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CHAPTER 2
THE ELEMENTS: BASIC BUILDING BLOCKS OF GREEN
CHEMICALS
From Green Chemistry and the Ten Commandments of
Sustainability, Stanley E. Manahan, ChemChar Research,
Inc., 2006
[email protected]
2.1. Elements, Atoms, and Atomic Theory
Atoms are composed of subatomic particles
• Positively charged proton (+)
• Negatively charged electron (-)
• Electrically neutral neutron (n)
Properties of atoms determine matter’s chemical behavior
• Arrangement and energy levels of electrons in atoms
Elemental behavior varies periodically
with increasing atomic number
• Enables placing elements in the periodic table
• As atomic number increases, electrons are added
incrementally to atoms
• Electrons occupy shells in atoms, which are filled with
a specific number of electrons
• As each shell is filled, a new shell is started, thus
beginning a new period (row) of the periodic table
• The construction of a simplified 20-element periodic
table is shown in this chapter
Green Aspects of Elements
• Nitrogen, oxygen, and carbon (contained in carbon
dioxide gas) from the “green” atmosphere
• Hydrogen and oxygen in water, the “greenest”
compound
• Sodium and chlorine in common table salt
• Silicon, calcium, and oxygen in soil that grows plants
supplying food to most organisms
• Hydrogen, oxygen, carbon, nitrogen, and sulfur in all
living material
Dalton’s Atomic Theory
Na
Na
Cl Cl
K
Two atoms in
Very similar, but
Cl
Cl2 molecule
not identical
1. Each element is composed of extremely
small particles with the same chemical
K
properties called atoms.
2. Atoms of different elements
do not have identical chemical
properties.
2H2 + O2  2H2O
H
H
+
H
O
H
O
+
H
O
H
H
+
O
H
3. Chemical compounds are formed by the combination of atoms
of different elements in definite, constant ratios that usually
can be expressed as integers or simple fractions.
Dalton’s Atomic Theory (2)
H
H
C
H
O
+
H
CH4
O
O
+
O
C
O
O
+
2O2

CO2
H
H
O
+
H
+
+
O
H
2H2O
4.Chemical reactions involve the separation and combination of
atoms as in the example above where bonds are broken between C
and H in CH4 and between O and O in O2, and bonds are formed
between C and O in CO2 and between H and O in H2O.
S
5. Atoms are not created, destroyed, or
changed to atoms of other elements
in ordinary chemical reactions.
C
S
C
Three Laws Explained by the
Atomic Theory
1. Conservation of Mass: There is no detectable change
of mass in ordinary chemical reactions.
2. Constant Composition: A specific chemical
compounds always contains the same elements in the
same proportions by mass.
3. Multiple Proportions: When two elements combine to
form two or more compounds, the masses of one
combining with a fixed mass of the other are in ratios
of small whole numbers.
The Nature of Atoms
Atoms are extremely small and light
• Individual masses are expressed in atomic mass units, u
• Size in picometers, picometer = 0.000 000 1 millimeters
• Atoms may be regarded as spheres 100-300 picometers in
diameter
Atoms are composed of three kinds of subatomic
particles
• Positively charged proton (+), mass essentially 1 u
• Neutral neutron (n), mass essentially 1 u
• Negatively charged electron (-), mass essentially 0
Atoms
Each atom of a specific element has the same number of
protons in its nucleus
• Atomic number
Isotopes are atoms of the same element that have different
numbers of neutrons and, therefore, different masses
• Isotopes are represented by special symbols:
Mass number
12
Element symbol
6
Atomic number
C
• The mass number is the sum of protons and neutrons
in the nucleus
The average mass of all atoms of an element is the atomic
mass
• Normally not integers
Electrons in Atoms
Behavior of electrons in atoms determines chemical
properties
Electrons are strongly attracted to the atom’s nucleus,
but do not come to rest on it
• Energy levels
• Orientations in space
• Electron configuration
2.2. Hydrogen, The Simplest Atom
Lewis symbols show electrons (outer shell electrons) in
atoms
Lewis symbol of the
hydrogen atom
H
Lewis formulas can be used to show electrons in
molecules, such as H2
H.
.H
Elemental hydrogen
does not exist as
individual H atoms
H2
Instead, it exists as
molecules, each
composed of 2 H
atoms with the
chemical formula H2
H:H
The covalent bond
holding the two H
atoms together
consists of 2 shared
electrons shown in
the Lewis formula of
H2 above.
Properties and Uses of Elemental H2
Elemental H2 is a colorless, odorless gas
• Lowest density of any pure substance
• Liquid hydrogen boils at -253C
Hydrogen gas is widely used in the chemical industry to react
chemically with a large number of substances
Burns readily with a large release of energy; mixtures of hydrogen
with oxygen or air are extremely explosive
2H2 + O2  2H2O + energy
Hydrogen is a very green element because when it is used to generate
energy, the reaction product is simply water, H2O
Properties and Uses of H2 (2)
Hydrogen can be produced by
2H2O (Electrical current)  2H2 + O2
or by steam reforming of methane or other hydrocarbons at high
temperatures and pressures:
CH4 + H2O (800C T, 30 atm P)  CO + 3H2
Hydrogen is used to manufacture a number of chemicals, for example
methyl alcohol, CH3OH):
CO + 2H2  CH3OH
Methanol can be used as a fuel or blended with gasoline to run
internal combustion engines
Now methane is broken down to elemental hydrogen and carbon
dioxide to produce hydrogen used in fuel cells
Hydrogen in Fuel Cells
e-
H2
Electrical curr ent that
e- can be us ed for motors ,
lighting, other purposes
O2
Net reaction 2H 2 + O 2  2H 2O
Anode, r eaction
2H 2  4H+ + 4e -
Cathode, reaction
O2 + 4H+ + 4e-  2H2O
Movement of H+ from anode to cathode
through cation-permeable membrane
2.3. Helium, The First Noble Gas
Helium, He, atomic number 2
• Predominantly
• Some
Helium is a noble gas meaning that it exists only as atoms of the
elements that are never bonded to other atoms
The Lewis symbol of helium is simply He with 2 dots:
-
He
2+
A helium atom has a filled
electron shell containing
2 electrons.
It can be r epresented by the
Lewis symbol above.
Helium (2)
Electrons added at various levels known as electron shells
• The one electron in hydrogen, H, goes into the first electron shell,
the one with the lowest possible energy
The lowest electron shell can contain a maximum of only 2 electrons
• So helium has a filled electron shell making it a noble gas
Helium is a nontoxic, odorless, tasteless, colorless gas with a very
low density of only 0.164 g/L at 25C and 1 atm pressure
Helium comes from some sources of natural gas containing up to 10%
helium by volume
• Helium was first observed in the light spectrum of the sun by the
specific wavelengths of light emitted by hot helium atoms
Uses of Helium
Helium gas used for
• Inert atmosphere
• Weather balloons
• Breathing by divers
Super-cold liquid helium at 4.2 K (-269°C)
• Cryogenics (very low temperatures)
• Superconductor
Hydrogen Wants to be Like Helium
Elemental hydrogen exists as diatomic molecules, formula H2
Hydrogen comes just before helium in the periodic table
Hydrogen acquires a noble gas electron configuration by two H atoms
sharing electrons as shown below:
2.4. Lithium, The First Metal
Lithium, Li, atomic number 3, atomic mass 6.941
• Most abundant lithium isotope is
having 4 neutrons in its nucleus
• A few percent of lithium atoms are the
isotope, which has 3
neutrons
Lithium’s lowest electron shell is filled with 2 electrons
The third electron in lithium goes into a second shell, an outer shell
Inner
electrons
Outer electron
Nucleus
3+
-
The lithium atom showing both inner shell
and outer shell electrons
Inner Shell and Outer Shell Electrons
Two of lithium’s 3 electrons are inner electrons contained in an inner
shell
• As in the immediately preceding noble gas helium
Inner electrons
• Stay on average relatively close to the nucleus
• Very tightly held
• Not exchanged or shared in chemical bonds
The third electron in lithium is an outer electron in the atom’s outer
shell farther from, and less strongly attracted to, the nucleus
Loss of Outer-Shell Electrons to Produce
Cations
Lewis symbols normally show outer-shell electrons
Li
Lithium loses its outer shell electron to become like helium:
• No longer a neutral atom, but has become a positively charged Li+
cation
• In losing an electron, the lithium atom is said to be oxidized
• Li+ cations are attracted to negatively charged anions in ionic
compounds
Lithium is the First Metal
Metals
•
•
•
•
•
1-3 outer-shell electrons
Form +1, +2, or +3 cations
Luster (shine)
Malleable
Conduct electricity
Uses of Lithium
Lithium has several important uses
• Li2CO3 to treat manic-depressive and schizoaffective mental
disorder, starting material for the preparation of other lithium
compounds, ingredient of specialty glasses and enamels
• Lithium hydroxide, LiOH, is used to formulate some kinds of
lubricant greases and in some long-life alkaline storage batteries
• In combination with iodine to power cardiac pacemakers lasting up
to 10 years
2.5. The Second Period of the Periodic Table
First period consists of only hydrogen and helium
Second period consists of elements 3-10
All atoms in the second period have 2 inner-shell electrons like helium
and 3-8 outer-shell electrons
Beryllium
Beryllium, atomic number 4, atomic mass 9.012
Be
4 protons and 5 neutrons in Be nuclei
Formation of Be2+ cation
Be:  Be2+ + 2eBeryllium is used in alloys mixed with other metals
Beryllium alloys
• Hard and corrosion-resistant
• Good electrical conductors, nonsparking when struck
• Specialty springs, switches, small electrical contacts
• Aircraft brake components
Beryllium is not a very green element
• Cause of berylliosis, a diseased marked by lung deterioration
Boron, a Metalloid
Boron, B, atomic number of 5, atomic mass 10.81
• Most boron atoms have 6 neutrons in addition to 5 protons in their
nuclei
• A less common isotope has 5 electrons
• Two of boron’s 5 electrons are in a helium core and 3 are outer
electrons as shown by the Lewis symbol
B
Boron is a metalloid
• Among the first 20 elements, silicon is also a metalloid
• Metalloids are semiconductors
Boron(2)
Boron is a high-melting substance (2190C)
• Alloyed with copper, aluminum, and steel metals to improve their
properties
• Absorbs neutrons in nuclear reactors
• Boron nitride, BN, is extraordinarily hard
• Boron oxide, B2O3, in heat-insulating fiberglass
• Boric acid, H3BO3, is used as a flame retardant in cellulose insulation
Carbon, The Element of Life
Carbon, C, atomic number 6
C
98.9% 162C, 1.1%163C
Detectable amounts of radioactive carbon-14, designated
14
6C
are produced by nuclear processes high in the atmosphere
• Radioactivity of carbon-14 used to date carbon-containing artifacts
Carbon is the “element of life”
Carbon is involved in organic compounds, thus forming the basis of
organic chemistry
Carbon Atoms Bonded to Each Other in Straight
Chains, Branched Chains, Rings
H H H H H H H H
H C C C C C C C C H An 8-carbon straight-chain
hydrocarbon, formula C 8 H18
H H H H H H H H
H
H
H
C
C
H C H
H
H
C H HH
C H
H
H
H
H
C
C
H C
C
C H
H
H
C
C
H H
H
H
H H
H
H
H C H
C
C
H C H
H
H
H
An 8-carbon cyclic hydroAn 8-carbon branched-chain
hydrocarbon, formula C 8 H18
carbon, formula C
8 H16
Elemental Carbon
Elemental carbon has some important uses
• Very finely divided carbon black used in tires, inks, and printer toner
• Graphite atoms bonded in large, flat molecules used as a dry
lubricant
• Activated carbon produced by reacting carbon with steam or carbon
dioxide used to purify foods, remove organic pollutants from water,
and remove pollutant vapors from air
• Composites consisting of carbon fibers bonded together with epoxy
resins
• Carbon in very hard and rigid structure of diamond
Green Carbon From The Air
Air is about 0.038% CO2 by volume, serving as a carbon source for
photosynthesis:
6CO2 + 6H2O (Sunlight energy)  C6H12O6 + 6O2
Organic carbon generated by photosynthesis produced petroleum,
coal, and other fossil fuels
There is much current interest in photosynthesis to provide carbon
raw material and fuel
Nitrogen From The Atmosphere
Nitrogen, N, atomic number 7, atomic mass 14.01
N
Diatomic N2 comprises 78% by volume of air
• Isolated from air by distillation of cold liquid air and by adsorption
processes
The molecules of elemental nitrogen are extremely stable NN
Elemental nitrogen is chemically rather unreactive
Liquid nitrogen boils at -190C
• Used in cryogenics to quick-freeze foods, for drying materials in
freeze-drying processes, preserve biological materials, such as
semen used in artificical breeding of animals or embryos used in in
vitro fertilization
Nitrogen, A Green Raw Material From The
Atmosphere
Inexhaustible nitrogen in the atmosphere, but hard to convert to
nitrogen compounds
N2 + 3H2  2NH3
Bacteria such as Rhizobium bacteria fix atmospheric nitrogen
Nitrogen is an essential life element
• Proteins, hemoglobin, chlorophyll, enzymes, and other life
molecules
• Nitrogen cycle
Oxygen, The Breath Of Life
Oxygen, atomic number 8, atomic mass 16.00
O
O2 molecules make up 21% of the volume of air
Oxygen may be regarded as a green element
• O2 is in the atmosphere for the taking
• From distillation of liquid air
Pure oxygen used for breathing, in chemical synthesis, oxyacetylene
torches
Oxygen (2)
Oxygen in the stratosphere (from Chapter 1)
• O2 (Ultraviolet radiation)  O + O
• O 2 + O  O3
• Protective ozone layer in the stratosphere
Ozone is detrimental and toxic in the atmosphere at sea level
Oxygen reacts with other substances to produce energy:
• Combustion of hydrocarbons at high temperatures to provide heat
or mechanical energy in an engine
2C8H18 + 25O2  16CO2 + 18H2O + energy
• Oxidation of glucose in an organism to provide energy
C6H12O6 (Glucose) + 6O2  6CO2 + 6H2O + energy (at 37C)
Fluorine, The Most Nonmetallic Element
Fluorine, F, atomic number 9, atomic mass 19.00 has 7 outer electrons
as shown by its Lewis symbol:
F
Elemental fluorine exists as diatomic F2
Fluorine is the most nonmetallic of all elements
Fluorine reacts violently with metals, organic matter, even glass!
Fluorine is a very corrosive poison that attacks flesh and produces
wounds that heal very poorly
Practice of green chemistry seeks to minimize the generation or use
of F2 and of highly corrosive HF
Fluorine is widely used in chemical synthesis, for example, to make
teflon and Freon substitutes, such as HFC-134a, CH2FCF3
2.6. The Magic Octet of 8 Outer-Shell
Electrons
Neon, atomic number 10, atomic mass 20.18
Most neon atoms have 10 neutrons, some have 12, and very few 11
Exists as individual Ne atoms, never combined with other atoms
About 2 parts per thousand by volume in air
• Neon is obtained from distillation of liquid air
• Most common use in glowing neon signs
Neon has a filled outer electron shell of 8 electrons
Ne
This filled shell makes neon a noble gas
Special Significance of the Octet
Noble gas elements other than neon are argon (atomic number 18),
krypton (atomic number 36), xenon (atomic number 54), and radon
(atomic number 86)
• Other than helium, which has a filled outer shell of 2 electrons, the
noble gases share a common characteristic of 8 outer-shell
electrons
The filled outer-shell electron configuration can by shown by the
following general Lewis symbol
X
• Represents an octet of electrons
The Octet Rule
The octet rule is the tendency of atoms to acquire stable octets
through chemical bonding as shown for elemental N2 below:
N
N
Two nitr ogen atoms, each
with 5 outer -shell electrons
•
•
•
•
N N
Diatomic N 2 molecule with the s table
octets of both atoms cir cled.
In N2 there are only 10 electrons potentially available for bonding
The 2 inner-shell electrons are not available for bonding
Therefore, 6 electrons have to be shared in a triple bond
The triple bond is extraordinarily strong accounting for the extreme
stability of the elemental N2 species
2.7. Completing the 20-Element Periodic Table
Ten more elements to complete the periodic table
Sodium, Na, atomic number 11, atomic mass 22.99
Soft, chemically very reactive metal
Below is a representation of the electrons in 2 inner shells and 1
outer shell of sodium and the Lewis symbol of sodium showing
the single outer-shell electron as a dot:
Na
Single electron in
an outer shell
Innermost electr on shell
with 2 electr ons
Second inner shell
with an octet of
electrons
Na
Lewis s ymbol of
sodium
Magnesium and Aluminum
Magnesium, Mg, atomic number 12, atomic mass 24.31 exists in the
elemental form as a strong lightweight metal
• Extension ladders
• Portable tools
• Aircraft
Aluminum, Al, atomic number 13, atomic mass 26.98 is a strong,
lightweight metal
• Aircraft • Automobiles • Electrical lines • Building construction
Aluminum metal forms a self-protecting oxide coating
Aluminum can be regarded as a green metal
• Strong, lightweight component in aircraft and automobiles
• Efficient transmission of electricity
• Abundant element
• May be extracted from fly ash left over from coal combustion
• Highly recyclable
• Recycling aluminum saves enormous amounts of energy required to
prepare aluminum metal from aluminum (bauxite) ore
Silicon
Silicon, Si, atomic number 14, atomic mass 28.09
• A metalloid and semiconductor
• Key element in semiconductor industry
• Second most abundant element in Earth’s crust
Silicon is a green element in electronics and signal transmission
• Vastly reduced bulk of electronic components, saving materials in
computers, radios, televisions, communications equipment
• Solid-state electronics consume only a fraction of the electricity
once used by vacuum tube based devices
• Silicon fiber optics: No bulky, expensive copper, less energy
Phosphorus
Phosphorus, P, atomic number 15, atomic mass 30.95
Most common elemental form is white phosphorus
• Chemically very reactive nonmetal that may catch fire
spontaneously in the atmosphere
• Toxic and causes deterioration of the bone and a condition called
“phossy jaw”
Phosphorus is an essential life element that is one of the components
of DNA, the macromolecule that directs life processes
Essential plant fertilizer
Ingredient of many industrial chemicals including some pesticides
Chemically related arsenic contaminates phosphorus
• Arsenic is toxic and must be removed from phosphorus put in food,
such as phosphoric acid added to soft drinks
Sulfur
Sulfur, S, atomic number 16, atomic mass 32.06
Essential nutrient for plants and animals, occurring in the amino acids
that compose proteins
Common air pollutant emitted as sulfur dioxide, SO2, in the
combustion of fossil fuels that contain sulfur
Much of the sulfur that is used is obtained from hydrogen sulfide, H2S,
that contaminates much of natural gas
2H2S + 3O2  2SO2 + 2H2O
2H2S + SO2  3S + 2H2O
Chlorine
Chlorine, Cl, atomic number 17, atomic mass 35.453
Has 7 outer-shell electrons, just 1 electron short of a full octet
Cl
Cl
Two chlorine atoms, each
lacking only 1 electr on for
a complete octet in their
outer shells
Na
Cl
A sodium atom donates its
outer-shell electron to a
chlorine atom, leaving the
resulting Na + cation with
its underlying second shell
electrons as its filled outer
shell octet
Cl Cl
share 2 electrons s o that
they are held together by
a single covalent bond in
the Cl 2 molecule.
Na
+-
Formation of ionic NaCl.
The Cl atom accepts an
electron to gain a stable
octet as the Cl- anion and
the Na atom loses an
electron leaving it as the
Na+ cation with a stable
octet.
Cl
to produce the ionic compound
sodium chloride (NaCl) in which
both the Na + cation and the Cl anion have filled outer electron
shells consisting of stable octets.
Greenish-yellow Cl2 gas
• Chlorine is an important industrial chemical used to make plastics
and solvents
Green aspects of chlorine
• Abundant
• Important for public health because of its use in water disinfection
• Polyvinyl chloride (PVC) used in water pipe and drain pipe replace
relatively scarce and expensive copper metal and toxic lead
Non-green Aspects of Chlorine
• Toxic substance (first military poison used)
• Organochlorine solvents pollute air and water and are somewhat
toxic
• Waste products of organochlorine compound manufacture are
pollutants
• Chlorine-containing vinyl chloride used to make PVC products is a
known human carcinogen
The practice of green chemistry minimizes the production and use of
elemental chlorine and generally attempts to minimize production of
organochlorine compounds and their dispersion to the environment
Argon
Argon, Ar, atomic number 18, atomic mass 39.95
Complete octet of outer-shell electrons makes argon a noble gas
Argon composes about 1% by volume of atmospheric air and is
recovered from distillation of liquid air
Uses of argon depend upon its chemically inert nature
• Argon is used to fill incandescent light bulbs to prevent evaporation
of white-hot tungsten atoms from the glowing lamp filament, thus
significantly extending bulb life
• Argon is used as a plasma medium in instruments employed for
inductively coupled plasma atomic emission analysis of elements in
environmental, biological, and other samples
Completing the Periodic Table
Potassium, K, atomic number 19, atomic mass 39.10
39
Ґ Mostly 19
K
Ґ Small fraction is radioactive
40
19K
Like sodium, potassium is a very reactive alkali metal
An essential element for life and a common crop fertilizer
Produces K+ ion
Calcium
Calcium, Ca, atomic number 20, atomic mass 40.08
Readily loses its 2 outer-shell electrons to produce Ca2+ cation
Alkaline earth metal
Elemental calcium metal is chemically reactive
Chemical properties very similar to those of magnesium
Essential for life, plant growth
Essential animal nutrient to form hydroxyapatite, Ca5OH(PO4)3 in teeth
and bones
Deficiency can cause disabling osteoporosis
2.8. The Brief Periodic Table is Complete
Elements Above Atomic Number 20
Placement of electrons in elements with atomic number 21 and higher
becomes complicated
Important elements above atomic number 20 include:
• Transition metals including chromium, manganese, iron, cobalt,
nickel, and copper
• Lanthanides and actinides including thorium, uranium, and
plutonium, which are important in nuclear energy and nuclear
weaponry
Aspects of the Periodic Table
Hydrogen is unique
Elements in vertical columns belong to groups with similar chemical
properties
First group on the left of the table — lithium, sodium, and potassium
— are alkali metals
• Very low density and so soft that they can be cut with a knife
• Exposed metal corrodes very rapidly
• Violent reaction with water to produce strong base metal hydroxides
2M + 2H2O  2MOH + H2
• React with elemental chlorine to produce ionic LiCl, NaCl, and KCl
The second group consists of alkaline earth metals: beryllium,
magnesium, calcium
• Highly reactive metals that produce Be2+, Mg2+, Ca2+ ions
Aspects of the Periodic Table (Cont.)
Halogens compose the second group from the right
• Diatomic gases in which the two atoms of F2 or Cl2 are held together
by a single covalent bond consisting of two shared electrons
• Most nonmetallic elements
• Readily gain electrons to complete their outer shell octets producing
F- and Cl- anions
The far right group consists of noble gases composed of single
atoms: helium, neon, and argon
He has a complete outer shell of 2e-, Ne and Ar have 8