Page 11 - Color Atlas of Biochemistry

P. 11

2 Basics

Periodic table tailed discussions of the subject are available
in chemistry textbooks.
The possible states of electrons are called
A. Biologically important elements
orbitals. These are indicated by what is
There are 81 stable elements in nature. Fifteen known as the principal quantum number
of these are present in all living things, and a and bya letter—s,p,ord. The orbitals are
further 8–10 are only found in particular or- filled one by one as the number of electrons
ganisms. The illustration shows the first half increases. Each orbital can hold a maximum of
of the periodic table, containing all of the bio- two electrons, which must have oppositely
logically important elements. In addition to directed “spins.” Fig. A shows the distribution
physical and chemical data, it also provides of the electrons among the orbitals for each of
information about the distribution of the ele- the elements. For example, the six electrons of
ments in the living world and their abun- carbon (B1) occupy the 1s orbital, the 2s orbi-
dance in the human body. The laws of atomic tal, and two 2p orbitals. A filled 1s orbital has
structure underlying the periodic table are the same electron configuration as the noble
discussed in chemistry textbooks. gas helium (He). This region of the electron
More than 99% of the atoms in animals’ shell of carbon is therefore abbreviated as
bodies are accounted for by just four ele- “He” in Fig. A. Below this, the numbers of
ments—hydrogen (H), oxygen (O), carbon (C) electrons in each of the other filled orbitals
and nitrogen (N). Hydrogen and oxygen are (2s and 2p in thecaseofcarbon) areshown on
the constituents of water, which alone makes the right margin. For example, the electron
up 60–70% of cell mass (see p.196). Together shell of chlorine (B2)consists ofthatof neon
with carbon and nitrogen, hydrogen and oxy- (Ne) and seven additional electrons in 3s and
gen are also the major constituents of the 3p orbitals. Iniron(B3), a transition metal of
organic compounds on which most living the first series, electrons occupy the 4s orbital
processes depend. Many biomolecules also even though the 3d orbitals are still partly
contain sulfur (S) or phosphorus (P). The empty. Many reactions of the transition met-
above macroelements are essential for all or- als involve empty d orbitals—e. g., redox reac-
ganisms. tions or the formation of complexes with
A second biologically important group of bases.
elements, which together represent only Particularly stable electron arrangements
about0.5% ofthe body mass,are presental- arise when the outermost shell is fully occu-
most exclusively in the form of inorganic ions. pied with eight electrons (the “octet rule”).
This group includes the alkali metals sodium This applies, for example, to the noble gases,
–
2
6
(Na) and potassium (K), and the alkaline earth as well as to ions such as Cl (3s 3p )and Na +
6
2
metals magnesium (Mg) and calcium (Ca). The (2s 2p ). It is only in the cases of hydrogen
halogen chlorine (Cl) is also always ionized in and helium that two electrons are already
the cell. All other elements important for life suf cient to fill the outermost 1s orbital.
are present in such small quantities that they
are referred to as trace elements.These in-
clude transitionmetals suchas iron(Fe), zinc
(Zn), copper (Cu), cobalt (Co) and manganese
(Mn). A few nonmetals, such as iodine (I) and
selenium (Se), can also be classed as essential
trace elements.

B. Electron configurations: examples
The chemical properties of atoms and the
types of bond they form witheachother are
determined by their electron shells. The elec-
tron configurations of the elements are there-
fore also showninFig. A.Fig. B explains the
symbols and abbreviations used. More de-

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