Environmental Engineering Reference
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FIGURE 1.1 Relevant energy level of the ground electronic state of the H atom and its ionized state
(H +  +  e). E is energy, n is the principal quantum number, r is the distance between the electron and
proton; −(1/ r ) is the Coulombic attraction between the electron and proton; and 13.6 eV corresponds to
the ionization energy of the H atom from its ground electronic state ( n  = 1 or 1s atomic orbital).
and electron that H contains. The abundance is 99.895%, 0.015%, and trace
amount, respectively, for H, D, and T. While the mass differs significantly
among the three isotopes, their electronic structures and properties are very
similar since the neutrons have essentially no effect on the electronic proper-
ties that are mainly determined by the electron and proton. Other highly
unstable nuclei ( 4 H to 7 H) have been synthesized in the laboratory but not
observed in nature.
The ionization energy for H atom is 13.6 eV or 1312.0 kJ mol −1 , equiva-
lent to a photon energy of 92 nm. Thus, H atom is highly stable under normal
conditions. The ionized form of the H atom is the proton, H + , which has
many interesting and unique properties of its own. It is the lightest and small-
est atomic ion. Figure 1.1 shows the relevant energy levels for the ground
electronic state of H atom relative to its ionized state (H +  + e). In water, the
proton is in the form of H 3 O + and plays a critical role in many biological
processes. The proton is also related to acids and bases, which are two essen-
tial classes of compounds in chemistry and important for chemical
Hydrogen atoms are reactive and can be combined with many elements
to form a huge number of different compounds, including most organic and
biological compounds, such as hydrocarbons, polymers, proteins, and DNA.
For most organic compounds, the hydrogen is bound to the atoms of carbon
and, to a lesser degree, nitrogen, oxygen, or other atoms, such as phosphorus
and sulfur. The H atom only forms a relatively strong single bond with these
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