Chemical combination , Types of bonds ( Chemical bonds & Physical bonds )
The most stable atoms are those of noble gases such as helium , neon , argon …..etc , Because their outermost energy level is completely filled with electrons , so , the atoms of these electrons do not undergo any chemical reaction ( at normal conditions ) with other elements or with each , their molecules are monoatomic .
Chemical reaction concept
All of the other elements are reactive to some extent , They undergo chemical reactions to complete their outermost shell by accepting , losing or sharing a number of electrons to acquire an identical electron configuration as that of the nearest noble gas .
As the results of this change in the number of electrons in outermost shells of atoms , bonds are formed between atoms or bonds are broken to form new bonds , this is called a chemical reaction , Chemical reaction is the breaking of bonds between atoms of the reactants molecules to form new bonds between atoms of the products molecules .
Application on the chemical reaction concept : The mixture of iron filings with sulphur powder is not considered a chemical compound , because there is no chemical reaction occurs between them , If this mixture is heated to a high temperature , a chemical reaction occurs ( formation of chemical bond ) between iron & sulphur producing the compound iron ( II ) sulphide .
Lewis electron-dot symbols
The valence electrons have an important role in the nature of the formed bond , So , the scientist Lewis had set a simple way to represent the valence electrons by using dots :
Application : Dot-representation of the valence electrons of the oxygen atom 8O
The electron configuration of oxygen atom : 1s² , 2s² , 2p4 , At the first , the valence electrons are distributed singularly on the four sides of the symbol , then pairing takes till completing the configuration .
The scientist Lewis had differentiated between the lone pair & bond pair of electrons , Lone pair is the electron pair which is found in one of the outer orbitals and does not share in bond formation , Bond pair is the electron pair which is responsible for the bond formation .
Types of bonds
Metals : They are characterized by their large atomic volumes and their low ionization energies , Therefore , Their atoms tend to lose the electrons of the outermost shell and change into cations ( positive ion ) with an identical electron structure to the nearest noble gas precedes them in the periodic table .
Nonmetals : They are characterized by their small atomic volumes and their high electron affinities , Therefore , Their atoms tend to gain electrons ( those lost by metal atoms ) and change into anions ( negative ion ) with an identical electron structure to the nearest noble gas follows them in the periodic table .
The ionic bonding and electronegativity
The properties of compounds differ according to the difference in electronegativity between their elements , The difference in electronegativity between the bonded elements play a role in the characteristics of the ionic bond , whereas the horizontal distance between the bonded elements – in the periodic table – increases , the difference in electronegativity increases , then the strength of ionic bond increases .
In general , when the difference in electronegativity is more than 1.7 then the formed compound is ionic , The ionic bond is the chemical bond which is formed between metals and nonmetals , the difference in electronegativity between them is more than 1.7 .
Molten sodium chloride conducts electricity more than molten magnesium chloride , because the ionic bond in NaCl is stronger than that of MgCl2 , where the difference is electronegativity between sodium and chlorine is greater than that between magnesium and chlorine atoms .
Although chlorine is a nonmetal and aluminum is a metal , aluminum chloride shows the properties of the covalent compounds , Because the difference in electronegativity between aluminum and chlorine atoms is less than 1.7 .
Sodium chloride has high melting and boiling points , due to the strong ionic bond between sodium and chlorine atoms which is resulted from the high difference in electronegativity between them ( 2.1 ) .
The covalent bond is -mostly-formed between atoms of nonmetals of the same electronegativity ( atoms of the same element ) or those of close electronegativities by sharing of electrons , where each atom shares by a certain number of electrons of the outermost shell equals the number of electrons required to complete this shell , forming a pair or more of electrons being in the vicinity of each atom .
Classification of covalent bond
The covalent bond is classified according to the difference in electronegativity between the bonded atoms into three types , which are : Pure covalent bond , Non-polar covalent bond , Polar covalent bond .
Pure covalent bond
When the two bonded atoms are of the same nonmetal element ( have the same electronegativity ) : Each atom in the molecule has the same ability to attract the two shared electrons ( electron pair ) of the bond , thus the electron pair spends the same time in the vicinity of each atom and the net charge for each of them equals zero , It is described as a pure covalent bond .
Pure covalent bond is the bond formed between two atoms , the difference in electronegativity between them equals zero , Examples : The bond in hydrogen molecule ( H − H ) , The bond in the fluorine molecule ( F − F ) .
Non-polar covalent bond
Non-polar covalent bond is the bond formed between two atoms of two different nonmetal elements , the difference in electronegativity between them is greater than zero till 0.4 , Example : The bond in methane molecule ( C − H ) .
Polar covalent bond
When the difference in electronegativity between the bonded atoms of two nonmetal elements is somewhat high ( greater than 0.4 and less than 1.7 ) , the more electronegative atom has a greater attraction for the electron pair of the bond , so , the electrons spend more time in the vicinity of it .
As a result , the atom acquires a partial negative charge ( δ– ) – not a complete one – and because of this unequal sharing of the electron pair towards it , the less electronegative atom acquires a partial positive charge ( δ+ ) , The produced molecule is then described as a polar molecule and the bond is described as a polar covalent bond .
As the difference in the electronegativity between the bonded elements in the polar molecule increases , the strength of the covalent bond increases , Example : The bond in hydrogen chloride molecule , The bond in water molecule , The bond in ammonia molecule .
Application : The polar covalent bond in the hydrogen chloride molecule HCl
In hydrogen chloride molecule the more electronegative chlorine atom has a greater attraction for the electron pair of the covalent bond , So , the electron pair spends more time in the vicinity of the chlorine atom .
As a result , the chlorine atom acquires the partial negative charge ( δ– ) , As of this unequal sharing of the electron pair towards chlorine , the hydrogen atom acquires the partial positive charge ( δ+ ) .
The bond in water molecule is polar covalent , while in chlorine molecule is pure covalent bond , because the difference in the electronegativity between the hydrogen and oxygen atoms in water molecule is somewhat high ( greater than 0.4 and less than 1.7 ) , while the difference in the electronegativity between the two chlorine atoms in chlorine molecule equals zero .
NH3 molecule is polar , Because the difference in the electronegativity between nitrogen and hydrogen atoms ( N − H ) is greater than 0.4 and less than 1.7 , also sum of polar pair of the molecule does not equal zero .