Green Chemistry and the Ten Commandments

2H 2 + O 2 → 2H 2 O + electrical energy (2.2.5)

e- e

-

Electrical current that

can be used for motors,

lighting, other purposes

Anode, reaction

2H 2 → 4H+ + 4e-

Cathode, reaction

O 2 + 4H+ + 4e- → 2H 2 O

Movement of H+ from

anode to cathode through

cation-permeable membrane

H 2 O 2

Net reaction 2H 2 + O 2 → 2H 2 O

Figure 2.2. Cross-sectional diagram of a fuel cell in which elemental hydrogen can be reacted with
elemental oxygen to produce electricity directly with water as the only chemical product.

2.3. Helium, the First Noble Gas

The second element in the periodic table is helium, He, atomic number 2. All helium
atoms have 2 protons in their nuclei and 2 electrons. There are two isotopes of helium
of which 24 Hethat contains 2 neutrons in the nucleus is by far the predominant one, with
much smaller numbers of the lighter isotope, 23 Hewhich has 2 protons and 1 neutron in
its nucleus and a mass number of 3.
Helium is a noble gas meaning that it exists only as atoms of the elements that are
never bonded to other atoms. Figure 2.3 is a representation of the helium atom showing
its 2 electrons. The Lewis symbol of helium is simply He with 2 dots. This shows a
very important characteristic of atoms. As electrons are added to atoms with increasing

-

-

2+

A helium atom has a filled It can be represented by the

electron shell containing Lewis symbol above.

2 electrons.

He

Figure 2.3. Two representations of the helium atom having a filled electron shell

Chap. 2, The Elements: Basic Building Blocks of Green Chemicals 33