Semi-metals

◄ Semi-Metals ►

The semimetals are elements and stand between the metals and the non- metals in the periodic table . In terms of electrical conductivity and appearance, they cannot be assigned to either metals or non-metals. All semi-metals are solids under normal conditions .

Semiconductor is an umbrella term and includes semimetals and compound semiconductors alike. In the past, semimetals were also known as metalloids , although this name was also used for non-metals .

The excitation of a semiconductor by thermal energy.

definition

Older definition

The original definition of semimetals is now considered out of date in chemistry. It defines the semimetals as a series of elements that are in the 3rd to 6th main group of the periodic table .

The following elements belong to the semimetals:

B.	boron	typical semi-metal
Si	Silicon	typical semi-metal
Ge	Germanium	is more of a metal
As	arsenic	typical semi-metal
Sb	antimony	is more of a metal
Bi	Bismuth	is more of a metal
Se	selenium	is more of a non-metal
Te	Tellurium	typical semi-metal
Po	polonium	is a metal

These elements have properties of metals as well as properties of classic non-conductors . They usually come in two allotropic crystal modifications , one of which is metallic and the other non-metallic. In general, their electrical conductivity is quite small at room temperature , but increases sharply with increasing temperature , unlike metals. That is why semi-metals are also referred to as elementary semiconductors .

In fact, many of these elements are classic semiconductors such as germanium or silicon . Their outer electron shell is occupied by three to six electrons , which is why they can both donate and accept electrons.

In the periodic table , the semi-metals are found in a diagonal between boron and astatine . Elements that are to the right above this line are non-metals . Elements below this line to the left are metals.

H																		Hey
Li	Be												B.	C.	N	O	F.	No
N / A	Mg												Al	Si	P	S.	Cl	Ar
K	Approx	Sc		Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr	Y		Zr	Nb	Mon	Tc	Ru	Rh	Pd	Ag	CD	In	Sn	Sb	Te	I.	Xe
Cs	Ba	La	*	Hf	Ta	W.	re	Os	Ir	Pt	Au	Ed	Tl	Pb	Bi	Po	At	Marg
Fr.	Ra	Ac	**	Rf	Db	Sg	Bra	Hs	Mt	Ds	Rg	Cn

			*	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	He	Tm	Yb	Lu
			**	Th	Pa	U	Np	Pooh	At the	Cm	Bk	Cf	It	Fm	Md	No	Lr

Newer definition

The more recent definition of semimetals, which is increasingly used in chemistry and especially in physics , refers more to the band structure of the solid than just to the physical and chemical properties. According to this, semimetals are crystalline solids with a band gap that has the value zero or a value in the range of thermal energy , where the Boltzmann constant and the absolute temperature . Typical representatives from the field of elements are the carbon modification graphite and gray tin (α-Sn). ${\ displaystyle k _ {\ mathrm {B}} T}$ ${\ displaystyle k _ {\ mathrm {B}}}$ ${\ displaystyle T}$

However, an exact assignment is very difficult because, as mentioned, the different crystal modifications usually have different properties. White tin (β-Sn) is a metal and gray tin (α-Sn) is a semiconductor , but the latter can also be referred to as semi-metal. The assignment should therefore be based on the most common modification at room temperature and normal pressure .

H																		Hey
Li	Be												B.	C.	N	O	F.	No
N / A	Mg												Al	Si	P	S.	Cl	Ar
K	Approx	Sc		Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr	Y		Zr	Nb	Mon	Tc	Ru	Rh	Pd	Ag	CD	In	Sn	Sb	Te	I.	Xe
Cs	Ba	La	*	Hf	Ta	W.	re	Os	Ir	Pt	Au	Ed	Tl	Pb	Bi	Po	At	Marg
Fr.	Ra	Ac	**	Rf	Db	Sg	Bra	Hs	Mt	Ds	Rg	Cn

			*	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	He	Tm	Yb	Lu
			**	Th	Pa	U	Np	Pooh	At the	Cm	Bk	Cf	It	Fm	Md	No	Lr

B, C (graphite), Si, P (black), Ge, Se (gray), Te are semiconductors according to this classification .
As, Sb, Po, At are semimetals according to this classification.
Al, Ga, Sn, Bi, In, Pb, Tl are metals .

properties

Physical Properties

The chemical elements are called semimetals, the properties of which are intermediate between metals and non-metals . They have semiconducting and amphoteric properties, but are not identical to the semiconducting group of substances . Semi-metals are solids , some of which form both metallic and non-metallic modifications. To do this, consider u. a. the electrical conductivity of the crystalline solids. Many semimetals have semiconductor properties; In other words, they have an average electrical conductivity that increases with increasing temperature . However, the terms semiconductor and semimetal are not synonyms . There are semiconducting oxides and polymers that are not semimetals but compounds . The specific conductivity of semiconductors such as silicon or germanium is several orders of magnitude lower than that of metals.

The electrical properties of semiconductors can be explained using the ribbon model . According to this model, the electrons are in the valence band , which is separated from the higher conduction band by a band gap . In semiconductors, practically all electrons are in the valence band at low temperatures . In the case of the elements of the 4th main group such as silicon and germanium , this is then completely occupied, so that no current can flow. But even at room temperature , the electrons have enough energy to overcome the band gap and transition into the conduction band. The conductivity then increases sharply with the temperature.

The values refer to the most common modification at room temperature and normal pressure.
element	boron	carbon	Silicon	phosphorus	Germanium	arsenic	selenium	antimony	Tellurium	Astatine
Melting point in ° C (1013 hPa)	2076	3642	1410	44.2	938.3	613	221	630.63	449.51	575
Boiling point in ° C (1013 hPa)	3930	3642	3260	280	2830	613	685	1635	990	610
Density in g / cm³ (20 ° C, 1013 hPa)	2,460	2.26	2,336	1.83	5.323	5.73	4.819	6.697	6.24
Mohs hardness	9.3	0.5	6.5		6.0	3.5	2.0	3.0	2.25
Electrical conductivity in S / m	1.0 · 10 ⁻⁴	3 · 10 ⁶	2.52 · 10 ⁻⁴	1.0 · 10 ⁻⁹	1.45-2.2	3.03 · 10 ⁶	1.0 · 10 ⁻¹⁰	2.5 · 10 ⁶	1 · 10 ⁴
Thermal conductivity in W / ( m · K )	27		150	0.236	60	50	0.52	24	3	2
Dissociation energy in kJ / mol				314.8		302.7	308	262.5	225
Band gap in eV			1.12		0.67		1.74
Atomic number	5	6th	14th	15th	32	33	34	51	52	85
Atomic mass in u	10.81	12.011	28.085	30.974	72.630	74,922	78.971	121.760	127.60
Electronegativity	2.04	2.55	1.90	2.19	2.01	2.18	2.55	2.05	2.1	2.2
Modifications ⁽¹⁾		Graphite diamond		white black red purple		gray yellow black	gray black red	gray black explosive
Crystal system ⁽²⁾	P 3	Graphite : h Diamond : A4	A4	cl	A4	P 3	H	P 3	H	cF

⁽¹⁾ The most common modification is marked in bold.

⁽²⁾cl: body-centered cubic , cF: face-centered cubic , P 3 : rhombohedral , oP: orthorhombic , A4: diamond structure , h: hexagonal

Chemical properties

The intermediate position of the semi-metals is also shown in the amphoteric character of the elements and their oxides . Most of the semi-metals dissolve in acids as well as bases . The oxides can also form salts with acids and bases . Most semimetals have both metallic and non-metallic modifications . The semimetals also occupy a middle position in the periodic table of the elements.

literature

Graham Chedd: Semi-Metals. Verlag DVA, Stuttgart, 1971, ISBN 3-421-02246-1 .
Franz Zach: Power Electronics - A Manual. 5th edition, Springer Verlag, Wiesbaden, 2015 ISBN 978-3-658-04898-3 .

Web links

Wiktionary: Semi-metal - explanations of meanings, word origins, synonyms, translations

Individual evidence

↑ Graham Chedd: semimetals. Publisher DVA. Stuttgart 1971, ISBN 3-421-02246-1 .
^ Franz Zach: Power Electronics - A Handbook . 5th edition. Springer Verlag, Wiesbaden 2015, ISBN 978-3-658-04898-3 , pp. 1827 .
↑ ^a ^b ^c Entry on semimetals. In: Römpp Online . Georg Thieme Verlag, accessed on October 23, 2013.
^ Peter Paetzold: Chemistry: An introduction . Walter de Gruyter, 2009, ISBN 3-11-021135-1 , p. 32 ( limited preview in Google Book search).
↑ ^a ^b Duden Learnattack GmbH: Semimetals
↑ ^a ^b ^c P. Häussinger, R. Glatthaar, W. Rhode, H. Kick, C. Benkmann, J. Weber, H.-J. Wunschel, V. Stenke, E. Leicht, H. Stenger: Noble Gases. In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley-VCH, Weinheim 2006 ( doi : 10.1002 / 14356007.a17_485 ).

[1] Graham Chedd: semimetals. Publisher DVA. Stuttgart 1971, ISBN 3-421-02246-1 .

[2] Franz Zach: Power Electronics - A Handbook . 5th edition. Springer Verlag, Wiesbaden 2015, ISBN 978-3-658-04898-3 , pp. 1827 .

[roempp-3] Entry on semimetals. In: Römpp Online . Georg Thieme Verlag, accessed on October 23, 2013.

[Peter_Paetzold-4] Peter Paetzold: Chemistry: An introduction . Walter de Gruyter, 2009, ISBN 3-11-021135-1 , p. 32 ( limited preview in Google Book search).

[:0-5] Duden Learnattack GmbH: Semimetals

[ullmann-6] P. Häussinger, R. Glatthaar, W. Rhode, H. Kick, C. Benkmann, J. Weber, H.-J. Wunschel, V. Stenke, E. Leicht, H. Stenger: Noble Gases. In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley-VCH, Weinheim 2006 ( doi : 10.1002 / 14356007.a17_485 ).