Self and the Beginning of Life 17“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityelectrons and protons are equal in number, producing electrical neutrality,
whereas neutrons vary in number even for the same element. Elements
having different number of neutrons are called isotopes.
All chemical compounds are formed by interaction of electrons,
which are grouped by energy levels into electron shells, from the lowest
level (innermost shell) to the highest (outermost shell). Pauli’s exclusion
principle dictates that no two electrons in the same atom can simulta-
neously occupy the same quantum state, denoted by a set of four quan-
tum numbers.1,2 This ordering rule demands that each electron finds its
“niche” in the shells surrounding the nucleus. Those in the outermost
orbit, called valence electrons, constantly “rub shoulders” with neighbor-
ing atoms and negotiate by giving away, taking in, or sharing electrons,
in effect creating chemical bonds to form molecules.^3 Simply put, outer
shell electrons determine what molecules can be found in the universe.
Out of the four fundamental forces of nature (strong and weak interac-
tions, electromagnetism, and gravity), only electromagnetic force, being
responsible for chemical bond formation, plays a direct and major role
in life.^4 Thanks to the electrons, a kaleidoscopic world unfolds before
our eyes and gives us everything we encounter — dandelions, bacteria,
butterflies, babies, and even you and me.
Out of the 92 natural elements present in the universe, only a hand-
ful is needed to form life. The major ones are (in decreasing order of
abundance) oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus,
potassium, sulfur, sodium, chlorine, magnesium, and traces of metals.
The first four are of lower atomic weight, located in the first two rows of
the periodic table, and make up 96% by weight of a living body (Table 3.1).
Hydrogen is interesting in that it is the lightest element, consisting of
only one proton and one electron, and was the first to form in the begin-
ning of the universe 13.7 billion years ago. All other elements of higher
atomic weight up to iron were latecomers, being products of nuclear
fusion taking place in stars. Elements heavier than iron were results of
supernova explosion.^5 In the present state of the visible universe, hydro-
gen makes up 80% of matter, most of the remaining 20% being helium.