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The chemical formula is a mathematical representation of the molecular structure of a compound indicating each atomic element by its symbol with the number of atoms of that element in the compound indicated in subscript. For example, the formula for neodymium chloride is NdCl3, evidencing that the compound's molecule is composed of one neodymium atom and three chlorine atoms.


The compound formula or Hill notation (following the Hill system) provides a unique formula order commonly used by chemical databases such as the U.S. Patent Office.


Chemical Abstracts Service Registry Numbers or CAS Numbers are unique numerical identifiers assigned to every chemical described in the open scientific literature. Elements, metals, minerals, isotopes, organic and inorganic compounds, organometallics, alloys, coordination compounds, and salts; as well as compounds, standard mixtures, polymers; biological sequences including proteins, nucleic acids, nuclear particles, and nonstructurable materials are also included.


Commercially available chemical substances are assigned a seven-digit code by the European Chemicals Agency (ECHA) known as the EC number. A list on substances available within the European Union containing an EC number is called the EC Inventory. The EC inventory is comprised of the EINECS, ELINCS and NPL numbers. The EC number begins with 3 numbers which identifies it as an EINECS, ELINCS or an NPL number followed by a dash, three more numbers, another dash, and one single number which represents a check digit.

  • EINECS - European Inventory of Existing Commercial Chemical Substances - Substances, excluding polymers, that were commercially available in the EU from January 1, 1971 to September 18, 1981. EINECS numbers being with the number 200. There are currently over 100,200 entries.
  • ELINCS - European List of Notified Chemical Substances - Substances that became commercially available after September 18, 1981. ELINCS numbers begin with the number 400. There are currently 4,381 entries.
  • NLP - The "No-longer Polymers" list - In April, 1992 a list called the NLP-list was made as a result of the changed definition of polymers which includes substances that were commercially available between after September 19, 1981 and October 31, 1993. NLP numbers begin with the number 500. There are currently over 700 entries.


For each reaction and variation, the Method Detection Limit or MDL number contains a distinctive identification code. The first letter indicates the reaction, the next three letters indicate which database contains the reaction record and these letters are followed by an 8-digit number.


The Beilstein Registry Number is a unique number assigned to compounds as a method of identification, similar to the CAS registry number. It is the unique identifier for compounds in the Beilstein database.


The PubChem Substance Identification Number or Pubchem SID is a unique registryID in the PubChem database of complexes, extracts, mixtures, and uncharacterized substances.


The PubChem Compound Identification Number or PubChem CID is a non-zero integer PubChem accession identifier for a unique chemical structure.


Simplified Molecular Input Line Entry System or SMILES is a chemical line notation for entering and representing molecules.


The IUPAC Name (International Union of Pure and Applied Chemistry) is a systematic name for chemical compounds. Preferably, all organic compound should have a name from which an unambiguous structural formula can be drawn.


InChI names are freely usable and non-proprietary, can be computed from structural information, and do not have to be assigned by an organization, and most of the information in an InChI is human readable.


The InChI Key was developed as an alternative to the lengthy full version. The InChIKeys consist of 14 characters resulting from a hash of the connectivity information of the InChI, followed by a hyphen, followed by 9 characters resulting from a hash of the remaining layers of the InChI, followed by a single character indication the version of InChI used, another hyphen, followed by single checksum character.



Molecular weight or molar mass is the mass of one mole (6.02 x 1023 molecules) of a given substance.


Appearance provides the characteristics of a material that would be discernable upon visual inspection only. It does not include characteristics evident only under a microscope or other visual aid unless otherwise indicated.


Density or true density is the measure of mass per unit of volume of a substance.


Bulk density is the mass of many particles of the material divided by the total volume they occupy.


Exact mass is the calculated mass of an ion or molecule containing a single specified isotope of each atom.


The monoisotopic mass is the sum of the masses of the atoms in a molecule calculated using the mass of the most abundant naturally occurring isotope of each element, rather than using the standard atomic mass for that element, which is a average of the masses of all naturally occurring isotopes, weighted by their relative abundance.


The boiling point of a liquid is point at which the vapor pressure of the liquid equals the atmospheric pressure surrounding the liquid. This point varies with atmospheric pressure, and thus the normal boiling point-- the temperature at which the vapor pressure of a liquid is equal to 1 atm (the atmospheric pressure at sea level)--is typically used.


The melting point at a substance is the temperature at which it changes phase from liquid to solid. This temperature varies slightly by atmospheric pressure, and standard values for melting point are usually specified at 1 atm of pressure.


Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature, through heat transfer.


The Poisson ratio is the fraction (or percent) of expansion divided by the fraction (or percent) of compression, for small values of these changes. Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions transverse to the direction of stretching.


Vickers hardness is a measure of the hardness of a material, calculated from the size of an impression produced under load by a pyramid-shaped diamond indenter. The indenter employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex at an angle of 136ยบ.


A measure of elasticity, equal to the ratio of the stress acting on a substance to the strain produced.


Solubility is a quantitative term. The terms soluble and insoluble are relative. A substance is said to be soluble if more than 0.1 g of that substance dissolves in 100 mL solvent.


Tensile strength is a measurement of the maximum stress that a material can withstand while being stretched or pulled before breaking. This is an intensive property of a material, and therefore does not depend on the size of the specimen being tested, but it does vary significantly based on factors such as microstructure of the material, surface defects, and the temperature of the material and the testing environment.


Thermal conductivity is an intrinsic property of a material that quantifies how efficiently said material conducts energy in the form of heat. The SI unit for thermal conductivity is watts per meter kelvin (W/(m·K)).


Electrical resistivity is an intrinsic property of a material that quantifies how strongly said material opposes the flow of electrical current. The SI unit of electrical resistivity is the ohm-meter (Ω·m)


Electronegativity describes the tendency of an atom or group of atoms within a molecule to attract electrons towards itself. It cannot be directly measured, but is rather calculated based on other atomic or molecular properties. The most commonly used method of calculation results in a dimensionless value for electronegativity on a relative scale running from approximately 0.7 to 3.98. This method was proposed by Linus Pauling, and thus the resulting scale is referred to as the Pauling scale.


Specific heat is the amount of heat per unit mass required to raise the temperature of a material by one degree Celsius.


Heat of vaporization refers to the amount of heat required to convert a given quantity of a liquid at its boiling point into vapor. The value is usually referenced to a unit of mass, in which case it may be referred to as the specific heat of vaporization, which may be given as calories/gram or Joules/gram.


Heat of fusion is the amount of energy required to convert a given quantity of a solid at its melting point to a liquid. The value is usually referenced to a unit of mass, in which case it may be referred to as the specific heat of fusion, which may be given as calories/gram or Joules/gram.


The average particle size is a concept established for comparing dimensions of solid, liquid, or gaseous particles. The particle size of a spherical object is defined by its diameter.


The size range (or particle diameter) for powdered materials typically ranges from nanometers (10-9 m) to micrometers (10-6 m). At the small end of the size range, nanoparticles are often referred to as clusters.


Particle surface area helps determine the rate at which solids dissolve, burn, and react with other materials.


Particles can take on distinct shapes that are often named after objects they resemble, for example, nanospheres, nanoboxes and nanoreefs. These morphologies may arise as an effect of templating or directing agent present in the synthesis, or may result from the innate crystallographic growth patterns of the materials themselves.


Electric charge is a fundamental property of matter that dictates its electrostatic attraction or repulsion in the presence of other matter. Protons have positive charges while electrons have negative charges. Opposite charges, such as protons and electrons, attract one another while like charges repel each other.


Many chemical compounds take the form of crystals, which exhibit highly ordered patterned arrangements of atoms. This molecular structure plays a significant role in determining many physical properties of a material including cleavage patterns, electronic band structure, and optical transparency.


A (Material) Safety Data Sheet or (M)SDS is used to provide information on a chemical or chemical mixture to be used by workers and emergency personnel who may come into contact with it. An MSDS will include both basic physical attributes of the product, such as melting point, boiling point, and flash point, along with safety information such as toxicity and reactivity and explanations of proper procedures for storage, disposal, ordinary use, and spill clean-up.