GREEN CHEMISTRY

Chap. 2, The Elements: Basic Building Blocks of Green Chemicals 33
2H + O 2 2 → 2H 2O + electrical energy (2.2.5)
e -
e - Electrical current that
can be used for motors,
lighting, other purposes
Movement of H + from
anode to cathode through
cation-permeable membrane
H 2
O 2
Anode, reaction
2H 2 → 4H + + 4e - Cathode, reaction
O 2 + 4H + + 4e - → 2H 2 O
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
4
of which 2
He that contains 2 neutrons in the nucleus is by far the predominant one, with
3
much smaller numbers of the lighter isotope, 2
He which 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+
-
He
A helium atom has a filled It can be represented by the
electron shell containing Lewis symbol above.
2 electrons.
Figure 2.3. Two representations of the helium atom having a filled electron shell