in precise ways. Some molecules are nonsuperimposable on their mirror images and are said to be chiral that is, they are handed. Chirality matters. 

Molecules are constructed from atoms. Atoms, in turn, are constructed from more basic particles protons, neutrons, and electrons. Protons are very, very small objects that bear a very small positive charge. Neutrons weigh just slightly more than protons but carry no charge. Protons and neutrons are clustered together in the 

As we noted above, an element is a substance that cannot be decomposed into simpler substances by any chemical reaction. There are 92 elements that occur in nature, in widely varying amounts, and several more have been created in particle accelerators. In order to understand the molecules of life, we need to understand something about several of these elements, above all about carbon. 

Elements are defined by the number of protons in the nucleus of each atom. The number of nuclear protons is equal to the number of electrons orbiting the nucleus. The nucleus of carbon contains six protons. This value is known as the atomic number for carbon. In nature, carbon occurs largely in a form in which the nucleus also contains six neutrons. The atomic mass of carbon is defined as the sum of the number of protons pins neutrons. Consequently, this form of carbon is called carbon-12, or About 98.9% of carbon in nature is Most of the rest is carbon-13, and contains seven neutrons in the nncleus. Smaller amounts of carbon occur that contain five or eight neutrons. These are known, respectively, as carbon-11, and carbon-14, These variations on the theme of carbon are called isotopes. Carbon-11 and carbon-14 are radioactive and decay spontaneously carbon-12 and carbon-13 are stable. 

Elemental carbon, whether it is soot, diamond, graphite, buckyballs, or graphene, contains only carbon atoms, each of which has exactly six protons in its nucleus. Lead (Pb) is a metallic element. Lead metal contains only lead atoms, each of which contains exactly 82 protons in its nnclens. Neon gas, familiar in neon lights, contains only neon atoms and each of these has jnst 10 protons in its nucleus. Elements are the bnilding blocks ont of which all matter is constitnted. 

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