Difference between revisions of "Gold"

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'''Gold''' is a chemical element with the symbol '''Au''' and atomic number 79. It is a transitional metal, part of Group 11, the same group as [[silver]] and [[copper]]. It's well known for its corrosion resistance and its high economic value. Gold is mainly used in jewels, electronics and as exchange.
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{{Infobox element
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<!-- General properties -->
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|name= Gold
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|symbol= Au
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|pronounce=
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|pronounce ref=
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|pronounce comment=
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|pronounce 2=
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|alt name=Aurum
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|alt names=
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|allotropes=
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|appearance= Metallic dark yellow
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<!-- Periodic table -->
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|above= [[Silver|Ag]]
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|below= Rg
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|left= [[Platinum]]
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|right= [[Mercury]]
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|number= 79
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|atomic mass= 196.966569(5)
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|atomic mass 2=
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|atomic mass ref=
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|atomic mass comment=
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|series= Transition metal
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|series ref=
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|series comment=
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|series color=
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|group=
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|group ref=
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|group comment=11
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|period= 6
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|period ref=
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|period comment=
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|block=d
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|block ref=
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|block comment=
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|electron configuration= [Xe] 4f<sup>14</sup> 5d<sup>10</sup> 6s<sup>1</sup>
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|electron configuration ref=
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|electron configuration comment=
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|electrons per shell= 2, 8, 18, 32, 18, 1
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|electrons per shell ref=
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|electrons per shell comment=
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<!-- Physical properties -->
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|physical properties comment=
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|color= Metallic yellow
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|phase= Solid
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|phase ref=
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|phase comment=
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|melting point K=1337.33
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|melting point C=1064.18
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|melting point F=1947.52
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|melting point ref=
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|melting point comment=
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|boiling point K=3243
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|boiling point C=2970
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|boiling point F=5378
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|boiling point ref=
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|boiling point comment=
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|sublimation point K=
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|sublimation point C=
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|sublimation point F=
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|sublimation point ref=
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|sublimation point comment=
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|density gplstp=
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|density gplstp ref=
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|density gplstp comment=
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|density gpcm3nrt=19.30
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|density gpcm3nrt ref=
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|density gpcm3nrt comment=
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|density gpcm3nrt 2=
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|density gpcm3nrt 2 ref=
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|density gpcm3nrt 2 comment=
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|density gpcm3nrt 3=
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|density gpcm3nrt 3 ref=
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|density gpcm3nrt 3 comment=
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|density gpcm3mp=17.31
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|density gpcm3mp ref=
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|density gpcm3mp comment=
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|density gpcm3bp=
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|density gpcm3bp ref=
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|density gpcm3bp comment=
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|molar volume=
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|molar volume unit =
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|molar volume ref=
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|molar volume comment=
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|triple point K=
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|triple point kPa=
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|triple point ref=
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|triple point comment=
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|triple point K 2=
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|triple point kPa 2=
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|triple point 2 ref=
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|triple point 2 comment=
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|critical point K=
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|critical point MPa=
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|critical point ref=
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|critical point comment=
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|heat fusion=12.55
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|heat fusion ref=
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|heat fusion comment=
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|heat fusion 2=
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|heat fusion 2 ref=
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|heat fusion 2 comment=
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|heat vaporization=342
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|heat vaporization ref=
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|heat vaporization comment=
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|heat capacity=25.418
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|heat capacity ref=
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|heat capacity comment=
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|heat capacity 2=
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|heat capacity 2 ref=
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|heat capacity 2 comment=
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|vapor pressure 1= 1646
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|vapor pressure 10= 1814
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|vapor pressure 100= 2021
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|vapor pressure 1 k= 2281
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|vapor pressure 10 k= 2620
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|vapor pressure 100 k= 3078
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|vapor pressure ref=
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|vapor pressure comment=
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|vapor pressure 1 2=
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|vapor pressure 10 2=
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|vapor pressure 100 2=
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|vapor pressure 1 k 2=
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|vapor pressure 10 k 2=
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|vapor pressure 100 k 2=
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|vapor pressure 2 ref=
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|vapor pressure 2 comment=
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<!-- Atomic properties -->
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|atomic properties comment=
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|oxidation states= 5, '''3''', 2, '''1''', −1, −2, −3
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|oxidation states ref=
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|oxidation states comment=(an amphoteric oxide)
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|electronegativity=2.54
 +
|electronegativity ref=
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|electronegativity comment=
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|ionization energy 1=890.1
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|ionization energy 1 ref=
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|ionization energy 1 comment=
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|ionization energy 2=1980
 +
|ionization energy 2 ref=
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|ionization energy 2 comment=
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|ionization energy 3=
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|ionization energy 3 ref=
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|ionization energy 3 comment=
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|number of ionization energies=
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|ionization energy ref=
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|ionization energy comment=
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|atomic radius=144
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|atomic radius ref=
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|atomic radius comment=
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|atomic radius calculated=
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|atomic radius calculated ref=
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|atomic radius calculated comment=
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|covalent radius=136±6
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|covalent radius ref=
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|covalent radius comment=
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|Van der Waals radius=166
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|Van der Waals radius ref=
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|Van der Waals radius comment=
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<!-- Miscellanea -->
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|crystal structure=
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|crystal structure prefix=
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|crystal structure ref=
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|crystal structure comment= ​Face-centered cubic (fcc)
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|crystal structure 2=
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|crystal structure 2 prefix=
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|crystal structure 2 ref=
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|crystal structure 2 comment=
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|speed of sound=
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|speed of sound ref=
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|speed of sound comment=
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|speed of sound rod at 20=
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|speed of sound rod at 20 ref=
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|speed of sound rod at 20 comment=
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|speed of sound rod at r.t.=2030
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|speed of sound rod at r.t. ref=
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|speed of sound rod at r.t. comment=
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|thermal expansion=
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|thermal expansion ref=
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|thermal expansion comment=
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|thermal expansion at 25=14.2
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|thermal expansion at 25 ref=
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|thermal expansion at 25 comment=
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|thermal conductivity=318
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|thermal conductivity ref=
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|thermal conductivity comment=
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|thermal conductivity 2=
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|thermal conductivity 2 ref=
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|thermal conductivity 2 comment=
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|thermal diffusivity=
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|thermal diffusivity ref=
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|thermal diffusivity comment=
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|electrical resistivity=
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|electrical resistivity unit prefix=
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|electrical resistivity ref=
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|electrical resistivity comment=
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|electrical resistivity at 0=
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|electrical resistivity at 0 ref=
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|electrical resistivity at 0 comment=
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|electrical resistivity at 20=22.14
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|electrical resistivity at 20 ref=
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|electrical resistivity at 20 comment=
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|band gap=
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|band gap ref=
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|band gap comment=
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|Curie point K=
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|Curie point ref=
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|Curie point comment=
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|magnetic ordering= Diamagnetic
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|magnetic ordering ref=
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|magnetic ordering comment=
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|tensile strength=120
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|tensile strength ref=
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|tensile strength comment=
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|Young's modulus=79
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|Young's modulus ref=
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|Young's modulus comment=
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|Shear modulus=27
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|Shear modulus ref=
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|Shear modulus comment=
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|Bulk modulus=180
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|Bulk modulus ref=
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|Bulk modulus comment=
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|Poisson ratio=0.4
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|Poisson ratio ref=
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|Poisson ratio comment=
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|Mohs hardness=2.5
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|Mohs hardness ref=
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|Mohs hardness comment=
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|Mohs hardness 2=
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|Mohs hardness 2 ref=
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|Mohs hardness 2 comment=
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|Vickers hardness=188–216
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|Vickers hardness ref=
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|Vickers hardness comment=
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|Brinell hardness=188–245
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|Brinell hardness ref=
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|Brinell hardness comment=
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|CAS number=7440-57-5
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|CAS number ref=
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|CAS number comment=
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<!-- History -->
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|naming=from Latin ''aurum'' (gold)
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|predicted by=
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|prediction date ref=
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|prediction date=
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|discovered by=
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|discovery date ref=
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|discovery date=~6000 BCE (Middle East)
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|first isolation by=
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|first isolation date ref=
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|first isolation date=
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|discovery and first isolation by=
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|named by=
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|named date ref=
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|named date=
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|history comment label=
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|history comment=
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<!-- Isotopes -->
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|isotopes=
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|isotopes comment=
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|engvar=
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}}
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'''Gold''' is a chemical element with the symbol '''Au''' and atomic number 79. It is a transitional metal, part of Group 11, the same group as [[silver]] and [[copper]]. It's well known for its corrosion resistance and its high economic value. Gold is mainly used in jewels, electronics, catalyst and as exchange.
 +
 
 +
The symbol Au comes from the latin name of gold, '''aurum''', and derivatives of this term are used in many countries as designation for gold, most often in Romance-speaking countries.
  
 
==Properties==
 
==Properties==
 
===Chemical===
 
===Chemical===
 
Gold is very resistant to acid and alkali attacks and does not react with oxgen or halogens at standard conditions. However a mixture of [[hydrochloric acid]] and [[nitric acid]] known as [[aqua regia]] will dissolve gold.
 
Gold is very resistant to acid and alkali attacks and does not react with oxgen or halogens at standard conditions. However a mixture of [[hydrochloric acid]] and [[nitric acid]] known as [[aqua regia]] will dissolve gold.
 +
 
:Au + HNO<sub>3</sub> + 4 HCl → HAuCl<sub>4</sub> + NO + 2 H<sub>2</sub>O
 
:Au + HNO<sub>3</sub> + 4 HCl → HAuCl<sub>4</sub> + NO + 2 H<sub>2</sub>O
  
Gold can also be dissolved by cyanides, such as [[sodium cyanide]], a process used in gold extraction, when the gold concentration is low. Mercury dissolves gold forming an amalgam.
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Gold can also be dissolved by cyanides, such as [[sodium cyanide]], a process used in gold extraction, when the gold concentration is low. [[Mercury]] dissolves gold forming an amalgam.
 +
 
 +
Gold resists the attack of molten [[sodium hydroxide]], however, at temperatures above 700 °C, there is visible corrosion of the metal, and traces of gold flakes and gold oxide can be observed in the alkali melt. Small amounts of metallic [[sodium]] have also been observed, which rapidly form an alloy with the gold, which is stable enough that it doesn't readily react with water or acids.<ref>http://pubs.acs.org/doi/pdf/10.1021/ja01601a004</ref>
  
 
===Physical===
 
===Physical===
Gold is a bright yellow dense, soft, malleable and ductile metal. Very pure gold (24 carat) is soft enough to be dent by biting it, a practice occasionally seen in gold diggers and Olympic athletes, who traditionally bit their gold medals. Gold is the most malleable of all metals, one gram can be beaten into a sheet of 1 square meter. It has high thermal and electric conductivity, properties that gives it many uses in electronics. It's density of 19.3 g/cm<sup>3</sup> is slightly higher than the one of [[tungsten]] and [[uranium]].
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[[File:Gold.jpg|thumb|250px|Small sample of gold foil]]
 +
Gold is a bright yellow dense, soft, malleable and ductile metal. Very pure gold (24 carat) is soft enough to be dent by biting it, a practice occasionally seen in gold diggers and Olympic athletes, who traditionally bit their gold medals. Gold is the most malleable of all metals, one gram can be beaten into a sheet of 1 square meter. It has high thermal and electric conductivity, properties that gives it many uses in electronics. Its density of 19.3 g/cm<sup>3</sup> is slightly higher than that of [[tungsten]] and [[uranium]].
  
 
==Availability==
 
==Availability==
Gold can be found in nature as nuggets, either pure or mixed with [[silver]] or platinum group metals. During the Gold Rush, very large nuggets were dug up from the rivers. Nowadays, nuggets tend to be rarer, instead grain sized gold is more often found, as previous extraction methods focused on large nuggets.
+
Gold can be found in nature as nuggets, either pure or mixed with [[silver]] or platinum group metals. During the Gold Rush, very large nuggets were dug up from the rivers. Nowadays, nuggets tend to be rarer, instead grain sized gold is more often found, as previous extraction methods focused on large nuggets. Extracting gold from gold-rich soil/sand is very intensive and may not be 100% legal depending on where you live.
  
 
Gold can be extracted from jewelry, but doing so often destroys jewelry that would cost more than the gold it is made of. Gold bullions and coins are also a source of gold, albeit an expensive one.
 
Gold can be extracted from jewelry, but doing so often destroys jewelry that would cost more than the gold it is made of. Gold bullions and coins are also a source of gold, albeit an expensive one.
Line 19: Line 297:
  
 
However the most sought source of gold are scrap electronics. Extracting gold from old electronics such as finger and socket contacts, pins, CPUs, RAM chips, board plating, adjustable switches, etc. is one of the most known aspects of amateur chemistry. Usually the older the electronic device is, the more gold it has. Extracting the gold is done by various methods: dissolving the copper circuit with a PCB etchant, such as [[Iron(III) chloride|ferric chloride]] and collecting the gold foil by filtering the solution, which is later purified by dissolving it in [[aqua regia]] and melted; dissolving the boards in cyanide solution, reducing the gold cyanide compound and melting the powder; dissolving the gold with [[mercury]] and extracting the gold; electrochemical separation. The amount of gold obtained is low, but it's a cheap source.
 
However the most sought source of gold are scrap electronics. Extracting gold from old electronics such as finger and socket contacts, pins, CPUs, RAM chips, board plating, adjustable switches, etc. is one of the most known aspects of amateur chemistry. Usually the older the electronic device is, the more gold it has. Extracting the gold is done by various methods: dissolving the copper circuit with a PCB etchant, such as [[Iron(III) chloride|ferric chloride]] and collecting the gold foil by filtering the solution, which is later purified by dissolving it in [[aqua regia]] and melted; dissolving the boards in cyanide solution, reducing the gold cyanide compound and melting the powder; dissolving the gold with [[mercury]] and extracting the gold; electrochemical separation. The amount of gold obtained is low, but it's a cheap source.
 +
 +
Gold itself is usually found uncombined in nature, but when found as a chemical compound, it is most often combined with tellurium, in the form of calaverite and krennerite (two different polymorphs of AuTe<sub>2</sub>), petzite (Ag<sub>3</sub>AuTe<sub>2</sub>) and sylvanite (AgAuTe<sub>4</sub>).
  
 
==Preparation==
 
==Preparation==
Gold can be reduced from its salts by reducing it with a reducing compound. Since gold sits close to the bottom of the reactivity scale, any common metal will reduce it to its elemental form. In case of [[chloroauric acid]], it is usually reduced with [[sodium metabisulfite]].
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Gold can be reduced from its salts by reducing it with a reducing compound. Since gold sits close to the bottom of the reactivity scale, any common metal will reduce it to its elemental form. In case of [[chloroauric acid]], [[sodium metabisulfite|sodium]] or [[potassium metabisulfite]] are commonly used as reducing agents, as they're cheap and readily available.
  
 
==Projects==
 
==Projects==
 
*Gold plating
 
*Gold plating
*Gold colloids
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*Make gold colloids
*Gold electrode
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*Gold electrode in water electrolysis
 +
*Catalyst
 +
*Make jewelry
 +
*Element collection
  
 
==Handling==
 
==Handling==
 
===Safety===
 
===Safety===
Pure gold is non-toxic and it's even used in medical implants. However, its compounds, especially the salts are toxic and they should be handled with proper protection.
+
Pure gold is non-toxic and it's even used in medical implants. On the other hand, most gold compounds (especially the salts) are toxic and they should be handled with proper protection.
  
 
===Storage===
 
===Storage===
No special storage is required for bulk and powdered gold.
+
No special storage is required for bulk and powdered gold. Though given the value of gold, it's best to keep it in a hidden place or a safe.
  
 
===Disposal===
 
===Disposal===
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[[Category:Minerals]]
 
[[Category:Minerals]]
 
[[Category:Solids]]
 
[[Category:Solids]]
 +
[[Category:Noble metals]]
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[[Category:Inert chemicals]]
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[[Category:Coinage metals]]

Latest revision as of 21:35, 7 April 2019

Gold,  79Au
General properties
Name, symbol Gold, Au
Alternative name Aurum
Appearance Metallic dark yellow
Gold in the periodic table
Ag

Au

Rg
PlatinumGoldMercury
Atomic number 79
Standard atomic weight (Ar) 196.966569(5)
Group, block 11; d-block
Period period 6
Electron configuration [Xe] 4f14 5d10 6s1
per shell
2, 8, 18, 32, 18, 1
Physical properties
Metallic yellow
Phase Solid
Melting point 1337.33 K ​(1064.18 °C, ​1947.52 °F)
Boiling point 3243 K ​(2970 °C, ​5378 °F)
Density near r.t. 19.30 g/cm3
when liquid, at  17.31 g/cm3
Heat of fusion 12.55 kJ/mol
Heat of 342 kJ/mol
Molar heat capacity 25.418 J/(mol·K)
 pressure
Atomic properties
Oxidation states 5, 3, 2, 1, −1, −2, −3 ​(an amphoteric oxide)
Electronegativity Pauling scale: 2.54
energies 1st: 890.1 kJ/mol
2nd: 1980 kJ/mol
Atomic radius empirical: 144 pm
Covalent radius 136±6 pm
Van der Waals radius 166 pm
Miscellanea
Crystal structure ​​Face-centered cubic (fcc)
Speed of sound thin rod 2030 m/s (at )
Thermal expansion 14.2 µm/(m·K) (at 25 °C)
Thermal conductivity 318 W/(m·K)
Electrical resistivity 22.14 Ω·m (at 20 °C)
Magnetic ordering Diamagnetic
Tensile strength 120 MPa
Young's modulus 79 GPa
Shear modulus 27 GPa
Bulk modulus 180 GPa
Poisson ratio 0.4
Mohs hardness 2.5
Vickers hardness 188–216 MPa
Brinell hardness 188–245 MPa
CAS Registry Number 7440-57-5
History
Naming from Latin aurum (gold)
Discovery ~6000 BCE (Middle East)
· references

Gold is a chemical element with the symbol Au and atomic number 79. It is a transitional metal, part of Group 11, the same group as silver and copper. It's well known for its corrosion resistance and its high economic value. Gold is mainly used in jewels, electronics, catalyst and as exchange.

The symbol Au comes from the latin name of gold, aurum, and derivatives of this term are used in many countries as designation for gold, most often in Romance-speaking countries.

Properties

Chemical

Gold is very resistant to acid and alkali attacks and does not react with oxgen or halogens at standard conditions. However a mixture of hydrochloric acid and nitric acid known as aqua regia will dissolve gold.

Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O

Gold can also be dissolved by cyanides, such as sodium cyanide, a process used in gold extraction, when the gold concentration is low. Mercury dissolves gold forming an amalgam.

Gold resists the attack of molten sodium hydroxide, however, at temperatures above 700 °C, there is visible corrosion of the metal, and traces of gold flakes and gold oxide can be observed in the alkali melt. Small amounts of metallic sodium have also been observed, which rapidly form an alloy with the gold, which is stable enough that it doesn't readily react with water or acids.[1]

Physical

Small sample of gold foil

Gold is a bright yellow dense, soft, malleable and ductile metal. Very pure gold (24 carat) is soft enough to be dent by biting it, a practice occasionally seen in gold diggers and Olympic athletes, who traditionally bit their gold medals. Gold is the most malleable of all metals, one gram can be beaten into a sheet of 1 square meter. It has high thermal and electric conductivity, properties that gives it many uses in electronics. Its density of 19.3 g/cm3 is slightly higher than that of tungsten and uranium.

Availability

Gold can be found in nature as nuggets, either pure or mixed with silver or platinum group metals. During the Gold Rush, very large nuggets were dug up from the rivers. Nowadays, nuggets tend to be rarer, instead grain sized gold is more often found, as previous extraction methods focused on large nuggets. Extracting gold from gold-rich soil/sand is very intensive and may not be 100% legal depending on where you live.

Gold can be extracted from jewelry, but doing so often destroys jewelry that would cost more than the gold it is made of. Gold bullions and coins are also a source of gold, albeit an expensive one.

Gold leaf, used in food decorations are also a source of gold, albeit the quantity is small and it's usually a gold alloy.

However the most sought source of gold are scrap electronics. Extracting gold from old electronics such as finger and socket contacts, pins, CPUs, RAM chips, board plating, adjustable switches, etc. is one of the most known aspects of amateur chemistry. Usually the older the electronic device is, the more gold it has. Extracting the gold is done by various methods: dissolving the copper circuit with a PCB etchant, such as ferric chloride and collecting the gold foil by filtering the solution, which is later purified by dissolving it in aqua regia and melted; dissolving the boards in cyanide solution, reducing the gold cyanide compound and melting the powder; dissolving the gold with mercury and extracting the gold; electrochemical separation. The amount of gold obtained is low, but it's a cheap source.

Gold itself is usually found uncombined in nature, but when found as a chemical compound, it is most often combined with tellurium, in the form of calaverite and krennerite (two different polymorphs of AuTe2), petzite (Ag3AuTe2) and sylvanite (AgAuTe4).

Preparation

Gold can be reduced from its salts by reducing it with a reducing compound. Since gold sits close to the bottom of the reactivity scale, any common metal will reduce it to its elemental form. In case of chloroauric acid, sodium or potassium metabisulfite are commonly used as reducing agents, as they're cheap and readily available.

Projects

  • Gold plating
  • Make gold colloids
  • Gold electrode in water electrolysis
  • Catalyst
  • Make jewelry
  • Element collection

Handling

Safety

Pure gold is non-toxic and it's even used in medical implants. On the other hand, most gold compounds (especially the salts) are toxic and they should be handled with proper protection.

Storage

No special storage is required for bulk and powdered gold. Though given the value of gold, it's best to keep it in a hidden place or a safe.

Disposal

Due to gold's price and rarity, it's best to try and recycle as much gold as possible.

References

  1. http://pubs.acs.org/doi/pdf/10.1021/ja01601a004

Relevant Sciencemadness threads