instance_of abelsonite
Porphyrins are a ___MASK72___ of heterocyclic macrocycle organic compounds , composed of ___MASK69___ modified pyrrole subunits interconnected at their  carbon atoms via methine bridges (= CH ). The parent porphyrin is porphin , and substituted porphines are called porphyrins . The porphyrin ___MASK41___ structure is aromatic , with a total of 26 electrons in the conjugated ___MASK2___ . Various analyses indicate that not all atoms of the ___MASK41___ are involved equally in the conjugation or that the ___MASK68___ ' s overall nature is substantially based on several smaller conjugated systems . One result of the large conjugated ___MASK2___ is that porphyrin molecules typically have very intense absorption bands in the visible region and ___MASK38___ be deeply colored ; the name " porphyrin " comes from the Greek word  (" porphyra "), meaning " purple ". A macrocycle is , as defined by IUPAC , " a cyclic macromolecule or a macromolecular cyclic portion of a ___MASK68___ ". In the chemical literature , macrocycles varyingly include molecules containing rings of 8 or more atoms , or 12 or more atoms . Well - known examples are the group of drugs known as macrolides . The IUPAC definition notes that a " cyclic macromolecule has no end - groups but ___MASK38___ nevertheless be regarded as a chain ", and that "" macrocycle " is sometimes used [ in the literature ] for molecules of low relative molecular mass that are not considered macromolecules . Many macrocycles are employed as macrocyclic ligands . A ___MASK68___ is an electrically neutral group of two or more atoms held together by chemical bonds . Molecules are distinguished from ions by their lack of electrical charge . However , in quantum physics , organic chemistry , and biochemistry , the term " ___MASK68___ " is often used less strictly , also being applied to polyatomic ions . Abelsonite is a nickel porphyrin mineral with ___MASK76___ formula C31H32N4Ni . It was discovered in 1969 in the U . S . State of Utah and described in 1975 . The mineral is named after geochemist Philip H . Abelson . It is the only known crystalline geoporphyrin . An ___MASK57___ is an atom or a ___MASK68___ in which the total number of electrons is not equal to the total number of protons , giving the atom or ___MASK68___ a net positive or negative electrical charge . Ions can be created , by either chemical or physical means , via ionization . Pyrrole is a heterocyclic aromatic ___MASK88___ , a five - membered ring with the formula CHNH . It is a colorless volatile liquid that darkens readily upon exposure to air . Substituted derivatives are also called pyrroles , e . g ., " N "- methylpyrrole , CHNCH . Porphobilinogen , a trisubstituted pyrrole , is the biosynthetic precursor to many natural products such as heme . In chemistry , a conjugated ___MASK2___ of connected p - orbitals with delocalized electrons in molecules with alternating ___MASK19___ and multiple bonds , which in general ___MASK38___ lower the overall energy of the molecule and increase stability . Lone pairs , radicals or carbenium ions ___MASK38___ be part of the ___MASK2___ . The compound ___MASK38___ be cyclic , acyclic , linear or mixed . The connectivity of a ___MASK68___ 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 ___MASK68___ . An atom is the smallest constituent unit of ___MASK58___ matter that has the properties of a ___MASK76___ element . Every solid , ___MASK25___ , ___MASK21___ , and plasma is composed of neutral or ionized atoms . Atoms are very small ; typical sizes are around 100 picometers ( a ten - billionth of a meter , in the short scale ). An ___MASK88___ is virtually any ___MASK16___ that contains carbon , although a consensus definition remains elusive and likely arbitrary . Organic compounds are rare terrestrially , but of central importance because all known life is based on organic compounds . The most basic petrochemicals are considered the building blocks of organic chemistry . Porphin , sometimes spelled porphine , is the parent ___MASK16___ for types of biochemically significant compounds called porphyrins . The chemical formula of porphin is CHN . Porphin is an organic compound that is aromatic and heterocyclic since its chemical structure , shown at right , essentially consists of four pyrrole rings joined together by four methine (= CH ) groups to form a larger macrocycle ring . The compound itself is a solid . An isomer ( from Greek  , isomerès ; isos = " equal ", méros = " ___MASK22___ ") is a ___MASK68___ with the same molecular formula as another ___MASK68___ , but with a different chemical structure . That is , isomers contain the same number of atoms of each element , but have different arrangements of their atoms . Isomers do not necessarily share similar properties , unless they also have the same functional groups . There are two main forms of isomerism ( or ): structural isomerism ( or constitutional isomerism ) and stereoisomerism ( or spatial isomerism ). The structural ___MASK73___ of a ___MASK16___ is a graphic representation of the molecular ___MASK55___ , showing how the atoms are arranged . The chemical bonding within the molecule is also shown , either explicitly or implicitly . Unlike chemical formulas , which have a limited ___MASK12___ of symbols and are capable of only limited descriptive power , structural formulas provide a complete geometric representation of the molecular ___MASK55___ . For example , many chemical compounds exist in different isomeric forms , which have different enantiomeric structures but the same chemical ___MASK73___ . A structural ___MASK73___ is able to indicate arrangements of atoms in three dimensional space in a way that a chemical ___MASK73___ may not be able to do .