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MOLECULAR AND STRUCTURAL FORMULA S For example Methane , a simple molecule consisting of one Carbon atom bonded to four Hydrogen atoms has the chemical formula: : CH4 and glucose with six Carbon atoms, twelve Hydrogen atoms and six Oxygen atoms has the chemical formula: : C6H12O6. A chemical formula may also supply information about the types and spatial arrangement of bonds in the chemical, though it does not necessarily specify the exact Isomer . For example Ethane consists of two carbon atoms single-bonded to each other, each having three hydrogen atoms bonded to it. Its chemical formula can be rendered as CH3CH3. If there were a double bond between the carbon atoms (and thus each carbon only had two hydrogens), the chemical formula may be written: CH2CH2, and the fact that there is a double bond between the carbons is assumed. However, a more explicit and correct method is to write H2C:CH2 or H2C=CH2. The two dots or lines indicate that a double bond connects the atoms on either side of them. A triple bond may be expressed with three dots or lines, and if there may be ambiguity, a single dot or line may be used to indicate a single bond. Molecules with multiple functional groups that are the same may be expressed in the following way: (CH3)3CH. However, this implies a different structure from other molecules that can be formed using the same atoms ( Isomer s). The formula (CH3)3CH implies a chain of three carbon atoms, with the middle carbon atom bonded to another carbon: and the remaining bonds on the carbons all leading to hydrogen atoms. However, the same number of atoms (10 hydrogens and 4 carbons, or C4H10) may be used to make a straight chain: CH3CH2CH2CH3. The alkene but-2-ene has two isomers which the chemical formula CH3CH=CHCH3 does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (''cis'' or ''Z'') or on the opposite sides from each other.(''trans'' or ''E'') POLYMERS For Polymer s, parentheses are placed around the repeating unit. For example, a Hydrocarbon molecule that is described as: CH3(CH2)50CH3, is a molecule with 50 repeating units. If the number of repeating units is unknown or variable, the letter ''n'' may be used to indicate this: CH3(CH2)''n''CH3. IONS For Ion s, the charge on a particular atom may be denoted with a right-hand superscript. For example Na+, or Cu2+. The total charge on a charged molecule or a Polyatomic Ion may also be shown in this way. For example: hydronium, H3O+ or sulfate, SO42-. ISOTOPES Although Isotope s are more relevant to Nuclear Chemistry or Stable Isotope chemistry than to conventional chemistry, different isotopes may be indicated with a left-hand Superscript in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is 32PO43-. Also a study involving stable isotope ratios might include 18O:16O. A left-hand subscript is sometimes used to indicate redundantly, for convenience, the Atomic Number . EMPIRICAL FORMULA In Chemistry , the Empirical Formula of a chemical is a simple expression of the relative number of each type of atom or ratio of the elements in it. Empirical formulas are the standard for ionic compounds, such as CaCl2, and for macromolecules, such as SiO2. An empirical formula makes no reference to Isomer ism, structure, or absolute number of atoms. The term empirical refers to the process of Elemental Analysis , a technique of Analytical Chemistry used to determine the relative percent composition of a pure chemical substance by element. For example, Hexane could have a Chemical Formula of CH3CH2CH2CH2CH2CH3, implying that it has a straight chain structure, 6 Carbon atoms, and 14 Hydrogen atoms. However the empirical formula for the same molecule would be C3H7. NON-STOICHIOMETRIC FORMULAS ''Main article: Non-stoichiometric Compound '' Chemical formulas most often use Natural Number s for each of the elements. However, there is a whole class of compounds, called Non-stoichiometric Compound s, that cannot be represented by well-defined natural numbers. Such a formula might be written using Real Number s, as in Fe0.95O, or it might include a variable part represented by a letter, as in Fe1–xO. SEE ALSO |
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