Covalent bond
Covalent bond is formed by an electron pair shared by both atoms. Two mechanisms of covalent bond formation are recognized: exchange mechanism and donor-acceptor mechanism.
1. Exchange mechanism. Each atom donates one unpaired electron to their common electron pair:
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2. Donor-acceptor mechanism. One atom (donor) donates the electron pair, and another one (acceptor) donates a free orbital for this pair:
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Two atoms may share several electron pares. Bonds of this kind are called multiple bonds:
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(или N º N) - тройная связь |
If electron density is distributed symmetrically between atoms, the covalent bond is called non-polar. If the electron density is shifted in the direction of one of the atoms, the bond is called polar. The larger is the difference in electro negativity of the bonded atoms, the higher is polarity of the bond.
Electro negativity is the ability of an atom to draw electron density from other atoms to itself. The most electronegative element is fluorine; the most electro positive one is francium.
Ionic bond
Ions are charged particles formed as a result of atoms losing or acquiring electrons:
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(sodium fluoride consists of sodium ions Na+ and fluoride ions F-).
If the difference in electro negativity of atoms is high, the bonding electron pair completely shifts to one of the atoms, and both atoms become ions.
Chemical bond between ions formed by means of electrostatic attraction is called ionic bond.
Hydrogen bond
Hydrogen bond is a bond formed by a positively charged hydrogen atom belonging to one molecule and a negatively charged atom of another molecule. Hydrogen bond is by its nature partly electrostatic and partly coordinate (donor-acceptor).
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(Hydrogenbondsareshownbydots) |
Presence of H-bonds explains high boiling points of water, alcohols, and carbonic acids.
Metal bond
Valence electrons in metals are sufficiently loosely held by their nuclei and may easily part with them. Hence a metal is composed of a number of positive ions arranged into a crystal lattice and a large number of electrons, freely moving through the whole crystal. Electrons in metal hold all the atoms together.
Hybridization of orbitals
Hybridization of orbitals is a transformation of some orbitals during formation of covalent bonds enabling more effective overlapping of orbitals.
1. sp3-Hybridization. One s-orbital and three p-orbitals combine to form four equivalent "hybrid" orbitals. Axes of these orbitals form angles of 109°28'.
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Molecules formed by sp3-hybridized atoms have tetrahedral symmetry (CH4, NH3).
2. sp2-Hybridization. One s-orbital and one p-orbital combine to form two equivalent "hybrid" orbitals, whose axes form an angle of 120 degrees.
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If the chemical bond is formed by overlap of orbitals along the line connecting nuclei, it is called s-bond. If the orbitals overlap outside the line connecting nuclei, p-bond is formed. Three sp2-orbitals can form three sigma bonds (BF3, AlCl3). Yet another bond (p-bond) may form if an orbital not taking part in hybridization has an electron (ethylene C2H4).
Molecules formed by an sp2-hybridized atom have a flat structure.
3. sp-Hybridization. One s-orbital and one p-orbital combine to form two equivalent "hybride" orbitals, whose axes form an angle of 180 degrees.
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These two sp-orbitals can form two s-bonds (BeH2, ZnCl2). Two additional p-bonds may form if another two p-orbitals not taking part in hybridization have electrons (acetylene C2H2).
Molecules formed by sp-hybridized atoms are linear.