Metallic bond is a kind of chemical bond, which mainly exists in metals.It is composed of free electrons and electrostatic attraction between metal ions arranged in a lattice.Because of the free movement of electrons, metal bonds have no fixed direction, so they are non-polar bonds.Metal bonds have many characteristics, such as: the melting point and boiling point of general metals increase with the strength of metal bonds.The strength of metal bond is generally inversely related to the radius of metal ions, and is related to the internal strength of metalFree electron densityPositive correlation (it can be roughly regarded as positive correlation with the number of peripheral electrons of the atom).In the complex (polymeric), in order to achieve 18 electronic structure, metals are connected with each other by covalent bonds, also called metal bonds.
Chinese name
Metal bond
Foreign name
metallic bond
Discipline
Chemistry
Definition
Chemical bond that makes metal atoms form metal crystals
Metal atoms in the condensed state contribute their valence electrons as the common electrons of the whole atomic matrix.Metal bond is similar to covalent bond in essence, but at this time, the degree of co ownership of its outer electrons is far greater than that of covalent bond.These shared electrons are also called free electrons, which form the so-called electron cloud or electron gas and move in the periodic field of the lattice according to the quantum mechanical law.The metal atoms that have lost their valence electrons become positive ions, embedded in this electronic cloud, and combine with each other by electrostatic interaction with these shared electrons. This combination mode is called metal bond.For example, the aluminum atom loses its outermost three valence electrons and becomes an aluminum ion with three positive charges composed of the nucleus and inner electrons.
Since these lost valence electrons are no longer fixed at a certain atomic position, the materials bound by metal bonds have good conductivity.Under the action of applied voltage, these valence electrons will move and form current in the closed circuit.
Metal bonds have no directionality, and changing the relative position between positive ions will not destroy the combination between electrons and positive ions, so metals have good plasticity.Similarly, the substitution of a metal positive ion by another metal positive ion will not destroy the bonding bond. The ability of this metal to dissolve (called solid solution) is also an important characteristic of metals.In addition, the electrical conductivity, thermal conductivity, compact arrangement and positive resistance temperature coefficient of metals all directly result from metal bonding.[1]
Free electron theory
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In a metal crystal, free electrons shuttle, which do not belong to a certain metal atom but are shared by the whole metal crystal.These free electrons interact with all metal ions to form a certain combination, which is called metal bond.Since only a few valence electrons of metals can be used for bonding, metals tend to form extremely compact structures when forming crystals, so that each atom has as many adjacent atoms as possible (metal crystals generally have high coordination numbers and compact stacking structures). In this way, electronic energy levels can be overlapped as much as possible, thus forming metal bonds.[2]
The above hypothetical model is called the free electron model of metalModified covalent bond theory。This theory is a hypothesis put forward by Drude and others in 1900 to explain the conductivity and thermal conductivity of metals.This theory has been improved and developed by Lorentz (1904) and Sommerfeld (1928), and many important properties of metals have been explained.For example:
1. Because onMetallic crystalThe free electrons shuttle through the metal, so under the action of the external electric field, the free electrons move directionally to generate current.When heating, because the vibration of metal atoms intensifies, which prevents free electrons from shuttle motion, the metal resistivity is generally positively correlated with temperature.
2. When the metal crystal is deformed by external force, although the metal atoms have shifted, the connection of free electrons has not changed, and the metal bond has not been destroyed, so the metal crystal has ductility.
3. Free electrons are easily excited, so they can absorb light above the cut-off frequency of photoelectric effect and emit various visible light, so most metals are silver white.
4. Temperature is a measure of the average kinetic energy of molecules, and the vibration of metal atoms and free electrons can easily be transmitted one by one, so the vibration of metal local molecules can be quickly transmitted to the whole, so the thermal conductivity of metal is generally good.
However, due to the oversimplification of the free electron model of metals, it is impossible to explain why metal crystals have binding force, or why metal crystals are divided into conductors, insulators and semiconductors.With the development of science and production, mainly the development of quantum theory, the energy band theory has been established.
Energy band theory
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Metal bondedEnergy band theoryIt uses the viewpoint of quantum mechanics to explain the formation of metal bonds.Therefore, the energy band theory, also known as the quantum mechanical model of metal bonds, has five basic viewpoints:
1. To make metalatomMinority valence electrons (1, 2 or 3) can adapt to highcoordination numberWhen bonding, valence electrons must be "delocalized" (that is, no longer subordinate to any specific atom), and all valence electrons should belong to the wholeMetal latticeThe atoms of are shared.
2. The atoms in the metal lattice are very dense and can form many molecular orbitals, and the energy difference between adjacent molecular orbitals is very small. It can be considered that the energy changes between energy levels are basically continuous.
3. The energy band formed by the molecular orbital can also be seen as the overlap of the electronic energy levels of closely packed metal atoms. This energy band belongs to the entire metal crystal.For example, the 1S energy levels of lithium atoms in the metal lithium overlap each other to form the 1S energy band in the metal lattice, and so on.Each energy band can include many similar energy levels, so each energy band will include a considerable energy range, sometimes up to 418 kJ/mol.
4. According to the different atomic orbital energy levels, metal crystals can have different energy bands (such as the 1s and 2s energy bands in the above metallic lithium). The low-energy energy band formed by the atomic orbital energy levels that have been filled with electrons is called "full band";The high energy band formed by the atomic orbital energy level not full of electrons is called the "conduction band".The energy difference between these two types of energy bands is so large that it is almost impossible for electrons in the low energy band to transition to the high energy band. Therefore, the energy gap between these two types of energy levels is called "band gap".For example, metallic lithium (electronic layer structure is 1stwo2sone）The 1s orbital of is full of electrons, the 2s orbital is not full of electrons, the 1s energy band is a full band, and the 2s energy band is a conduction band. The energy difference between the two is quite different, and the spacing between them is a forbidden band, which can not be crossed by electrons (that is, electrons cannot jump from the 1s energy band to the 2s energy band).However, the electrons in the 2S band can move freely in the adjacent energy levels in the band under the condition of receiving external energy.
5. The adjacent energy bands in the metal can also overlap each other, such as beryllium (electronic layer structure is 1stwo2stwo）The 2s orbital of is full of electrons. The 2s energy band should be a full band. It seems that beryllium should be a non conductor.However, due to the fact that the 2s energy band of beryllium is very close to the empty 2p energy band and can overlap, the electrons in the 2s energy band can be upgraded into the 2p energy band for movement, so beryllium is still a metal with good conductivity, and has the general property of metal.
Conductivity
The energy difference between metal energy bands and the state of electron filling in the energy band determine whether the substance is a conductor, a non conductor or a semiconductor (i.e., metal, non-metal or quasi metal).If all the energy bands of a substance are full and the energy gap between the energy bands is large, the substance will be a non conductor;If the energy band of a substance is partially filled with electrons, or there is an empty energy band with a small energy gap that can overlap with the adjacent (electronic) energy band, it is a conductor.The energy band structure of semiconductors is that the full band is filled with electrons, the conduction band is empty, and the band gap is very narrow. In general, because the electrons on the full band cannot enter the conduction band, the crystal is not conductive (especially at low temperatures).Because the band gap is very narrow, under certain conditions, the electrons on the full band can easily transition to the conduction band, so that the original empty conduction band can also fill some electrons, and at the same time, there are also vacancies (usually called holes) left on the full band, so that the conduction band and the original full band are not full of electrons, so they can conduct electricity.
physical property
When an external electric field is applied to a metal, the electrons in the conduction band will transition to higher energy levels in the energy band and move through the lattice along the direction of the external electric field, which indicates the conductivity of the metal.The electrons in the energy band can absorb light energy, and also can emit the absorbed energy, which indicates that metal luster and metal are excellent reflectors of radiant energy.Electrons can also transmit heat energy, indicating that metal has thermal conductivity.When applying stress to metal crystals, because the electrons in the metal are delocalized (that is, they do not belong to any atom but belong to the metal as a whole), the metal bond in one place is broken, and the metal bond can be formed in another place. Therefore, mechanical processing will not damage the metal structure, but can only change the shape of the metal, that is, the metal is ductile, malleablePlasticity and other common machinability reasons.The more unpaired valence electrons provided by metal atoms for the formation of energy band, the stronger the metal bond, the higher the melting point and boiling point of the reaction in physical properties, and the greater the density and hardness.
The energy band theory is difficult to explain some problems, for example, some transition metals have high hardness, high melting point and other properties. Some people think that the secondary electrons of atoms participate in the formation of partial covalent metal bonds.Therefore, the metal bond theory is still developing.
