Dr. Alexander Krasev
Digitally signed by Dr. Alexander Krasev DN: CN = Dr. Alexander Krasev, C = US, O = Home, OU = Home Date: 2005.02.09 10:48:13 -05'00'
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The 7 essential mineral elements are the most abundant substances dissolved in the extracellular and intracel: lular fluids. Most of the body's calcium and phosphorus - atoms, however, make up the solid matrix of bone tissue. The 13 essential trace elements are present in extremely small quantities, but they are nonetheless essential for normal growth and function. For example, iron i plays a critical role in the transport of oxygen by the blood. - ~dditionaltrace elements will likelv be added to this list as (,,
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electrically neutral, the atomic number is also equal to the number of electrons in the atom.
Atomic Weight Atoms have very little mass. A single hydrogen atom, for g. The atomic example, has a mass of only 1.67 x weight scale is a scale that indicates an atom's mass relative to the mass of other atoms. This scale is based upon assigning the carbon atom a value of 12. On this scale a hydrogen atam has an atomic weight of approximately 1, indicating that it has one-twelfth the mass of a carbon atom; a rntfgnesium atom, with an atomic weight of 24, has twice the mass of a carbon atom.' Since almost all the mass of an atom is due to its neutrons and protons, and since hydrogen, with a single proton, has an atomic weight of 1, the atomic weight of an atom is approximately equal to the sum of the number of protons and neutrons it contains. Its atomic weight is not exactly equal to this sum because electrons do have some mass and because protons and neutrons do not have exactly equal masses. s of a chemical element is the grams that is equal to the nuight. Thus, 12 g of carbon is 1 gram atomic mass bf carbon, and 1g of hydrogen is 1 gram atomic mass of hydrogen. One gram atomic mass of any e l m t contaralns t h s a m number of atoms. For example, L g of hydrogen contains 6 x lom atom, and 12 g of carbon, whose atoms have 12 times the mass of&&% b%&od gen atom, also has 6 x lom atoms. Just four of the body's ess; al elements (Table 21)hydrogen, oxygen, carb~n,and nitrogen-account for over 99 percent of the atoms in the body. ,& A b S~~STW& MI am an -&-.
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2 ~ i ~the c e&&c weight s&is a r d o of atomic masses, it has no units. The unit of atomic mass is known as a dalton [ l dalton (d) is equal to one-twelfth the mass of a single carbon atom]. Thus, carbon has an atomic weight of 12, and a single carbon atom has an atomic mass of 12 daltons. zf fos13Sa
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enter through the foods we eat and the k r we breathe but do not have any known essential chemical function. Some elements, for example, mercury, are not required for normal function and can be toxic. I
wo or more atoms bonded together make up a molecule. For example, - a molecule of water contains two hydrogen atoms and one oxygen atom, which can be represented by H20. The atomic composition of glucose, a sugar, is C6H1206rindicating that the molecule contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Such formulas, however, do not indicate which atoms are linked to which in the molecule.
Covalent Chemical Bonds The atoms in molecules are held together by chemical bonds, which are formed when electrons are transferred from one atom to another or are shared between two atoms. The strongest chemical bond between two atoms, a covalent bond, is formed when one electron in the outer electron orbit of each atom is shared between the two atoms. The atoms in most molecules found in the body are linked by covalent bonds. The atoms of some elements can form more than one covalent bond and thus become linked simultaneously to two or more other atoms (Figure 2-1). Each type of atom has a characteristic number of covalent bonds it can form, which depends on the number of electrons present in its outermost electron orbit. The number of chemical bonds formed by the four most abundant atoms in the body are hydrogen, 1; oxygen, 2; nitrogen, 3; and carbon, 4. When the structure of a molecule is diagramed, each covalent bond is represented by a line indicating a pair of shared electrons. The covalent bonds of the four elements mentioned above can be represented as