8. CHEMICAL COMMUNICATION. COVALENT UNPOLAR AND POLAR BONDS.

Formation of chemical compounds is caused by emergence of chemical bond between atoms in molecules and crystals.
The chemical bond is mutual coupling of atoms in molecule and crystal lattice as a result of action between atoms of electric forces of attraction.
Emergence of the atomic model of Bohr which for the first time has explained structure of electron shell promoted creation of idea of chemical bond and its electronic nature. According to Bohr's model electrons can hold positions to which certain power states, i.e. energy levels answer in atom. In 1915 the German physicist Kossel has offered explanation of chemical bond in salts, and in 1916 the American scientist Lewis has offered interpretation of chemical bond in molecules. They proceeded from ideas that atoms of elements possess tendency to achievement of electron configuration of rare gases (full filling of external electron layer). Kossel and Lewis's representations have received names of the electronic theory of valency.
Valency of elements of the main subgroups of the Periodic system depends on number of the electrons which are on external electron layer. Therefore it is accepted to call these external electrons valent. For elements of secondary subgroups both electrons of external layer, and electrons of internal subtotals can act as valence electrons.
Distinguish three main types of chemical bond: covalent, ionic, metal.

Table. Types of chemical bond and their main distinctive signs.

Chemical bond	The connected atoms	Character of elements	Process in electron shell	The formed particles	Crystal lattice	Character of substance	Examples
Ionic	Atom of metal and atom of nonmetal	Electrohollow zhitelny and electro- negative	Transition of valence electrons	Positive and negative ions	Ionic	Soleobraz- ny	NaCl CaO NaOH
Covalent	Atoms of nonmetals (is more rare - atoms of metals)	Elektrootritsa- telny is more rare electrohollow zhitelny	Formation of the general electronic couples, filling of molecular orbitals	Molecules	Molecular	Flying or nonvolatile	Br2 CO2 C6H6
				---------	Atomic	Алмазоподоб ny	Si SiC diamond
Metalliches Kaya	Atoms of metals	Electrohollow zhitelny	Return of valence electrons	Positive ions and electron gas	Metal	Metalliches- Kaya	Metals and alloys

COVALENT LINKAGE.

Covalent linkage is formed at the expense of the general electronic couples arising in covers of the connected atoms.

It can be formed by atoms of one total element and then it is unpolar; for example, such covalent linkage exists in molecules of single-element gases H2, O2, N2, Cl2, etc.

Covalent linkage can be formed by atoms of the different elements similar on chemical character, and then it is polar; for example, such covalent linkage exists in the molecules H2O, NF3, CO2. Covalent linkage is formed between atoms of elements,

Neokhodimo to enter concept electronegativity. Electronegativity is ability of atoms of chemical element to delay to itself the general electronic couples participating in chemical bonding.

number of elektrootritsatelnost

Relative electronegativity of elements (across Polingu)

group	I	II	III	IV	V	VI	VII	VIII
period
1	H 2,1										He -
2	Li 0,97	Be 1,47	B 2,01	With 2,50	N 3,07	O 3,5	F 4,10				Ne -
3	Na 1,01	Mg 1,23	Al 1,47	Si 1,74	P 2,1	S 2,6	Cl 2,83				Ar -
4	K 0,91	Ca 1,04	Sc 1,20	Ti 1,32	V 1,45	Cr 1,56	Mn 1,60	Fe 1,64	Co 1,70	Ni 1,75
	Cu 1,75	Zn 1,66	Ga 1,82	Ge 2,02	As 2,20	Se 2,48	Br 2,74				Kr -
5	Rb 0,89	Sr 0,99	Y 1,11	Zr 1,22	Nb 1,23	Mo 1,30	Tc 1,36	Ru 1,42	Rh 1,45	Pd 1,35
	Ag 1,42	Cd 1,46	In 1,49	Sn 1,72	Sb 1,82	Te 2,01	I 2,21				Xe -
6	Cs 0,86	Ba 0,97	La * 1,08	Hf 1,23	Ta 1,33	W 1,40	Re 1,46	Os 1,52	Ir 1,55	Pt 1,44
	Au 1,42	Hg 1,44	Tl 1,44	Pb 1,55	Bi 1,67	Po 1,76	At 1,90				Rn -
7	Fr 0,86	Ra 0,97	Ac ** 1,00	* Lanthanides - 1,08 - 1,14 ** Actinoids - 1,11 - 1,20

 

Elements with bigger electronegativity will delay the general electrons from elements with smaller electronegativity.

For the evident image of covalent linkage in chemical formulas points are used (each point answers valence electron, and also line answers the general electronic couple).
Example. Bonds in _{the molecule Cl2} can be represented so:

Such records of formulas are equivalent. Covalent linkages possess space orientation. As a result of covalent binding of atoms either molecules, or atomic crystal lattices with strictly certain geometrical arrangement of atoms are formed. To each substance there corresponds the structure.
From position of the theory of Bohr formation of covalent linkage is explained by tendency of atoms to transform the external layer to octet (full filling to 8 electrons). Both atoms represent for formation of covalent linkage on one not coupled electron, and both electrons become the general.
Example. Formation of molecule of chlorine.

Points have designated electrons. At arrangement it is necessary to observe pravilo:elektrona are put in certain sequence - at the left, from above, on the right, from below on one, then add on one, not coupled electrons also take part in bonding.

New electronic couple which has arisen from two not coupled electrons becomes the general for two chlorine atoms. There are several ways of formation of covalent linkages at the expense of overshoot of electronic clouds.

s-s-communication
s-p-communication
p-p-communication
p-p-communication

σ - communication is much stronger than π-communication, and π-communication can be only with σ-communication, due to this communication double and acetylene multiple bonds are formed.

Polar covalent bindings are formed between atoms with different electronegativity.

At the expense of the shift of electrons from hydrogen to chlorine chlorine atom is loaded partially negatively, hydrogen - is partially positive.

 

  

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