Selenium

Selenium,  ₃₄Se

General properties
Name, seembol	selenium, Se
Appearance	black an reid allotropes
Pronunciation	/sᵻˈliːniəm/ si-LEE-nee-əm
Selenium in the periodic cairt
S ↑ Se ↓ Te arsenic ← selenium → bromine
Atomic nummer (Z)	34
Group, block	group 16 (chalcogens), p-block
Period	period 4
Element category	polyatomic nonmetal, sometimes considered a metalloid
Staundart atomic wicht (±) (Ar)	78.971(8)
Electron configuration	[Ar] 3d10 4s2 4p4
per shell	2, 8, 18, 6
Pheesical properties
Phase	solid
Meltin pynt	494 K ​(221 °C, ​430 °F)
Boilin pynt	958 K ​(685 °C, ​1265 °F)
Density near r.t.	(gray) 4.81 g/cm3 (alpha) 4.39 g/cm3 (vitreous) 4.28 g/cm3
when liquid, at m.p.	3.99 g/cm3
Creetical pynt	1766 K, 27.2 MPa
Heat of fusion	(gray) 6.69 kJ/mol
Heat o	95.48 kJ/mol
Molar heat capacity	25.363 J/(mol·K)
pressur P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 500 552 617 704 813 958
Atomic properties
Oxidation states	6, 4, 2, 1, -2 (strangly acidic oxide)
Electronegativity	Pauling scale: 2.55
Atomic radius	empirical: 120 pm
Covalent radius	120±4 pm
Van der Waals radius	190 pm
Miscellanea
Crystal structur	​hexagonal
Speed o soond thin rod	3350 m/s (at 20 °C)
Thermal expansion	(amorphous) 37 µm/(m·K) (at 25 °C)
Thermal conductivity	(amorphous) 0.519 W/(m·K)
Magnetic orderin	diamagnetic
Young's modulus	10 GPa
Shear modulus	3.7 GPa
Bulk modulus	8.3 GPa
Poisson ratio	0.33
Mohs haurdness	2.0
Brinell haurdness	736 MPa
CAS Nummer	7782-49-2
History
Discovery	Jöns Jakob Berzelius an Johann Gottlieb Gahn (1817)
First isolation	Jöns Jakob Berzelius an Johann Gottlieb Gahn (1817)
Maist stable isotopes o selenium
iso NA hauf-life DM DE (MeV) DP 72Se syn 8.4 d ε - 72As γ 0.046 - 74Se 0.87% - (β+β+) 1.2094 74Ge 75Se syn 119.779 d ε - 75As γ 0.264, 0.136, 0.279 - 76Se 9.36% 76Se is stable wi 42 neutrons 77Se 7.63% 77Se is stable wi 43 neutrons 78Se 23.78% 78Se is stable wi 44 neutrons 79Se trace 3.27×105 y β− 0.151 79Br 80Se 49.61% - (β−β−) 0.1339 80Kr 82Se 8.73% 1.08×1020 y β−β− 2.995 82Kr
Decay modes in parentheses are predictit, but hae nae yet been observed

Selenium is a chemical element wi seembol Se an atomic nummer 34. It is a nonmetal wi properties that are intermediate atween those o its periodic cairt column-adjacent chalcogen elements sulphur an tellurium. It rarely occurs in its elemental state in naitur, or as pure ore compoonds. Selenium (Greek σελήνη selene meanin "Muin") wis discovered in 1817 bi Jöns Jacob Berzelius, who notit the seemilarity o the new element tae the previously kent tellurium (named for the Yird).

References