53
CHAPTER TWO
Chemical Basis of Life
a form of energy similar to X-radiation and is the most pene-
trating form of atomic radiation. From Science to Technology
2.2 examines how radiation that moves electrons can affect
human health.
PRACTICE
3
What is the relationship between matter and elements?
4
Which elements are most common in the human body?
5
Where are electrons, protons, and neutrons located within an
atom?
6
What is an isotope?
7
What is atomic radiation?
Molecules and Compounds
Two or more atoms may combine to form a distinctive type
of particle called a
molecule.
A
molecular formula
is short-
hand used to depict the numbers and types of atoms in a
molecule. It consists of the symbols of the elements in the
molecule with numerical subscripts that indicate how many
atoms of each element are present. For example, the molecu-
lar formula for water is H
2
O, which indicates two atoms of
hydrogen and one atom of oxygen in each molecule. The
molecular formula for the sugar glucose, C
6
H
12
O
6
, indicates
six atoms of carbon, twelve atoms of hydrogen, and six
atoms of oxygen.
If atoms of the same element combine, they produce mol-
ecules of that element. Gases of hydrogen (H
2
), oxygen (O
2
),
and nitrogen (N
2
) consist of such molecules. If atoms of dif-
ferent elements combine, molecules of compounds form.
Two atoms of hydrogen, for example, can combine with one
atom of oxygen to produce a molecule of the compound water
weight. For example, all oxygen atoms have eight protons
in their nuclei. Some, however, have eight neutrons (atomic
weight 16), others have nine neutrons (atomic weight 17),
and still others have ten neutrons (atomic weight 18). Atoms
that have the same atomic numbers but different atomic
weights are called
isotopes
(i
so-to
¯pz) of an element. A sam-
ple of an element is likely to include more than one isotope,
so the atomic weight of the element is often considered to be
the average weight of the isotopes present.
(See Appendix A,
Periodic Table of the Elements, p. 939.)
The ways atoms interact refl
ect their numbers of elec-
trons. An atom has the same number of electrons and pro-
tons, so all the isotopes of a particular element have the same
number of electrons and chemically react in the same man-
ner. For example, any of the isotopes of oxygen can have the
same function in metabolic reactions.
Isotopes of an element may be stable, or they may have
unstable atomic nuclei that decompose, releasing energy or
pieces of themselves until they reach a stable form. Such
unstable isotopes are called
radioactive,
and the energy or
atomic fragments they emit are called
atomic radiation.
Elements that have radioactive isotopes include oxygen,
iodine, iron, phosphorus, and cobalt. Some radioactive iso-
topes are used to detect and treat disease (From Science to
Technology 2.1).
Atomic radiation includes three common forms called
alpha (
α
), beta (
β
), and gamma (
γ
). Each type of radioactive
isotope produces one or more of these forms of radiation.
Alpha radiation consists of particles from atomic nuclei,
each of which includes two protons and two neutrons, that
move slowly and cannot easily penetrate matter. Beta radia-
tion consists of much smaller particles (electrons) that travel
faster and more deeply penetrate matter. Gamma radiation is
TABLE
2.3
|
Atomic Structure of Elements 1 Through 12
Electrons in Shells
Element
Symbol
Number
Approximate Atomic Weight
Protons
Neutrons
First
Second
Third
Hydrogen
H
1
1
1
0
1
Helium
He
2
4
2
2
2 (inert)
Lithium
Li
3
7
3
4
2
1
Beryllium
Be
4
9
4
5
2
2
Boron
B
5
11
5
6
2
3
Carbon
C
6
12
6
6
2
4
Nitrogen
N
7
14
7
7
2
5
Oxygen
O
8
16
8
8
2
6
Fluorine
F
9
19
9
10
2
7
Neon
Ne
10
20
10
10
2
8 (inert)
Sodium
Na
11
23
11
12
2
8
1
Magnesium
Mg
12
24
12
12
2
8
2
For more detail, see Appendix A, Periodic Table of the Elements.
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