Biomedical Engineering Reference
In-Depth Information
according to the law of mass action. The arrow in Eq. (8.1) shows the direction of the reac-
tion that occurs spontaneously, and the reaction rate constant describes how quickly the
reaction occurs. The reaction rate constant is a function of temperature, whereby an increase
in temperature generally increases
K.
For our purposes, temperature is constant and so is
K.
8.1.1 Chemical Bonds
Individual atoms rarely appear in nature, but they appear within a molecule. Molecules
are collections of atoms that are held together by strong chemical bonds in which electron(s)
are shared or electron(s) are transferred from one atom to another. Forces between molecules
are relatively weak, allowing molecules to act independently of one another. A compound
is a molecule that contains at least two different atoms, whereas a molecule can contain just
one type of atom. Molecules such as hydrogen,
O
2
, are not compounds
because they consist of a single type of atom. For simplicity in presentation, we will use
the term
H
2
, and oxygen,
to describe both molecules and compounds.
An atom is an electrically neutral particle that consists of an equal number of protons
and electrons. An atom with an unbalanced number of electrons or protons is called an
ion, which makes it a positively or negatively charged particle. Examples of positively
charged ions, called cations, include
molecules
Na
þ
,
K
þ
, and
Ca
þ2
;
Na
þ
and
K
þ
have each lost one
Ca
þ2
has lost two electrons. An example of a negatively charged ion, called
electron, and
Cl
, which has gained an extra electron.
There are many types of chemical bonds that join atoms and molecules together. They
vary in strength, with ionic and covalent bonds displaying the strongest bonds, and the
hydrogen bond displaying the weakest bond.
Many molecules are composed of positively and negatively charged ions that are bound
by an ionic bond. These bonds are extremely strong due to the electrostatic attraction
between the oppositely charged ions. Ionic bonds usually involve an atom that has few elec-
trons in the outer shell, with another atom that has an almost complete set of electrons in
the outer shell. Ionic bonds involve a transfer of electrons from one atom to another atom.
Consider sodium chloride, where sodium has one electron in the outer shell and chloride
has seven electrons in its outer shell. The bond is formed by sodium transferring its
electron to chloride, resulting in a molecule consisting of
an anion, is
Na
þ
Cl
ð
NaCl
Þ:
Consider magnesium and oxygen atoms forming an ionic bond. Magnesium has two elec-
trons in its outer field that are transferred to oxygen, forming
usually written as
Mg
þ2
O
2
:
Covalent bonds involve two atoms sharing an electron pair that increases the stability of
the molecule. The atoms do not have to be the same type, but they must be the same electro-
negativity. For instance,
H
2
is formed by a covalent bond. Consider carbon dioxide, a mol-
ecule that consists of one carbon and two oxygen atoms. These atoms share the electrons,
with carbon having four electrons in its outer shell, and oxygen having six electrons in its
outer shell. The four electrons in carbon are used in the outer shell of the two oxygen atoms.
A hydrogen bond involves the force between a hydrogen atom in one molecule and an
electronegative atom in another molecule, typically with oxygen in
H
O
, nitrogen in
2
NH
Note that the force experienced in a hydrogen bond is not within
a molecule but between molecules. The small size of hydrogen allows it to become very
close to another molecule with a small atomic radii like the ones previously mentioned.
3
, and fluorine in
HF
:
