Geology Reference
In-Depth Information
layers of tiny mineral crystals around some irritant, perhaps
a sand grain. Most pearls are lustrous white, but some are sil-
ver gray, green, or black (Figure 3.1c).
From our discussion so far, we have a formal defi nition
of the term mineral and we know that minerals are the basic
constituents of rocks. Now let's delve deeper into what min-
erals are made of by considering matter, atoms, elements,
and bonding.
a symbol—for example, oxygen (O), aluminum (Al), and
potassium (K) (
Figure 3.3).
At the center of an atom is a tiny nucleus made up of one
or more particles known as protons , which have a positive elec-
trical charge, and neutrons , which are electrically neutral. The
nucleus is only about 1/100,000 of the diameter of an atom, yet
it contains virtually all of the atom's mass. Electrons , particles
with a negative electrical charge, orbit rapidly around the nu-
cleus at specifi c distances in one or more electron shells . The
electrons determine how an atom interacts with other atoms,
but the nucleus determines how many electrons an atom has,
because the positively charged protons attract and hold the
negatively charged electrons in their orbits.
The number of protons in its nucleus determines an at-
om's identity and its atomic number . Hydrogen (H), for in-
stance, has one proton in its nucleus and thus has an atomic
number of 1. The nuclei of helium (He) atoms possess
2 protons, whereas those of carbon (C) have 6, and uranium
(U) 92, so their atomic numbers are 2, 6, and 92, respectively.
Atoms also have an atomic mass number , which is the sum
of protons and neutrons in the nucleus (electrons contrib-
ute negligible mass to atoms). However, atoms of the same
chemical element might have different atomic mass num-
bers because the number of neutrons can vary. All carbon
(C) atoms have six protons—otherwise they would not be
carbon—but the number of neutrons can be 6, 7, or 8. Thus
we recognize three types of carbon, or what are known as iso-
topes (
MATTER—WHAT IS IT?
Matter is anything that has mass and occupies space, and ac-
cordingly includes water, plants, animals, the atmosphere,
and minerals and rocks. Physicists generally recognize three
states or phases of matter: liquids , gases , and solids. * Liquids,
such as surface water and groundwater, as well as atmo-
spheric gases, are important in our considerations of sev-
eral surface processes, such as running water and wind, but
here our main concern is with solids because, by defi nition,
minerals are solids.
Atoms and Elements
Matter is made up of chemical elements , which, in turn, are
composed of atoms , the smallest units of matter that retain
the characteristics of a particular element (
Figure 3.2). That
is, elements cannot be changed into different substances ex-
cept through radioactive decay (discussed in Chapter 17).
Thus, an element is made up of atoms, all of which have the
same properties. Scientists have discovered 92 naturally oc-
curring elements, some of which are listed in Figure 3.2, and
several others have been made in laboratories. All naturally
occurring elements and most artifi cial ones have a name and
Figure 3.4), each with a different atomic mass number.
The isotopes of carbon, or those of any other ele-
ment, behave the same chemically; carbon 12 and carbon 14
are both present in carbon dioxide (CO 2 ), for example.
However, some isotopes are radioactive, meaning that they
Distribution of Electrons
First
Shell
Nucleus
Electrons
Atomic
Number
Second
Shell
Third
Shell
Fourth
Shell
Element
Symbol
Hydrogen
Helium
Carbon
Oxygen
Neon
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Potassium
Calcium
Iron
H
He
C
O
Ne
Na
Mg
Al
Si
P
S
Cl
K
Ca
Fe
1
2
6
8
10
11
12
13
14
15
16
17
19
20
26
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
4
6
8
8
8
8
8
8
8
8
8
8
8
1
2
3
4
5
6
7
8
8
14
1
2
2
HYDROGEN
1 p , 1 e
HELIUM
2 p , 2 e
OXYGEN
8 p , 8 e
NEON
10 p , 10 e
SILICON
14 p , 14 e
IRON
26 p , 26 e
Figure 3.2 Shell Models for Common Atoms The shell model for several atoms and their electron
confi gurations. A blue circle represents the nucleus of each atom, but remember that atomic
nuclei are made up of protons and neutrons, as shown in Figure 3.4.
*Actually, scientists also recognize a fourth state of matter known as
plasma , an ionized gas as in fl uorescent and neon lights and matter in the
Sun and stars.
 
 
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