CHAPTER 2THE ELEMENTS: BASIC BUILDING BLOCKS OF GREEN

CHEMICALS

From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research,

Inc., 2006manahans@missouri.edu

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

KNaClCl NaK Cl

HHHHOO++ OHHOHH+

Two atoms in Cl2 molecule

Very similar, but not identical

Very different

Very different1. Each element is composed of extremely small particles with the same chemical properties called atoms.

2. Atoms of different elements do not have identical chemical properties.

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.

2H2 + O2 2H2O

Dalton’s Atomic Theory (2)

OO+ OO+CHHHH COO+OHHOHH+

SCS C

CH4 + 2O2 CO2 + 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.

5. Atoms are not created, destroyed, or changed to atoms of other elements in ordinary chemical reactions.

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 AtomsAtoms 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

AtomsEach 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:126CMass number

Atomic numberElement symbol

• 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 AtomLewis symbols show electrons (outer shell electrons) in atoms

Lewis formulas can be used to show electrons in molecules, such as H2

HLewis symbol of the hydrogen atom

H. H2 H:H.HElemental hydrogen does not exist as individual H atoms

Instead, it exists as molecules, each composed of 2 H atoms with the chemical formula H2

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 -253CHydrogen 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 (800C T, 30 atm P) CO + 3H2 Hydrogen is used to manufacture a number of chemicals, for example methyl alcohol, CH3OH):

CO + 2H2 CH3OHMethanol can be used as a fuel or blended with gasoline to run internal combustion enginesNow methane is broken down to elemental hydrogen and carbon dioxide to produce hydrogen used in fuel cells

Hydrogen in Fuel Cellse-e-Electrical current thatcan be used for motors,lighting, other purposesAnode, reaction2H2 →4H++4e- ,Cathode reactionO2+4H++4e-→2H2OO2H2 Net r 2eaction H2+O2→2H2O Movement of H+fr om anode to cathode - through cation permeable membrane

2.3. Helium, The First Noble GasHelium, 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 atomsThe Lewis symbol of helium is simply He with 2 dots:--2+A helium atom has a filled It can be represented by theelectron shell containing Lewis symbol above.2 electrons.

He

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 was first observed in the light spectrum of the sun by the specific wavelengths of light emitted by hot helium atoms

Helium is a nontoxic, odorless, tasteless, colorless gas with a very low density of only 0.164 g/L at 25C and 1 atm pressureHelium comes from some sources of natural gas containing up to 10% helium by volume

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 HeliumElemental hydrogen exists as diatomic molecules, formula H2

Hydrogen comes just before helium in the periodic tableHydrogen acquires a noble gas electron configuration by two H atoms sharing electrons as shown below:

2.4. Lithium, The First Metal

Lithium’s lowest electron shell is filled with 2 electronsThe third electron in lithium goes into a second shell, an outer shell

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

3+Outer electronNucleusInnerelectrons---The lithium atom showing both inner shell and outer shell electrons

Inner Shell and Outer Shell ElectronsTwo of lithium’s 3 electrons are inner electrons contained in an inner shell

• As in the immediately preceding noble gas heliumInner 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

• 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

LiLithium loses its outer shell electron to become like helium:

Lithium is the First MetalMetals

• 1-3 outer-shell electrons• Form +1, +2, or +3 cations• Luster (shine)• Malleable• Conduct electricity

Uses of LithiumLithium 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 TableFirst period consists of only hydrogen and heliumSecond period consists of elements 3-10All atoms in the second period have 2 inner-shell electrons like helium and 3-8 outer-shell electrons

BerylliumBeryllium, atomic number 4, atomic mass 9.012

4 protons and 5 neutrons in Be nucleiFormation of Be2+ cation Be: Be2+ + 2e-

Beryllium is used in alloys mixed with other metalsBeryllium alloys• Hard and corrosion-resistant• Good electrical conductors, nonsparking when struck• Specialty springs, switches, small electrical contacts• Aircraft brake componentsBeryllium is not a very green element• Cause of berylliosis, a diseased marked by lung deterioration

Be

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

Boron is a metalloid• Among the first 20 elements, silicon is also a metalloid• Metalloids are semiconductors

B

Boron(2)Boron is a high-melting substance (2190C)• 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 LifeCarbon, C, atomic number 6 CDetectable amounts of radioactive carbon-14, designated

are produced by nuclear processes high in the atmosphere• Radioactivity of carbon-14 used to date carbon-containing artifactsCarbon is the “element of life”Carbon is involved in organic compounds, thus forming the basis of organic chemistry

6C1498.9% 6C, 1.1% 6C1312

Carbon Atoms Bonded to Each Other in Straight Chains, Branched Chains, Rings

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C CH

H

H

H

H

H

H

H

H

C

CH H

H H

H

H

CH H

H

C

C C

C

C

CC

C

H H

HH

H H

H H

H HHH

HHHH

An 8-carbon straight-chainhydrocarbon, formula C 8 H18

An 8-carbon branched-chainhydrocarbon, formula C 8 H18

An 8-carbon cyclic hydro-carbon, formula C8 H16

Elemental CarbonElemental 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 AirAir 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 fuelsThere is much current interest in photosynthesis to provide carbon raw material and fuel

Nitrogen From The AtmosphereNitrogen, N, atomic number 7, atomic mass 14.01

Diatomic N2 comprises 78% by volume of air

N• Isolated from air by distillation of cold liquid air and by adsorption

processes

The molecules of elemental nitrogen are extremely stable NNElemental nitrogen is chemically rather unreactiveLiquid nitrogen boils at -190C• 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 nitrogenNitrogen is an essential life element• Proteins, hemoglobin, chlorophyll, enzymes, and other life

molecules• Nitrogen cycle

Oxygen, The Breath Of LifeOxygen, atomic number 8, atomic mass 16.00OO2 molecules make up 21% of the volume of airOxygen may be regarded as a green element• O2 is in the atmosphere for the taking• From distillation of liquid airPure oxygen used for breathing, in chemical synthesis, oxyacetylene torches

Oxygen (2)Oxygen in the stratosphere (from Chapter 1)

• O2 (Ultraviolet radiation) O + O

• O2 + O O3

• Protective ozone layer in the stratosphereOzone is detrimental and toxic in the atmosphere at sea levelOxygen 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 37C)

Fluorine, The Most Nonmetallic ElementFluorine, F, atomic number 9, atomic mass 19.00 has 7 outer electrons as shown by its Lewis symbol:

Elemental fluorine exists as diatomic F2

Fluorine is the most nonmetallic of all elementsFluorine reacts violently with metals, organic matter, even glass!Fluorine is a very corrosive poison that attacks flesh and produces wounds that heal very poorlyPractice of green chemistry seeks to minimize the generation or use of F2 and of highly corrosive HFFluorine is widely used in chemical synthesis, for example, to make teflon and Freon substitutes, such as HFC-134a, CH2FCF3

F

2.6. The Magic Octet of 8 Outer-Shell Electrons

Neon, atomic number 10, atomic mass 20.18Most neon atoms have 10 neutrons, some have 12, and very few 11Exists as individual Ne atoms, never combined with other atomsAbout 2 parts per thousand by volume in air• Neon is obtained from distillation of liquid air• Most common use in glowing neon signsNeon has a filled outer electron shell of 8 electrons

This filled shell makes neon a noble gas

Ne

Special Significance of the OctetNoble 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

• Represents an octet of electrons

X

The Octet RuleThe octet rule is the tendency of atoms to acquire stable octets through chemical bonding as shown for elemental N2 below:

• 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

NNNNTwo nitrogen atoms, eachwith 5 outer-shell electronsDiatomic N2 molecule with the stableoctets of both atoms circled.

2.7. Completing the 20-Element Periodic TableTen more elements to complete the periodic tableSodium, Na, atomic number 11, atomic mass 22.99Soft, chemically very reactive metalBelow 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:NaInnermost electron shellwith 2 electronsSecond inner shellwith an octet ofelectrons

Single electron inan outer shellNaLewis symbol ofsodium

Magnesium and AluminumMagnesium, Mg, atomic number 12, atomic mass 24.31 exists in the elemental form as a strong lightweight metal• Extension ladders • Portable tools • AircraftAluminum, Al, atomic number 13, atomic mass 26.98 is a strong, lightweight metal• Aircraft • Automobiles • Electrical lines • Building constructionAluminum metal forms a self-protecting oxide coatingAluminum 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

SiliconSilicon, Si, atomic number 14, atomic mass 28.09

• A metalloid and semiconductor• Key element in semiconductor industry• Second most abundant element in Earth’s crustSilicon 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

PhosphorusPhosphorus, P, atomic number 15, atomic mass 30.95Most 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 processesEssential plant fertilizerIngredient of many industrial chemicals including some pesticidesChemically 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, S, atomic number 16, atomic mass 32.06Essential 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

Sulfur

ChlorineChlorine, Cl, atomic number 17, atomic mass 35.453

Has 7 outer-shell electrons, just 1 electron short of a full octet

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.

ClClClClTwo chlorine atoms, eachlacking only 1 electron fora complete octet in theirouter shellsshare 2 electrons so thatthey are held together bya single covalent bond inthe Cl2 molecule.NaClNaA sodium atom donates itsouter-shell electron to achlorine atom, leaving theresulting Na+ cation withits underlying second shellelectrons as its filled outershell octet+Cl-to produce the ionic compoundsodium chloride (NaCl) in whichboth the Na+ cation and the Cl-anion have filled outer electronshells consisting of stable octets.

Greenish-yellow Cl2 gas• Chlorine is an important industrial chemical used to make plastics

and solventsGreen 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

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

• 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

ArgonArgon, Ar, atomic number 18, atomic mass 39.95Complete octet of outer-shell electrons makes argon a noble gasArgon composes about 1% by volume of atmospheric air and is recovered from distillation of liquid airUses 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 TablePotassium, K, atomic number 19, atomic mass 39.10• Mostly 19K 40• Small fraction is radioactive 19K39Like sodium, potassium is a very reactive alkali metalAn essential element for life and a common crop fertilizerProduces K+ ion

CalciumCalcium, Ca, atomic number 20, atomic mass 40.08Readily loses its 2 outer-shell electrons to produce Ca2+ cationAlkaline earth metalElemental calcium metal is chemically reactiveChemical properties very similar to those of magnesiumEssential for life, plant growth

Essential animal nutrient to form hydroxyapatite, Ca5OH(PO4)3 in teeth and bonesDeficiency can cause disabling osteoporosis

2.8. The Brief Periodic Table is Complete

Elements Above Atomic Number 20Placement of electrons in elements with atomic number 21 and higher becomes complicatedImportant 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 TableHydrogen is uniqueElements in vertical columns belong to groups with similar chemical propertiesFirst 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 KClThe 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.)

The far right group consists of noble gases composed of single atoms: helium, neon, and argonHe has a complete outer shell of 2e-, Ne and Ar have 8

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