Chemical bond（chemical bond）Two or more adjacent molecules or crystals of pure substanceatomInterionicInteraction forceThe general name of.To bind ions or atoms togetherForceIt is commonly referred to as chemical bond.
Ionic bond、covalent bond、Metal bondThey have different causes. The ionic bond is formed by the electrostatic action through the electron transfer between atoms to form positive and negative ions.covalent bondThe cause of formation is more complex. Lewis theory believes that,covalent bondIt is formed by sharing one or more pairs of electrons between atoms. Other explanations include valence bond theory,Valence shell electronMutual exclusion theory, molecular orbital theory andHybrid orbitTheory, etc.Metal bond is a kind ofModificationOfcovalent bondIt is formed by several atoms sharing some free flowing electrons.[1]
Two in one water moleculehydrogen atomAnd 1Oxygen atomIt is the combination of chemical bonds to form water molecules.becauseNucleusIt is positively charged and electrons are negatively charged, so we can say that all chemical bonds are formed by the simultaneous attraction of electrons by two or more atomic nuclei.There are three types of chemical bonds, namely ionic bondscovalent bond、Metal bond(Hydrogen bond is not a chemical bond, it is a kind of intermolecular force).
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Ionic bond
The process by which atoms gain or lose electrons from each other to form ionic bonds
Band reversalchargeThe interaction between ions is called ionic bond, and the essence of bonding is the electrostatic interaction between anion and cation.When the electronegativity between two atoms differs greatly, it is generally metal and nonmetal.For example, chlorine and sodium are synthesized by ionic bondingsodium chloride。Chlorine with high electronegativity will snatch an electron from sodium with low electronegativity to meetOctet。Then chlorine will exist in the form of - 1, while sodium will exist in the form of+1Coulomb electrostatic forceIt is also said that the ionic bond is the bonding mode for the combination of metal and nonmetal because positive and negative attract each other.The ionic bond can be extended, so there is no molecular structure.
There are also strong and weak ionic bonds.Its strength affects theIonic compoundMelting point, boiling point, solubility and other properties.The stronger the ionic bond, the higher its melting point.The smaller the ion radius or the more charges, the stronger the interaction between the anion and cation.For example, if the particle radius of sodium ion is smaller than that of potassium ion, the ionic bond in sodium chloride NaCl is stronger than that in potassium chloride KCl, so the melting point of sodium chloride is higher than that of potassium chloride.
Definition: ionic bond is formed by the passage of positive and negative ionsElectrostatic actionThe positive and negative ions are spherical or nearly spherical, and the charge is spherically symmetric, so the ion bond can have electrostatic effect in all directions, so there is nodirectionalOf.
The concept of ionic bond: the interaction between oppositely charged ions is called ionic bond.
Bonding essence: electrostatic effect.Electrostatic effects include electrostatic attraction between anions and cations, electrostatic repulsion between electrons and between atomic nuclei.(One suction, two repulsions)
Bonding reasons: ① atoms gain and lose electrons to form stable anions and cations. ②The attraction and repulsion between ions are atEquilibrium state。③The total energy of the system decreases.
Scope of existence: ionic bonds exist in most strong bases, salts andmetallic oxideMedium.
Chemical bond
An ion can attract and bond with multiple oppositely charged ions at the same time, althoughIonic crystalOne ion can only directly interact with several ions with opposite charges (for example, Na+in NaCl can directly interact with six Cl -), but this is due to space factors.In places far away, there is also a relatively weak effect, so there is no saturation.The concept of chemical bond was established and developed on the basis of long-term practical experience, which is used to summarize a large number of chemical facts observed, especially to explain why atoms are combined in a certain proportion into relatively stable and independent molecules with certain geometric shapes, whose properties are completely different from those of their constituent atoms.At the beginning, people drew a short line between the two atoms combined with each other as the symbol of chemical bond;After the discovery of electron, 1916G. N. LewisThe concept of forming ion and ion bond by filling the electron stable shell or forming covalent bond by sharing a pair of electrons of two atoms is proposed, and the electronic theory of chemical bond is established.
Quantum TheoryAfter its establishment, in 1927, W.HHeitlerAnd FW. Londonquantum mechanicsThe reason for the stable existence of hydrogen molecule is explained, and the essence of chemical bond is clarified in principle.Many people, especially LC. Pauline and RS.MalikenThe theoretical explanation of chemical bond has been gradually improved.
Chemical bonds are essentially electrical. When atoms form molecules, the outer electrons are redistributed（transfer、share、deviationSo as to produce a strong force between positive and negative electricity.However, the way and degree of this electrical action are different, so chemical bonds can be divided into ionic bonds, covalent bonds and metal bonds.Ionic bond is the anion generated after the atom gains or loses electronscationChemical bond formed by electrostatic interaction between.The essence of ionic bond is electrostatic action.Since electrostatic attraction has no directionality, the interaction between anion and cation can be in any direction, and the ionic bond has no directionality.As long as conditions permit, the cation can be attracted as much as possibleanionAnd vice versa, the ionic bond is not saturated.Different anions and cations have different radii and electrical properties, resulting in crystalsSpatial latticeNot the same.
covalent bond
1. Covalent bond refers to the sharing of electron pairs between atoms（Electronic cloudOverlap).The electrons that form overlapping electron clouds move around all bonded atoms.An atom with several unpaired electrons can interact with severalspinElectrons in opposite directions pair to form bonds,Covalent bond saturationIs due to the fact that when electron clouds overlap (electron pairing)Pauli exclusion principle。The overlap of electronic clouds can only occur in a certain direction, but not at will.The formation of covalent bond directionality is due to the fact that when forming covalent bonds, the larger the overlapping area of the electronic clouds, the more stable the covalent bonds formed. Therefore, when forming covalent bonds, they are always formed along the direction of the greatest overlap of the electronic clouds (this isMaximum overlap principle）。Covalent bonds have saturation and directivity.
2. After atoms form covalent bonds by sharing electron pairs, the total energy of the system decreases.
covalent bondIs formed by bonding electronsAtomic orbitalOverlap occurs, and to stabilize the covalent bond, the overlap must be the largest.Because in addition toS trackIn addition, other orbits have a certain extension direction, so when bonding, except the s-sσ bond(For example, H2) Except for the maximum overlap in any direction, the bonds formed by other orbits can only reach the maximum overlap in a certain direction.Classification of covalent bonds
There are different classifications of covalent bonds.
(1) According to the number of shared electronic pairs, there areSingle bond（Cl—Cl）、double bond（C=C）、Triple bond(N ∨ N, C ∨ C), etc.
(2) Classified by whether the shared electronic pair is offset, YesPolar bond(H-Cl) andNonpolar bond（Cl—Cl）。
(3) According to the way of providing electron pairs, there are normal covalent bonds and coordination bonds (the shared electron pairs are provided by one party, and the other party provides empty orbits.
For example, one of the N-H bonds in the ammonium ion belongs to the coordination bond).
Chemical bond
(4) According to the overlap mode of electron clouds, there are σ bonds (the electron clouds form bonds in the way of "head to head" along the bond axis, such as C-C.) andpi bond(The electronic cloud is keyed in the direction of "shoulder to shoulder" along both sides of the key axis. For example, in C=CBond energySmaller keyC=C has a σ bond and a π bond.)etc.
3. Old theory: The condition for the formation of covalent bond is that there must be a single electron in the atom, and the spin direction must be opposite. Since one single electron of an atom can only pair with another single electron, the covalent bond has saturation.For example, after H atom and Cl atom form HCl molecule, they can no longer form HCl2 with another Cl atom.
4. New theory: When covalent bonds are formed, the atomic orbits of the bonding electrons overlap and split, and the bonding electrons fill in the lower energy orbits, that isBonding orbital。If there are other atoms involved in bonding, the electrons provided by them will be filled with higher energyAntibonding orbitThe formed molecules will also be unstable.Compounds like HCl that share electron pairs to form molecules are calledCovalent compound。
Classification of compounds
Atoms rely on chemical bonds to form small organic molecules
1．Ionic compound: Compounds consisting of cations and anions.
Most salts (including all ammonium salts), strong bases, most metal oxides, metalshydride。livelymetallic elementAnd livelyNonmetalNot all compounds formed by elements are bound by ionic bonds, such as AICI3, FeCl3, BeCl2, etc.Nonmetallic elementCan also be formed betweenIonic compoundFor example, ammonium salts are ionic compounds.
2. Covalent compounds: compounds formed mainly by covalent bonds, called covalent compounds.
Nonmetallic oxide, acid, weak base, a small part of salt, non-metallic hydride.
3. Ionic compounds must contain ionic bonds, possibly covalent bonds.stayCovalent compoundThere must be no ionic bond in.
Metal bond
oneOverview: A kind of chemical bond, mainly existing in metals.fromfree electronAnd metal ions arranged in latticestatic electricityAttractive combination.Due to the free movement of electrons, the metal bond has no fixed direction, so it isNonpolar bond。Metal bonds have many properties of metals.For example, the melting point and boiling point of general metals increase with the strength of metal bonds.Its strength is usually the same as that of metalIonic radiusbecomeInverse correlation, and metal interiorFree electron densityIn positive correlation (it can be roughly regarded as being related to the periphery of the atomNumber of electronsPositive correlation).
twoModified covalent bond theory:Metallic crystalThe free electron shuttles around. It does not belong to a certain metal ion but is 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),Electronic energy levelAs much overlap as possible can be obtained to form metal bonds.The above hypothetical model is called metallicfree electronModel, calledModified covalent bond theory。In 1900, Drude et alheat conductionPerformance.This theory has been improved and developed by Lorentz (1904) and Sommerfeld (1928), and many important properties of metals have been explained.However, due to the oversimplification of the free electron model of metals, it is impossible to explain why metal crystals haveBinding force, nor can it explain why there are metallic crystalsconductor、insulatorAnd semiconductor.With the development of science and production, mainly the development of quantum theoryEnergy band theory。
Local key
Existing only between two atomscovalent bond。Containing only the local keyPolyatomic moleculeIt can be seen that the chemical bond between two relatively independent atoms connects the atoms, which ignores the influence of adjacent chemical bonds and describesDiatomic moleculeThe method of chemical bond in the middle is used on the localized bond of polyatomic molecule.asethyleneThere is one C-C, four C-H σ bonds, one C-CπKey.The localized bond has relatively constant bond properties.For example, the key length and key of a certain type of local keydipole moment, keysPolarity, keysForce constantThe bond energies of,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, are approximately unchanged in different molecules.Therefore, the relevant extensive properties of molecules can be approximately expressed as the sum of the corresponding bond properties.This characteristic of localized bonds has been widely used in chemistry, for example, to calculate molecularAtomization energyApproximate value.This model better reflects thatElectronic cloudThe determined molecular properties are as followsBond energy、Key length、Bond angle, bond dipole, bond polarization, etc.This molecular orbital around two atoms becomesLocalized orbit。
Polar bond
Chemical bond
In a compound molecule, different atoms formcovalent bondBecause the ability of two atoms to attract electrons is different, the shared electron pair must be biased to the atomic side with stronger electron attraction ability, so the atomic side with weaker electron attraction ability is relatively significantly positive.Such covalent bond is calledPolar covalent bond, referred to as polar bond.
For example: H-Cl bond in HCl molecule belongs to polar bond
There is a simple method to judge the polar bond and non-polar bond. Compare the atomic weight of each atom in the compound. Generally speaking, the atom with larger relative atomic mass has a stronger ability to attract electrons.However, it should be noted that compounds with polar bonds are not necessarily polar compounds, such as methane, which is non-polar molecules with polar bonds (because the positive and negative charge centers coincide).
Nonpolar bond
Formed between atoms of the same elementcovalent bond, callednonpolar covalent bond 。Homoatomic attractionShared electron pairThe bonding electron pairs are evenly distributed between the two nuclei without bias to any atom, and the bonding atoms are not sensitive to electricity.wrongPolar bondCan exist inElementary moleculeMedium (such as H-H bond in H2, O=O bond in O2, N ∨ N bond in N2), can also exist in compound molecules (such as C-C bond in C2H2).Bond of nonpolar bonddipole momentIs 0.The molecules formed by non-polar bond areNonpolar molecule。The bonds existing in non-polar molecules are not all non-polar bonds. If a polyatomic moleculepositive chargeThe geometric center coincides with the geometric center of the negative charge, so even if it is composed of polar bonds, it is also a nonpolar molecule.The crystals formed by non-polar bonding can beAtomic crystal, orMixed crystalorMolecular crystal。For example, there are three types of carbonAllotrope: It can be formed by C-C non-polar bondRegular tetrahedronSkeleton diamond (atomic crystal)StratotypeGraphite (mixed crystal) can also form spherical carbon moleculesfullerene C60 (molecular crystal).
For example: Cl Cl bond in Cl2 molecule belongs to non-polar bond
Coordination bond
Also called coordination covalent bond, it is a specialcovalent bond。When the electron pair used in the covalent bond is supplied by one of the atoms alone, it is calledCoordination bond。Once the coordination bond is formed, it is the same as the common covalent bond.The two electrons shared between two bonded atoms are not provided by each atom, but come from one atom.For example, ammonia and boron trifluoride can formCoordination compound: In the picture formula, → indicates coordination key.The pair of electrons between F and B comes from theLone pair electron。
Pauli principle
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Chemical bond
Pauli principleIt is found that the fact of high-resolution spectrum reveals that in addition to the space motion, there is a strange quantized motion of extranuclear electrons, which is called spin motion（spin m.q.n）Represents.This principle consists ofPauliFirst, put forward.
Pauli exclusion principleContent: Each orbit can accommodate at most two electrons with opposite spin, which is usually represented by "↑↓".
The nucleus can also have a net spin.Because of thermal equilibrium, these nuclei are usually randomly oriented.But for some specific elements, such as xenon-129, part of the nuclear spin may also be polarized, and this state is calledHyperpolarization, onMagnetic resonance imagingIt has very important applications.
Hunt rule
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Hund Rule content: when the electrons are arranged in the sameenergy levelFor different tracks,Ground state atomThe electrons in are always preferentially occupying a single orbit, and their spin states are the same.This rule was first proposed by Hunter and is called the Hunter rule.
