Space and time began about 13.7 billion years ago but it consisted almost exclusively of hydrogen until some stars grew sufficiently massive to create the heavier elements in gigantic explosions called supernovae.
The Sun formed about 4.6 billion...
All life on Earth depends on a small number of elements. Most of the mass of the human body (nearly 99%) consists of only six elements: oxygen 65%, carbon 18.5%, hydrogen 9.5%, nitrogen 3.2%, calcium 1.5%, and phosphorus 1.0%. Another 0.85% is composed of five elements: potassium 0.4%, sulfur 0.3%, sodium 0.2%, chlorine 0.2%, and magnesium 0.1%.
The remaining mass (less than 10 grams on average) consist of traces of iron (for blood cells), fluorine, zinc, copper, iodine (which is necessary for brain development and the proper function of the thyroid gland (for this reason it is added to table salt in the form of potassium iodide)), selenium, chromium, manganese (a vital component of all cells), molybdenum, lithium and cobalt (for vitamin B12).
This means life requires a total of 22 elements. However, the human body contains detectable traces of 60 elements (some of which may not be necessary).
Of course, elements are not used in a pure form but are combined in an enormous number of compounds (molecules) some of which are extremely complex. One of the simplest is water which makes up over half of the average adult human body and this is made of oxygen and hydrogen. The human body contains more oxygen by mass than any other element, but more hydrogen by the number of atoms. An average adult human body contains approximately 7×1027 atoms. (7×1027 is 7 billion, billion, billion atoms).
Alcohols, sugars, fats, lignans, chitins and other biologically active compounds are combinations of carbon, oxygen and hydrogen. Adding nitrogen and sulfur, forms antibiotics and amino acids. And with phosphorus, these compounds form ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) molecules, the code for all life, and adenosine triphosphate (ATP) (chemical formula: C10H16N5O13P3), the main energy-transfer molecule in every living cell. The formula means that one molecule of ATP contains 10 carbon, 16 hydrogen, 5 nitrogen, 13 oxygen, and 3 phosphorous atoms. The diagram shows how these are arranged.
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ATP (Adenosine triphosphate) consists of three components, a nitrogenous base (adenine), the sugar ribose, and a triphosphate. It is a precursor of DNA and RNA, used to make protein, and is an essential component of muscles and nerves (see part - 6).
The basic unit of each element is the atom which has a nucleus containing a fixed number of protons and a variable number of neutrons. Each nucleus is normally surrounded by a number of electrons equal to the number of protons, as the electrons have a negative charge and are attracted by the positively charged protons. The neutrons have no charge.
The electrons may be visualized occupying a series of shells at increasing distances from the nucleus. Shell 1 contains a maximum of 2 electrons. Shell 2 a maximum of 8. Shell 3 a maximum of 18 and shell 4 a maximum of 32.
