The scientific constants package also includes all information about the elements. You specify an element, and optionally an atomic mass if you wish a specific isotope, together with a property. This can then be uses as a constant, or it can be evaluated to a floating-point number.
[> evalf( Element( 'C', 'atomicweight' ) );
$1.994425018 \times 10^{-26}$
[> evalf( Element( 'C', 'density' ) );
$2200.0$
[> evalf( Element( 'C', 'electronaffinity' ) );
$2.023388855 \times 10^{-19}$
[> evalf( Element( 'C', 'electronegativity' ) );
$2.55$
[> evalf( Element( 'C', 'ionizationenergy' ) );
$1.804098941 \times 10^{-18}$
[> evalf( Element( 'Hg', 'meltingpoint' ) );
$234.32$
[> evalf( Element( 'Hg', 'boilingpoint' ) );
$629.88$
Some properties are only defined for isopopes, which can be specified using an atomic mass:
[> evalf( Element( 'C[12]', 'abundance' ) ); # Only defined for stable isotopes
$0.9893$
[> evalf( Element( 'C[14]', 'atomicmass' ) );
$2.325294625 \times 10^{-26}$
[> evalf( Element( 'C[14]', 'betadecayenergy' ) );
$2.507005868 \times 10^{-14}$
[> evalf( Element( 'C[14]', 'bindingenergy' ) );
$1.686843757 \times 10^{-11}$
[> evalf( Element( 'C[14]', 'halflife' ) );
$1.808211815 \times 10^{11}$
[> evalf( Element( 'C[14]', 'massexcess' ) );
$4.838400360 \times 10^{-13}$
This property is only defined for the two stable isotopes of hydrogen:
[> evalf( Element( 'H[1]', 'electronaffinityisotopic' ) );
$1.208353597 \times 10^{-19}$
[> evalf( Element( 'H[2]', 'electronaffinityisotopic' ) );
$1.208991263 \times 10^{-19}$
All of the above constants have SI units. You can change this by adding the option 'system' = 'SystemName' where the system name is one of 'Atomic', 'CGS', 'EMU', 'ESU', 'FPS', 'MKS', 'MTS' or 'SI'.