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Thematic Questions about Chemical Elements• Nature of the chemical elements

•What distinguishes elements from one another?• Occurrence of the range of elements

•Which elements occur in the universe and why?• Origin of specific elements

•How, why and when were the various elementsformed?

• Stability of individual elements•Why are some elements unstable?•What happens to them?

G302 Development of the Global Environment

Element Synthesis: Exploration ofChemical Fundamentals

Element Synthesis and Isotopes• Elemental Abundance and Isotopes

•distribution of elements in the universe•factors that define elemental differences

• Forms of Radioactivity•types of decay and processes

• Formation of Elements and their Stability•sequence of element formation in stars•role of binding in stabilizing atomic nucleii

• Elemental Composition of the Universe•products of formation and nuclear fusion

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Element Building: Nucleosynthesis

2

hydrogen

10 20 30 40 50 60 70 80 90Element (Atomic ) Number

10-2

100

102

104

106

108

1010helium

beryllium

oxygen

silicon iron

boron

leadbismuth

thorium uranium

Relative

abu

ndan

ce (Si

= 1

06)

Relative Abundance in the Sun• non-uniform trend

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Cosmic Abundance of Elements

•general decrease as atomicnumber increases; even > odd

•higher relative abundance ofiron and lead

•production or preservation?

Elemental Characteristics• Major components: protons, neutrons, electrons• Atomic Number = number of protons in nucleus• Name: Helium; Symbol: 3He2

• 2 = atomic number, which determines name• 3 = sum of protons and neutrons

• Elements may have variable numbers ofneutrons: 3He2 or 4He2; these are isotopes

• Individual isotopes may be stable or unstable• unstable = radioactive, decay to other elements• elements without stable isotopes don’t occur naturally

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Elements and Isotopes

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Examples of Characteristics• Names, symbols, isotopes (radioactive)

•Most elements occur as a number of isotopes

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Simplest Elements

Name Hydrogen Helium Lithium BerylliumSymbol H He Li BeAtomic No. 1 2 3 4Isotopes 1,2,3 3,4 6,7 9,10

Name Boron Carbon Nitrogen OxygenSymbol B C N OAtomic No. 5 6 7 8Isotopes 10,11 12,13,14 14,15 16,17,18

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Natural Distribution of Isotopes

Equal numbers ofprotons andneutrons

10 20 30 40 50 60 70 80 90Number of Neutrons

0

10

20

30

40

50

60

Natural trend ofelements, more

neutrons than protons

Num

bers

of

Prot

ons

100 110 120

70

80

4

Three Common Forms of Radioactive Decay• Nucleons constant, but changed in alpha decay

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Radioactivity

Beta decay

14N

14C

7

6

7 8

14C e- + 14N(n° e- + p+)

No. neutrons

No.

pro

tons

Electron capture

40K

40Ar

19

18

21 22

40K + e- 40Ar(p+ + e- n°)

No. neutrons

No.

pro

tons

Alpha decay 238U92

91

144 146

238U 234Th + 4He

No. neutrons

No.

pro

tons

90 234Th

145

Fusion Processes• Formation of Deuterium (2H1)

•subsequent focus on protons and neutrons

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Formation of Elements

positron

neutrino

1H1

1H12H1

proton

neutron

nucleons

5

Initial Stages of Fusion

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Formation of Elements

1H1

1H1

2H1

1H13He2

3He2

4He2

+

2 x 1H1

2 x 4He2

8Be412C6

4He24He2

4He2

16O8 20Ne10

1. hydrogen burning to helium (to 6 x 107K)

2. successive burning of He, C, O, and Ne (to 1.5 x 109K)

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4He2

16O8 20Ne10

3. Capture of 4He (α-particles; T = 109K)

4He24He2

24Mg1228Si14

28Si14 4He2 4He24He2

32S1636Ar18

40Ca20

Subsequent Stages of Fusion

Formation of Elements

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4. An equilibrium process that accounts for the highabundance of iron group elements (T = 4 x 109K)

5. Neutron capture by the slow (s) process with long time (102 - 105 years) intervals between successive captures allowing for intervening β decay

6. Neutron capture by the rapid (r) process in supernovawith short (0.01s - 1s) intervals between successive captures, enabling formation of elementsbeyond bismuth

Later Stages of Nucleosynthesis

Formation of Elements

not made by s-processmade by s-processradioisotopes along s-path

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• Slow process with the cores of stars (nuclear reactor)• Rapid process occurs during supernova (atomic bomb)

Comparison of Neutron Capture Processes

Formation of Elements

Se 34As 33Ge 32Ga 31Zn 30Cu 29Ni 28Co 27Fe 26

30 31 32 33

3734 35 36

41

neutrons

42 43

38 39 40

elem

ent/

prot

ons

s-processneutron captureelectron capturelater β-decay

elem

ent/

prot

ons

neutrons48 49 50 51 52

Mo 42Nb 41Zr 40Y 39Sr 38Rb 37Kr 36Br 35Se 34As 33Ge 32Ga 31

53 54 55 56

r-processneutron captureβ-decaylater β-decay

not made by r-processmade by r-processradioisotopes along r-path

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7. Proton capture (p-process) yielding rare light isotopesof heavy elements in an H-rich medium (T = 3 x 109K)

Final Process of Nucleosynthesis

Formation of Elements

Controls on Nucleosynthesis• Relationship to stability of nuclei

•binding energies of nucleons vs. atomic number•even numbers of nucleons more tightly bound

Occurrence of Nucleosynthesis• Nuclear fusion within stars, supernova

•related to star size

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4He2

16O8

20Ne10

24Mg12

28Si14 56Fe26

12C6

Most Stable Nucleii• Isotopes with low

binding energies•Matches cosmicabundance of majorelements (9/top 10)

Stability of the Elements

H He C (N) O% 77 21 0.3 (0.5) 0.8

Ne Mg Si S Fe% 0.2 0.06 0.07 0.04 0.1

32S16

1H1

8

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Stability• Low binding

energiesare morestable

• A functionof packingof nucleonswithinnucleus

Binding Energy of Atomic Nuclei

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Nucleon Number Systematics

Odd vs. Even• Smaller nuclear

mass is morestable

• Even numbersof nucleonspack tighterwithin nucleus

• Even numberednuclei are morestable Proton Number

42 43 44 45 46 47 48

60 59 58 57 56 55 54Neutron Number

O-OO-O

O-O

E-EE-E

Mass 102isobar

Neutron Number

42 43 44 45 46 47 48

61 60 59 58 57 56 55

E-O

E-O

E-OO-E

O-E

Mass 103isobar

Proton Number

Incr

easing

Mas

s

9

Dependent on Size and Temperature• Element burning from interior outward• Series of layers

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Element Formation within Stars

H-burning

He-burning

C-burning

Ne-burning

O-burning

Si-burning

H-burning

unburned H

He-burning

unburned He

The Sun Late Sun Large star as Red Giant

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Summary of Processes• Big bang formation of

atomic matter (H, He)• Sequential series of

nucleosyntheticprocesses within starsand supernova

• Resulting cosmicoccurrence of elements• function of synthesis

and stability

Abundance of Elements

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Formation from Atomic Remnants

Origins of Stars

Region of star formation

supernova

Material fromsupernova:

birthplace forstars