The following properties are applicable to elements of the periodic table in the ScientificConstants package.

atomicweight: The mass of an average atom in a sample of the element, weighted by abundances of stable or long-lived isotopes.  The precision is usually limited by variations in the abundances.  For radioactive elements in the ScientificConstants package, an error message is produced, indicating the mass number of the best known or most stable isotope (see Element).

boilingpoint: The normal boiling point of the element, or, the temperature at which the liquid form boils given a standard pressure of 101.325 kPa (one atmosphere).  The boiling point is given in absolute temperature units (for example, kelvins in the SI system).

A few elements (C, As) have no liquid state at standard pressure and hence no boiling point.  In these cases, the sublimation point at standard pressure can be accessed by giving the property in the parameterized form boilingpoint(sp).

density: For elements that are in the solid or liquid form at room temperature, the mass per volume, measured at or near room temperature. For those elements with several solid forms, the density is of the most stable crystal form. (Bk - hex, C - hex, P - cubic, Se - hex, S - ortho, Sn - tetra.)

For elements that are in the gaseous form at room temperature, the calculated ideal gas density (mass per volume) at 25 degrees Celsius and 101.325 kPa  can be accessed by giving the property in the parameterized form density(gas).

electronaffinity: The energy released when a neutral atom gains an extra electron. The positive values correspond to released energy.  In the ScientificConstants package, the electron affinities are in energy units per atom.

Note: Electron affinity has not been included for hydrogen because it differs significantly between the two stable isotopes; see the later electronaffinityisotopic description.

electronegativity: Pauling's measure of the ability of an atom to attract the electrons in a bond with another atom. It is used to predict the character of a bond.

ionizationenergy: The energy required to remove an electron from a neutral atom.  In the ScientificConstants package, the ionization energies are in energy units per atom.

meltingpoint: The temperature at which the solid form of the element melts, given a standard pressure of 101.325 kPa (one atmosphere).  The melting point is given in absolute temperature units (for example, kelvins in the SI system).

A few elements (C, As) have no liquid state at standard pressure and hence no melting point.  In these cases, the temperature of the triple point can be accessed by giving the property in the parameterized form boilingpoint(tp).

The following properties are applicable to isotopes of elements of the periodic table in the ScientificConstants package.

abundance: For isotopes of a given element, abundances are relative values corresponding to the isotopic composition of samples commonly available in the laboratory (not always corresponding to natural composition). This is defined for stable or long-lived isotopes.

atomicmass: The mass of a neutral atom of an isotope.

betadecayenergy: The difference in mass-energy between the initial and final nuclei in a beta decay process, also called the $Q$ value of the decay.  In the ScientificConstants package, the stored beta decay energies apply to the beta- (negative beta) decay, in which an electron is emitted and the atomic number of the nucleus increases by one. The beta decay energy is the maximum mass-energy the emitted electron can possess.

bindingenergy: The difference in mass-energy between a nucleus of an isotope and its separated constituent nucleons.

electronaffinityisotopic: Included for the two stable isotopes of hydrogen, see the previous electronaffinity description.

halflife: For a particular radioactive isotope sample containing a large number of nuclei, the time taken for half of the nuclei to decay (possibly via multiple modes).

massexcess: The mass-energy quantity $\left(-A{m}_u+m_{\mathrm{atom}}\right)c^2$, often denoted Delta, where $m_{\mathrm{atom}}$ is the mass of an atom of the isotope, $A$ is the mass number of the isotope, and $m_u$ is the atomic mass constant. Alternatively, with $c^2$ considered as the appropriate energy conversion factor, the mass excess can be given as $-A\mathrm{amu}+m_{\mathrm{atom}}$ where $m_{\mathrm{atom}}$ is measured in atomic mass units.

Note: Mass excess can be referred to using the name mass defect. However, the term mass defect is also used for another distinct quantity.