The Standard States of Elements
The standard states of elements are the forms that they adopt at a temperature of 25°C and
pressure of 1 atmosphere (1 atm).!
These forms of the elements are the reactants in the formation equations of multi-element
substances.
The heat of formation (∆Hf°) of an element in its standard state is zero.
Below is the list of the elements whose standard states you need to know for this course.
Please note that some elements have multiple possible forms at the standard state such as
carbon, which can be either graphite or diamond. However only one of these forms is the true
standard state; in those cases the proper state will be listed next to the formula for the element.
Gases
• Hydrogen H2(g)!
• Nitrogen N2(g)!
• Oxygen O2(g)!
• Fluorine F2(g)!
• Chlorine Cl2(g)
Liquids
• Bromine Br2(l)!
• Mercury Hg(l)
Solids
There are two distinct classes of elemental solids. Molecular solids, unlike other elemental
solids, have specific molecular structures (arrangements of bonded atoms) that are packed
close to each other (held by van der Waals forces) to form the solid. The atoms of a molecular
solid are only bonded to the other atoms in their specific molecule, unlike the elements in the
other elemental solids, which are bound to all their nearest neighboring elements.
Molecular Solids
Phosphorus P4 - white
Sulfur S8(s) - rhombic
Iodine I2(s)
Solids (Non-Metal)
Silicon#Si(s)
Carbon C(s) - graphite
