This document decribes in a human-readeable format the nomenclature used 
in AtomProperties.pm module. Most of it corresponds to the internationally 
recognised sources like IUPAC, but some have to be noted:

--  Lanthanides. All elements with atomic numbers [57-71] are considered 
    lanthanides as defined by IUPAC. All lanthanides are assigned a group 
    number of 19.
--  Actinides. All elements with atomic numbers [89-103] are considered 
    actinides as defined by IUPAC. All actinides are assigned a group number
    of 20.
--  Atomic weight/mass.These two terms are oftenly mistaken for each other.
    -- Atomic weight. In this document it is interpreted as standard atomic
       weight published by IUPAC in regular intervals.
    -- Atomic mass. In this document it is interpret as the exact mass 
       of the most common isotope of an element.

Next, some fields of the atom entries are described in detail:

--  covalent_radii. A hash of covalent radii values of the element in regard 
    to the number of electrons used in bonding. Accepted values are 
    "single", "double" and "triple" bonds. Measured in Angstroms.
--  covalent_radius. The longest of the radii given in "covalent_radii" field.
    Measured in Angstroms.
--  spin_state_radii. A  hash of covalent radii values of the element in regard 
    to the spin state. Accepted values are "l.s." for low spin and "h.s" for 
    high spin. Measured in Angstroms.
