CHAPTER 2 THE ELEMENTS: BASIC BUILDING BLOCKS OF GREEN CHEMICALS From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, Chem. Char Research, Inc. , 2006 manahans@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 Very diffe rent Two atoms in Very similar, but Cl 2 molecule not identical t 1. Each element is composed of extremely ren ffe small particles with the same chemical y di Ver properties called atoms. 2. Atoms of different elements do not have identical chemical properties. 2 H 2 + O 2 2 H 2 O 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) CH 4 + 2 O 2 CO 2 + 2 H 2 O 4. Chemical reactions involve the separation and combination of atoms as in the example above where bonds are broken between C and H in CH 4 and between O and O in O 2, and bonds are formed between C and O in CO 2 and between H and O in H 2 O. 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 Atoms are extremely small and light • Individual masses are expressed in atomic mass units, u • Size in picometers, picometer = 0. 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 Element symbol Atomic number • 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 Lewis formulas can be used to show electrons in molecules, such as H 2 Elemental hydrogen does not exist as individual H atoms Instead, it exists as molecules, each composed of 2 H atoms with the chemical formula H 2 The covalent bond holding the two H atoms together consists of 2 shared electrons shown in the Lewis formula of H 2 above.
Properties and Uses of Elemental H 2 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 2 H 2 + O 2 2 H 2 O + energy Hydrogen is a very green element because when it is used to generate energy, the reaction product is simply water, H 2 O
Properties and Uses of H 2 (2) Hydrogen can be produced by 2 H 2 O (Electrical current) 2 H 2 + O 2 or by steam reforming of methane or other hydrocarbons at high temperatures and pressures: CH 4 + H 2 O (800 C T, 30 atm P) CO + 3 H 2 Hydrogen is used to manufacture a number of chemicals, for example methyl alcohol, CH 3 OH): CO + 2 H 2 CH 3 OH 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
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:
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 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 H 2 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 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 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 Metals • • • 1 -3 outer-shell electrons Form +1, +2, or +3 cations Luster (shine) Malleable Conduct electricity
Uses of Lithium has several important uses • Li 2 CO 3 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, Li. OH, 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, atomic number 4, atomic mass 9. 012 4 protons and 5 neutrons in Be nuclei Formation of Be 2+ cation Be: Be 2+ + 2 e. Beryllium 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 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, B 2 O 3, in heat-insulating fiberglass • Boric acid, H 3 BO 3, is used as a flame retardant in cellulose insulation
Carbon, The Element of Life Carbon, C, atomic number 6 Detectable amounts of radioactive carbon-14, designated 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
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 is about 0. 038% CO 2 by volume, serving as a carbon source for photosynthesis: 6 CO 2 + 6 H 2 O (Sunlight energy) C 6 H 12 O 6 + 6 O 2 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 Diatomic N 2 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 N 2 + 3 H 2 2 NH 3 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 2 molecules make up 21% of the volume of air Oxygen may be regarded as a green element • O 2 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) • O 2 (Ultraviolet radiation) O + O • O 2 + O O 3 • 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 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O + energy • Oxidation of glucose in an organism to provide energy C 6 H 12 O 6 (Glucose) + 6 O 2 6 CO 2 + 6 H 2 O + 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: Elemental fluorine exists as diatomic F 2 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 F 2 and of highly corrosive HF Fluorine is widely used in chemical synthesis, for example, to make teflon and Freon substitutes, such as HFC-134 a, CH 2 FCF 3
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 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 • 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 N 2 below: • • In N 2 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 N 2 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:
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, 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, 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, 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, SO 2, in the combustion of fossil fuels that contain sulfur Much of the sulfur that is used is obtained from hydrogen sulfide, H 2 S, that contaminates much of natural gas 2 H 2 S + 3 O 2 2 SO 2 + 2 H 2 O 2 H 2 S + SO 2 3 S + 2 H 2 O
Chlorine, Cl, atomic number 17, atomic mass 35. 453 Has 7 outer-shell electrons, just 1 electron short of a full octet Formation of ionic Na. Cl. 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.
Greenish-yellow Cl 2 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, 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 Like sodium, potassium is a very reactive alkali metal An essential element for life and a common crop fertilizer Produces K+ ion
Calcium, Ca, atomic number 20, atomic mass 40. 08 Readily loses its 2 outer-shell electrons to produce Ca 2+ 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, Ca 5 OH(PO 4)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 2 M + 2 H 2 O 2 MOH + H 2 • React with elemental chlorine to produce ionic Li. Cl, Na. Cl, and KCl The second group consists of alkaline earth metals: beryllium, magnesium, calcium • Highly reactive metals that produce Be 2+, Mg 2+, Ca 2+ ions
Aspects of the Periodic Table (Cont. ) Halogens compose the second group from the right • Diatomic gases in which the two atoms of F 2 or Cl 2 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 2 e-, Ne and Ar have 8
Скачать презентацию CHAPTER 2 THE ELEMENTS BASIC BUILDING BLOCKS OF
03dfa2444e755c87268032ddc8929a3d.ppt