CHAPTER 1 - THE BUILDING BLOCKS OF MATTER

Elementary Particles:

The entire material world is composed of tiny fundamental particles called atoms, and

the traditional and simplest model of an atom is a tiny solar system with a central nucleus

consisting of protons (positive electrical charge) and neutrons (no electrical charge), and which

constitutes nearly the entire mass of the atom, surrounded by tiny orbiting electrons (negative

electrical charge.

The number of protons in the nucleus of an atom is the atomic number and defines the

element. For example, every atom of carbon has six protons in its nucleus and every atom with

six protons in is nucleus is an atom of carbon. The number of neutrons in an atom nucleus is

usually about equal to the number of protons, but may be a few more. Neutrons (no electrical

charge) neither alter the electrical charge of the nucleus nor participate in chemical reactions, but

neutrons do increase the mass of the atom, as they have about the same mass as a proton. The

number of protons plus the number of neutrons in the nucleus of an atom is the mass number of

that atom. A carbon atom with six neutrons in its nucleus is an atom of the common isotope of

carbon and is designated C12 (i.e. six protons and six neutrons). An atom of C13 or C14 would

contain seven and eight neutrons respectively and would represent additional isotopes of carbon.

If all carbon atoms were C12, the atomic weight of carbon would be 12.000. Natural carbon,

however, has small numbers of C13 and C14 atoms causing the measured atomic weight of

naturally occurring carbon to be 12.011 (See Table 1-1). Atoms of C14 are radioactive, meaning

that the nuclei of the atoms are too massive to be stable and will spontaneously break apart,

emitting nucleus fragments (i.e. radiation) and perhaps clusters of protons and neutrons

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representing the nuclei of elements of lower atomic number. This is a nuclear reaction and the

associated radiation may damage crystal structures (e.g. metamict zircon) or may cause color

centers (e.g. topaz). ).

Our major concern here are electrons and their configuration about the nucleus of the

atom, because these electrons are the “players” in chemical reactions, atomic bonding, light

absorption and transmission, color, index of refraction, fluorescence, hardness and other

characteristics of gem minerals.

Electrons are the negatively charged “planets” of the atomic “solar system”. They are tiny

with an insignificant mass compared to the mass of a neutron or proton. The electrical charge of

an electron, however, is equal and opposite that of a proton. An electrically neutral atom,

therefore, has an equal number of electrons in orbit and protons in its nucleus. An atom becomes

electrically unbalanced with the loss or gain of electrons creating ions. An atom which has more

orbiting electrons than nuclear protons is a negative ion or anion. An atom with fewer electrons

is a positive ion or cation. The effective size of an ion may be quite different from the effective

size of a neutral atom of the same element.

The Periodic Table of the Elements

Every electron exists in a so-called energy level, and the laws of quantum mechanics

allow only certain energy levels to exist and every electron must be in one of those energy levels.

It must occupy the lowest energy level available to it, unless it is excited to a higher level by

absorbing energy (e.g. light energy) from an external source. In their simplest form, the major

energy-levels, or shells, may be viewed as concentric onion-like layers about the atom nucleus.

The shell nearest the nucleus is called the K shell and contains only one sub-shell, the s sub-

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shell, which, in turn, has only one electron pair, or orbital (i.e. two electrons of opposite

“spin”). An electrically neutral hydrogen atom has an atomic number of 1 and one electron in

the s sub-shell of the 1st or K shell and its “electron configuration” is represented as 1s1 (i.e.

the first shell contains one electron in the s sub-shell). The electron configuration of a neutral

helium atom is expressed as 1s2, or two s electrons in the 1st shell. With helium, the 1st (K) shell is

filled and we must move out to the 2nd shell, or L shell, to add additional electrons. *

*In building the periodic table we assume electrically neutral atoms with an equal number of electrons and

protons. The addition of an electron does not change the element (i.e. increase the atomic number), but the addition

of a proton to balance it does change the element.

The 2nd shell (L) allows two sub-shells, the s sub-shell, which may contain only two

electrons, and the p sub-shell which may contain up to six electrons. Filling the 2nd shell

completes the second period (row) of the standard periodic table (Table 1-1), beginning with

lithium (1s22s1) and continuing with beryllium (1s22s2), boron (1s22s22p1), carbon (1s22s22p2),

nitrogen (1s22s22p3), oxygen (1s22s22p4), fluorine (1s22s22p5) and ending with neon (1s22s22p6).

Neon has complete s and p sub-shells with a total of eight electrons. This is a very stable electron

configuration, and neon will not form ions or participate in chemical reactions. Neon is an inert

gas of independent atoms as are all elements below it in the right column of the periodic table

(Ne, Ar, Kr, Xe, Rn).

Lithium (1s22s1) has only one electron in its outer (L) shell which orbits far from the

nucleus, owing to the weak positive charge of the nucleus (three protons). This single electron

can be taken from the lithium atom by a minor electrical force, forming a lithium ion (i.e. cation

Li+) and leaving the lithium atom with a complete outer electron shell (i.e. two 1s electrons

complete the 1st shell). A fluorine atom (1s22s22p5) requires only one additional electron to

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complete its 2nd shell of eight electrons. Its strong nuclear attraction (nine protons) pulls its two

electron shells nearer the nucleus and may well attract the electron freed from the lithium ion,

forming a negative fluorine ion (i.e. anion F─). Positive lithium ions and negative fluorine ions

unite with an ionic bond to form a strongly ionic compound lithium fluoride (LiF).

Table 1-1 Periodic Table of the Elements

Beryllium forms bivalent cations (Be2+) and oxygen forms bivalent anions (O 2─).

Beryllium oxide (BeO) is much harder than lithium fluoride because Be2+ and O 2─ share a double

ionic bond. Lithium oxide (Li2O) requires two lithium cations (Li+) for every oxygen anion (O 2─)

for charge balance. Those electrons in the outermost shell and which may participate in chemical

reactions are called valence electrons.4

For the third row of the periodic table we move out to the 3rd, or M, shell to add electrons

forming the elements of the third period. Atoms become larger with the addition of another shell

but shrink from left to right, sodium (1s22s22p63s1) to argon (1s22s22p63s23p6), as protons are

added to the nucleus until yet another shell, the 4th or N shell is begun. Argon, like helium, is an

inert gas with complete s and p sub-shells in its outer shell. The 3rd shell is not filled with

complete s and p sub-shells, however, as an additional ten electrons are yet to be added to the M

shell as a d sub-shell.

The 4th shell begins before the 3rd shell is complete, because the 4s sub-shell represents a

slightly lower energy level than the energy level of the 3d sub-shell. The 4th shell begins,

therefore, with two elements, potassium (1s22s22p63s23p64s1) and calcium (1s22s22p63s23p64s2),

filling the 4s sub-shell. The atom building then continues with ten electrons of the 3d sub-shell

from scandium (1s22s22p63s23p64s23d1) to zinc (1s22s22p63s23p64s23d10) by adding ten electrons to

the 3d sub-shell to complete the 3rd shell. The 4th row of the periodic table is completed by the

addition of the six electrons of the 4p sub-shell, yielding elements from gallium to another inert

gas krypton. Those ten elements from Sc to Zn are called transition metals and result from the

addition of ten 3d electrons added not to the outermost electron shell (4th shell) but to the

underlying 3rd shell. These elements are responsible for the most beautiful and most stable colors

in gemstones and for most color in rocks and nature in general (see Chapter 6). Because some 3d

electrons occupy energy levels near those of the two 4s electrons, transition elements may form

more than one kind of cation. Iron, for example, may form bivalent ferrous cations (Fe2+) or

trivalent ferric cations (Fe3+), which have different chemical characteristics and cause different

colors.

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We could continue this process of building up the periodic table by adding additional

electron shells, however, the above will suffice for our needs in future chapters. Note that the 5th

shell, or O shell, begins with two 5s electrons (Rb and Sr) followed by another transition series

of ten elements, Y to Cd, adding ten 4d electrons. The 6th shell, or P shell, begins with two 6s

electrons (Cs and Ba) and begins the third transition series with one 5d electron (Lanthanum).

This is followed by fourteen elements, called the rare earth elements, resulting from the

fourteen electrons of the 4f sub-shell, which completes the forth shell. Electrons forming the rare

earth elements are added two shells below the outermost shell, which yields fourteen elements of

almost identical chemical characteristics, since only electrons in the outer shell are involved in

chemical reactions. If we maintain electrically neutral atoms, one proton is added to the nucleus,

changing the element, for every electron added to the electron shells and sub-shells. Because the

electrons of the rare earth elements are added close to the nucleus, the strongly positive nucleus

pulls in the electron shells, and the atom size of the rare earth elements actually decreases

slightly across the series from La to Lu (lanthanide contraction). Atoms of those elements

immediately following this contraction are about the same size as those elements which lie

directly above them on the periodic table, even though they contain an additional electron shell.

Cations of Hf4+, for example, are actually slightly smaller than Zr4+ cations, and hafnium exists in

nature almost exclusively as a minor replacement of zirconium ions in such minerals as zircon.

Thorium cations Th4+, although two shells larger than zirconium cations, are still small enough to

replace a few Zr+4 cations, and the radioactive thorium cations become the source of radiation

that bombards the zircon structure producing metamict zircon.

Let us now change our attention from horizontal rows on the periodic table to vertical

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columns. Neutral atoms of elements in the 1st, or left-hand, column (Li, Na, K, Rb, Cs, Fr)

loosely hold one s electron in their outermost orbit, as a valence electron. This s electron is easily

removed with little applied energy, forming a single-charge positive ion which is large compared

to ions of elements in the column that follows to the right. Elements of the 1st column are called

alkali elements. They can form only single-charge cations and do not cause color. Elements of

the 2nd column (Be, Mg, Ca, Sr, Ba, Ra) are called alkaline earth elements. They can form only

double-charge cations and do not cause color. Elements of the last, or right-hand column (He,

Ne, Ar, Kr, Xe, Rn) are inert gases and do not enter into chemical combination with other

elements. Elements in the column to the left of the inert gasses (F, Cl, Br, I, At) need only one

additional electron to complete the octet configuration of the complete s and p sub-shells. These

elements are called halogen elements. They have a strong attraction for the additional p electron

and form only single-charge negative anions, which do not cause color, and which readily

combine with alkali cations (e.g. NaCl, KF, etc.) or alkaline earth cations (e.g. CaF2, MgCl2,

etc.).

Elements in the column to the left of the halogens (O, S, Se, etc.) form anions with

double negative charge and those in the next left column (N, P, As, etc.) may form triple

negative anions (N 3─) or positive

.five cations (N5+ as in NO3─). Complex ions like the nitrate ion (NO3

─), sulfate ion

(SO42─), carbonate ion (CO3

2─), phosphate ion (PO43─), etc. are actually held together more by

covalent bonding than by ionic bonds, however, these complex anions may unite with simple

cations to form ionic compounds (e.g. CaCO3 calcite).

Abundance of the Elements:

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Of the ninety-two naturally occurring elements in the periodic table, only eight exist in

the “earth’s crust”* in amounts over one weight percent, and two of those eight elements

(oxygen and silicon) make up almost three-fourths (74%) of the crust. Consequently, nearly all

abundant minerals are silicates and minerals containing elements other than the abundant eight

(i.e. oxygen, silicon, aluminum, iron, calcium, sodium, magnesium and potassium) are rare..

Various geologic processes may concentrate one or more of these rare minerals to form “ores” of

these trace elements. Hydrogen is concentrated in the water of the hydrosphere, carbon in

organic matter and carbonate sediments, and nitrogen makes up about three-fourths of the

atmosphere. All three are abundant and important in the chemistry at the earth’s surface, but

none rises to the level of one percent of the total crust.

*The earth’s crust is that outer layer of the earth, ranging in thickness from about 5 miles, beneath the oceans, to as

much as 40 miles, below the continents. The atmosphere, hydrosphere and biosphere may be considered part of the

earth’s crust and have little effect on the overall crustal composition, when they are included in a mix with the rocky

crust.

By far the most abundant element of the universe is hydrogen, i.e. uncombined

protons. A single proton is a hydrogen atom. Giant, cold clouds of protons and neutrons populate

interstellar space and form the raw materials of stars, and a young “main sequence star” is

almost completely hydrogen. In its thermonuclear furnace, protons and neutrons combine to form

hydrogen atoms of mass number 2 (i.e. 1H1 + n1 = 1H2) and hydrogen atoms fuse to yield helium

atoms of mass 4, with the release of thermonuclear energy.

1H2 + 1H2 2He4 8

In the helium-burning stage, or “red giant star”, the hydrogen fuel in the core of the star

is used up (i.e. fused to helium), and the outer zone of burning hydrogen expands to giant

proportions. Elements of higher atomic number are now formed largely by the fusion of helium

atoms (2He4), and we find that elements with even atomic numbers tend to be more abundant

than those with odd atomic numbers. Two helium nuclei, of mass 4, may combine to form

beryllium nuclei of mass 8, however, this mass number is highly unstable (i.e. radioactive) and

quickly dissociates again to the two helium nuclei.

Three helium nuclei (2He4) fuse to form a stable isotope of carbon 6C12.

2He4 + 2He4 + 2He4 6C12

By this process, elements of atomic numbers 3, 4, and 5 require the incorporation of a hydrogen

nuclei (1H2) are largely bypassed in the main stream of creation. Lithium, beryllium and boron

are rare in the universe, and minerals like spodumene [LiAl(SiO3)2], beryl [Be3Al2(SiO3)6] and

tourmaline [Na(Li,Al)3Al6(SiO3)6(BO3)3(OH)4] are consequently rare in the mineral kingdom.

Four helium nuclei (2He4) or one helium nucleus and one carbon nucleus (6C12) may fuse

to yield oxygen which is the most abundant element in the earth’s crust.

2He4 + 2He4 + 2He4 + 2He4 8O16 or 2He4 + 6C12 8O16

In the carbon-oxygen burning stage in the evolution of a star, the helium fuel is

exhausted, and the heavy elements of the universe are formed.

6C12 + 6C12 12Mg24 and 8O16 + 6C12 14Si28

The final products of this stage are largely silicon (14Si28) and magnesium (12Mg24), and again

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many less abundant elements are bypassed in the main sequence of creation. The remaining free

protons (1H1) and helium nuclei (2He4) are absorbed, however, to yield a variety of elements

between carbon (6C12) and calcium (20Ca40). Some nuclei of very abundant nuclides, like 12Mg24

and 14Si28, will absorb free protons (1H1) to form 13Al26 and 15P30 which are abundant elements of

odd atomic number, but elements of even atomic number are invariably more abundant than

adjacent elements with odd atomic numbers.

In the final fusion stages of a dying star, temperatures rise to billions of degrees and the

silicon (14Si28) and magnesium (12Mg24) nuclei fuse to stable nuclei of iron and nickle.

14Si28 + 12Mg24 26Fe52 or 14Si28 + 14Si28 28Ni56

Iron and nickel form the cores of planets like our Earth and are the most stable of all nuclei,

meaning that further thermonuclear fusion will not occur spontaneously at any temperature, to

radiate more stellar energy. When all the silicon and magnesium fuel is used up, a final

gravitational collapse may cause the star to explode as a spectacular supernova where those

heavy elements beyond iron and nickel are formed by neutron and proton capture, building

atomic numbers as high as 92 (uranium) and mass numbers of 238 or higher.

Beyond bismuth (83Bi209), all nuclei are too massive to be stable and are radioactive. They

decay, or divide, to nuclei with smaller, more stable atomic numbers and mass numbers (atomic

fission).

Table 1-2 - Element Abundance in the Earth’s Crust

Element Symbol Wt.% Ion Oxide Name of Oxide

Oxygen O 46.4% O ─2

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Silicon Si 27.6% Si+4 SiO2 Silica

Aluminum Al 8.1% Al+3 Al2O3 Alumina

Iron Fe 5.1% Fe+2 FeO Ferrous Oxide

Fe+3 Fe2O3 Ferric Oxide

Calcium Ca 3.6% Ca+2 CaO Lime

Sodium Na 2.8% Na+ Na2O Soda

Potassium K 2.6% K+ K2O Potash

Magnesium Mg 2.1% Mg+2 MgO Magnesia

98.3%

Phosphorous (P), Hydrogen (H), Manganese (Mn) and Titanium (Ti) combine for another

1% of the earth’s crust.

The Internet Encyclopedia of Science – Geology and Planetary Sciences

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