All chemical species in the reaction mechanism must be composed of chemical elements or isotopes of chemical elements. Each element and isotope must be declared as a one- or two-character symbol. The purpose of the element data is to associate atomic weights of the elements with their character symbol representations and to identify the order in which arrays of element information in the Gas-phase Kinetics Subroutine Library are referenced. For example, an array of atomic weights for the elements is in exactly the same order in which the elements were declared in the element data. In other words, if the atomic weights are stored in an array AWT, then AWT(3) is the atomic weight of the third element declared in the element data.

Element data must start with the word ELEMENTS (or ELEM), followed by any number of element symbols on any number of lines. Element symbols may appear anywhere on a line, but those on the same line must be separated by blanks. Any line or portion of a line starting with an exclamation mark (!) is considered a comment and will be ignored. Blank lines are ignored.

If an element is on the periodic chart, then only the symbol identifying the element need appear in the element data.

For the elements appearing on the periodic chart, the Ansys Chemkin pre-processors have the atomic weight (in grams per mole) stored internally. For isotopes, a one- or two- character symbol must be input by the user to identify each isotope, and a symbol and an atomic weight (in grams per mole) for each must be defined. The same symbol must be used in the thermodynamic data to identify the elemental composition of species involving the isotope. Once an isotope has been so defined, it is treated exactly as a new element. If an ionic species is used in the reaction mechanism (that is, OH+), an electron must be declared as the element E.

For an isotope, the atomic weight must follow the identifying symbol and be delimited by slashes (/). The atomic weight may be in integer, floating-point, or E format (for example, 2, 2.0, 0.2E1), but internally it will be converted to a floating point number. For example, the isotope deuterium may be defined as D/2.014/. If desired, the atomic weight of an element in the periodic chart may be altered by including the atomic weight as input just as though the element were an isotope.

Figure 3.1: Examples of Species Data shows several equivalent ways to describe element information. In this example the elements are hydrogen, oxygen, nitrogen, and the isotope deuterium.

ELEMENTS   H   D /2.014/  O   N   END

ELEM                                  ! ELEM is equivalent to ELEMENTS
   H
   D / 2.014 /
   O
   N
END                                   ! an END line is optional

ELEM H
ELEM D/2.014/
ELEM O
ELEM N

Table 3.1: Summary of the Rules for Element Data summarizes the rules for element data.