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Text of the page (random words):
y imply a correct structural formula for example ethanol may be represented by the condensed chemical formula ch 3 ch 2 oh and dimethyl ether by the condensed formula ch 3 och 3 these two molecules have the same empirical and molecular formulae c 2 h 6 o but may be differentiated by the condensed formulae shown which are sufficient to represent the full structure of these simple organic compounds condensed chemical formulae may also be used to represent ionic compounds that do not exist as discrete molecules but nonetheless do contain covalently bound clusters within them these polyatomic ions are groups of atoms that are covalently bound together and have an overall ionic charge such as the sulfate so 4 2 ion each polyatomic ion in a compound is written individually in order to illustrate the separate groupings for example the compound dichlorine hexoxide has an empirical formula clo 3 and molecular formula cl 2 o 6 but in liquid or solid forms this compound is more correctly shown by an ionic condensed formula clo 2 clo 4 which illustrates that this compound consists of clo 2 ions and clo 4 ions in such cases the condensed formula only need be complex enough to show at least one of each ionic species chemical formulae as described here are distinct from the far more complex chemical systematic names that are used in various systems of chemical nomenclature for example one systematic name for glucose is 2 r 3 s 4 r 5 r 2 3 4 5 6 pentahydroxyhexanal this name interpreted by the rules behind it fully specifies glucose s structural formula but the name is not a chemical formula as usually understood and uses terms and words not used in chemical formulae such names unlike basic formulae may be able to represent full structural formulae without graphs types empirical formula main article 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 the compound empirical formulae are the standard for ionic compounds such as cacl 2 and for macromolecules such as sio 2 an empirical formula makes no reference to isomerism 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 has a molecular formula of c 6 h 14 and for one of its isomers n hexane a structural formula ch 3 ch 2 ch 2 ch 2 ch 2 ch 3 implying that it has a chain structure of 6 carbon atoms and 14 hydrogen atoms however the empirical formula for hexane is c 3 h 7 likewise the empirical formula for hydrogen peroxide h 2 o 2 is simply ho expressing the 1 1 ratio of component elements formaldehyde and acetic acid have the same empirical formula ch 2 o this is also the molecular formula for formaldehyde but acetic acid has double the number of atoms like the other formula types detailed below an empirical formula shows the number of elements in a molecule and determines whether it is a binary compound ternary compound quaternary compound or has even more elements molecular formula isobutane structural formula molecular formula c 4 h 10 condensed formula ch 3 3 ch h c h h c h h c h h c h h h displaystyle ce h overset displaystyle h atop underset atop displaystyle h c overset displaystyle h atop underset atop displaystyle h c overset displaystyle h atop underset atop displaystyle h c overset displaystyle h atop underset atop displaystyle h c h n butane structural formula molecular formula c 4 h 10 condensed formula ch 3 ch 2 ch 2 ch 3 molecular formulae simply indicate the numbers of each type of atom in a molecule of a molecular substance they are the same as empirical formulae for molecules that only have one atom of a particular type but otherwise may have larger numbers an example of the difference is the empirical formula for glucose which is ch 2 o ratio 1 2 1 while its molecular formula is c 6 h 12 o 6 number of atoms 6 12 6 for water both formulae are h 2 o a molecular formula provides more information about a molecule than its empirical formula but is more difficult to establish structural formula main article structural formula in addition to indicating the number of atoms of each element in a molecule a structural formula shows or implies the molecule s connectivity which is the pattern of chemical bonds between the atoms there are several types of structural formula each emphasizing different aspects of the molecular structure the two diagrams show two molecules which are structural isomers of each other since they both have the same molecular formula c 4 h 10 but they have different structural formulas as shown condensed formula main article condensed formula the connectivity of a molecule often has a strong influence on its physical and chemical properties and behavior two molecules composed of the same numbers of the same types of atoms i e a pair of isomers might have completely different chemical and or physical properties if the atoms are connected differently or in different positions in such cases a structural formula is useful as it illustrates which atoms are bonded to which other ones from the connectivity it is often possible to deduce the approximate shape of the molecule a condensed or semi structural formula may represent the types and spatial arrangement of bonds in a simple chemical substance though it does not necessarily specify isomers or complex structures for example ethane consists of two carbon atoms single bonded to each other with each carbon atom having three hydrogen atoms bonded to it its chemical formula can be rendered as ch 3 ch 3 in ethylene there is a double bond between the carbon atoms and thus each carbon only has two hydrogens therefore the chemical formula may be written ch 2 ch 2 and the fact that there is a double bond between the carbons is implicit because carbon has a valence of four however a more explicit method is to write h 2 c ch 2 or less commonly h 2 c ch 2 the two lines or two pairs of dots indicate that a double bond connects the atoms on either side of them a triple bond may be expressed with three lines hc ch or three pairs of dots hc ch and if there may be ambiguity a single line or pair of dots may be used to indicate a single bond molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets for example isobutane may be written ch 3 3 ch this condensed structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions isomers the formula ch 3 3 ch implies a central carbon atom connected to one hydrogen atom and three methyl groups ch 3 the same number of atoms of each element 10 hydrogens and 4 carbons or c 4 h 10 may be used to make a straight chain molecule n butane ch 3 ch 2 ch 2 ch 3 crystal chemical formula chemical formulae for inorganic crystals typically omit structural information of value to crystallographers and mineralologists such as the linkage type dimensionality of subnetworks within the crystal the coordination polyhedron coordination geometry of the first coordination sphere of each atom and the structural connectivity between subunits 1 the international union of crystallography iucr has proposed a crystal chemical formula notation that extends standard formulae with optional annotations for these items 2 for example while quartz fibrous silica and stishovite all have the same formula unit sio 2 their crystal chemical formulas demonstrate the structural differences between these polymorphs of silica quartz may be described as 3 si 4 t 1 4 o 2 indicating it has a three dimensional network structure with tetrahedral coordination geometry at each si atom where each sio 4 tetrahedron is linked to four adjacent tetrahedra via single shared o atoms fibrous silica may be described as 1 si 4 t 2 2 o 2 indicating it consists of one dimensional chains of sio 4 tetrahedra each linked to two others via shared pairs of o atoms and stishovite may be described as 3 si 6 o 1 1 1 1 1 1 1 1 2 2 10 o 2 indicating it has a three dimensional network structure with octahedral coordination geometry at each silicon atom where each sio 6 octahedron is linked to ten adjacent octahedra eight via single shared o atoms and two via shared pairs of o atoms 2 chemical names in answer to limitations of chemical formulae main article chemical nomenclature the alkene called but 2 ene has two isomers which the chemical formula ch 3 ch chch 3 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 3 as noted above in order to represent the full structural formulae of many complex organic and inorganic compounds chemical nomenclature may be needed which goes well beyond the available resources used above in simple condensed formulae see iupac nomenclature of organic chemistry and iupac nomenclature of inorganic chemistry 2005 for examples in addition linear naming systems such as international chemical identifier inchi allow a computer to construct a structural formula and simplified molecular input line entry system smiles allows a more human readable ascii input however all these nomenclature systems go beyond the standards of chemical formulae and technically are chemical naming systems not formula systems 4 polymers in condensed formulae for polymers in condensed chemical formulae parentheses are placed around the repeating unit for example a hydrocarbon molecule that is described as ch 3 ch 2 50 ch 3 is a molecule with fifty repeating units if the number of repeating units is unknown or variable the letter n may be used to indicate this formula ch 3 ch 2 n ch 3 ions in condensed formulae for ions the charge on a particular atom may be denoted with a right hand superscript for example na or cu 2 the total charge on a charged molecule or a polyatomic ion may also be shown in this way such as for hydronium h 3 o or sulfate so 2 4 here and are used in place of 1 and 1 respectively for more complex ions brackets are often used to enclose the ionic formula as in b 12 h 12 2 which is found in compounds such as caesium dodecaborate cs 2 b 12 h 12 parentheses can be nested inside brackets to indicate a repeating unit as in hexamminecobalt iii chloride co nh 3 6 3 cl 3 here nh 3 6 indicates that the ion contains six ammine groups nh 3 bonded to cobalt and encloses the entire formula of the ion with charge 3 further explanation needed this is strictly optional a chemical formula is valid with or without ionization information and hexamminecobalt iii chloride may be written as co nh 3 6 3 cl 3 or co nh 3 6 cl 3 brackets like parentheses behave in chemistry as they do in mathematics grouping terms together they are not specifically employed only for ionization states in the latter case here the parentheses indicate 6 groups all of the same shape bonded to another group of size 1 the cobalt atom and then the entire bundle as a group is bonded to 3 chlorine atoms in the former case it is clearer that the bond connecting the chlorines is ionic rather than covalent isotopes although isotopes are more relevant to nuclear chemistry or stable isotope chemistry than to conventional chemistry different isotopes may be indicated with a prefixed superscript in a chemical formula for example the phosphate ion containing radioactive phosphorus 32 is 32 po 4 3 also a study involving stable isotope ratios might include the molecule 18 o 16 o a left hand subscript is sometimes used redundantly to indicate the atomic number for example 8 o 2 for dioxygen and 16 8 o 2 for the most abundant isotopic species of dioxygen this is convenient when writing equations for nuclear reactions in order to show the balance of charge more clearly trapped atoms traditional formula mc 60 the notation m c 60 main article endohedral fullerene the symbol at sign indicates an atom or molecule trapped inside a cage but not chemically bound to it for example a buckminsterfullerene c 60 with an atom m would simply be represented as mc 60 regardless of whether m was inside the fullerene without chemical bonding or outside bound to one of the carbon atoms using the symbol this would be denoted m c 60 if m was inside the carbon network a non fullerene example is as ni 12 as 20 3 an ion in which one arsenic as atom is trapped in a cage formed by the other 32 atoms this notation was proposed in 1991 5 with the discovery of fullerene cages endohedral fullerenes which can trap atoms such as la to form for example la c 60 or la c 82 the choice of the symbol has been explained by the authors as being concise readily printed and transmitted electronically the at sign is included in ascii which most modern character encoding schemes are based on and the visual aspects suggesting the structure of an endohedral fullerene non stoichiometric chemical formulae main article non stoichiometric compound chemical formulae most often use integers for each element however there is a class of compounds called non stoichiometric compounds that cannot be represented by small integers such a formula might be written using decimal fractions as in fe 0 95 o or it might include a variable part represented by a letter as in fe 1 x o where x is normally much less than 1 general forms for organic compounds a chemical formula used for a series of compounds that differ from each other by a constant unit is called a general formula it generates a homologous series of chemical formulae for example alcohols may be represented by the formula c n h 2 n 1 oh n 1 giving the homologs methanol ethanol propanol for 1 n 3 hill system the hill system or hill notation is a system of writing empirical chemical formulae molecular chemical formulae and components of a condensed formula such that the number of carbon atoms in a molecule is indicated first the number of hydrogen atoms next and then the number of all other chemical elements subsequently in alphabetical order of the chemical symbols when the formula contains no carbon all the elements including hydrogen are listed alphabetically by sorting formulae according to the number of atoms of each element present in the formula according to these rules with differences in earlier elements or numbers being treated as more significant than differences in any later element or number like sorting text strings into lexicographical order it is possible to collate chemical formulae into what is known as hill system order the hill system was first published by edwin a hill of the united states patent and trademark office in 1900 6 it is the most commonly used system in chemical databases and printed indexes to sort lists of compounds 7 a list of formulae in hill ...