# Semi-metals

Semi-Metals
Position in the periodic table

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The original definition of semimetals is now considered out of date in chemistry.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

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.

### 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.