The coordinate bond , Physical bonds ( hydrogen bond and the metallic bond )
The coordinate bond is considered as a special type of the covalent bonds which differs only in the origin of the electron pair which is shared between the atoms involved in the covalent bond , because the electron pair forming the ordinary covalent bond is produced by both of the two combined atoms contributing one electron to the bond , whereas the electron pair of the coordinate bond is produced only by one of the two combined atoms .
The coordinate bond
The coordinate bond is bond that is formed between a donor atom having a lone pair of electrons or more and an acceptor atom having a vacant orbital , The coordinate bond is represented by an arrow directs from the donor atom to the acceptor atom , The coordinate bond is formed between two atoms :
- The first : having a lone pair of electrons occupying one orbital , This atom is called the donor atom .
- The second : having a vacant orbital , needing this electron pair to acquire the stable electron structure , This atom is called the acceptor atom .
Application 1 : The coordinate bond formed in the ammonium ion ( NH4 )+
Ammonia gas dissolves in water forming a coordinate bond between :
- Nitrogen atom of ammonia molecule which contains a lone pair of electrons , so , the nitrogen atom represents the donor atom .
- Water proton ( positive hydrogen ion H+ ) which contains a vacant orbital accepts the lone pair of electrons from nitrogen atom , so , the proton H+ represents the acceptor atom .
Application 2 : The coordinate bond formed in hydronium ion ( H3O )+
Acids dissolve in water forming a coordinate bond between :
- Oxygen atom of water molecule which contains a lone pair of electrons , The oxygen atom represents the donor atom .
- Acid proton ( positive hydrogen ion H+ ) which contains a vacant orbital accepts the lone electron pair from oxygen atom , The proton H+ represents the acceptor atom , It is observed that the other lone electron pair remains as it is in the oxygen atom .
Ammonium hydroxide molecule ( NH4OH ) contains three types of the bonds , because it contains :
- Ionic bond as a result of an electrostatic attraction occurs between the positive ammonium ion ( NH4 )+ and the negative hydroxide ion ( OH− ) .
- Coordinate bond in ammonium ion , when the nitrogen atom of the ammonia molecule donates the lone electron pair to the positive proton ( H+ ) .
- Three covalent bonds in ammonia molecule resulting from the sharing of electrons between the nitrogen atom and the three hydrogen atoms .
Physical bonds are the hydrogen bond and the metallic bond .
The hydrogen bond
It is known that water boils at 100° C , This temperature is considered to be high for a compound of a relative low molecular mass ( 18 g/mol ) .
If this temperature is compared with the boiling point of hydrogen sulphide ( of molecular mass 34 g/mol ) which is – 61° C , we will find a great difference although oxygen precedes sulphur in the same group 6A in the periodic table .
In general , as relative molecular mass increases , the boiling point increases , but the boiling point of water is anomalous in the group ( VI ) hydrides , This anomalousness of the boiling point of water is explained by the attraction of water molecules to each other by what is known as the hydrogen bond or hydrogen bridge .
To form hydrogen bond , the hydrogen atom must become two atoms of high electronegativity such as ( N , O , F atoms ) , The hydrogen bond is the bond that is formed between hydrogen atom binds by a polar bond [ eg. : ( F − H ) , ( O − H ) , ( N − H ) ] with high electronegative bonded atom [ eg. : F , O , N ] , All the compounds of hydrogen bonds are polar compounds dissolve in polar solvents as water .
- The bond between hydrogen fluoride molecule HF .
- The bond between water molecules H2O .
- The bond between ammonia molecules NH3 .
Application : Hydrogen bonds between water molecules
- The oxygen atom in water carries a partial negative charge ( δ− − ) , The two hydrogen atoms carries a partial positive charge ( δ++ ) .
- The two hydrogen atoms becomes as a bridge between the two oxygen atoms which have a high electronegativity .
- The molecules get near to each other through the hydrogen atom which bonded the water molecules together .
The strength of hydrogen bond
Although the hydrogen bond clearly affects physical properties of the compounds ( as water ) , its bond strength is far weaker than the covalent bond , because the length of hydrogen bond is longer than that of covalent bond , as the length of the bond increases and the bond strength becomes weaker .
The strength of hydrogen bond increases , when the hydrogen bond becomes in a straight line with the polar covalent bond as in water molecules H2O , Hydrogen fluorine molecules HF , By increasing the difference in the electronegativity between hydrogen atom and the other atom in the polar covalent bond .
The hydrogen bonds between hydrogen fluoride molecules ( HF ) are stronger than that between water molecules , Because the difference in the electronegativity between fluorine & hydrogen is greater than that between oxygen & hydrogen , the strength of the hydrogen bond increases as the difference in the electronegativity between the bonded atoms increases .
The hydrogen bond between H2O molecules is stronger than that between NH3 molecules , Because the difference in the electronegativity between nitrogen & hydrogen atoms , In addition , the hydrogen bonds in water molecules are in a straight line with the polar covalent bond .
The forms of the compounds which have hydrogen bonds
The compounds which have hydrogen bonds take several forms , the compounds may form Straight line such as the hydrogen bonds between hydrogen fluorine molecules , closed ring such as the hydrogen bonds between hydrogen fluorine molecules , Open net such as the hydrogen bonds between water molecules .
The metallic bond
Each metal has a crystal lattice with a definite form in which the positive metal ions take a certain arrangement , The outermost shell electrons ( valence electrons ) of each atom are associated together forming an electron cloud with free movement to bind this great collection of positive metal ions by what is known as metallic bond .
The metallic bond is the bond produced from the electron cloud of valence electrons which decreases the repulsive forces between the positive metal ions in the crystal lattice , These free valence electrons account for the good electrical and thermal conductivity of metals .
The metallic bond strength
The number of valence electrons in the metal atom plays an important role in the strength of the metallic bond , as the number of valence electrons increases in the metal atoms , the atoms become more strongly bound and accordingly the metal becomes more hard and has a higher melting point .
Hardness : The strength of metal to resist the scratch , the scientist Friedrich Mohs had put a scale for it , The hardness of tale ( made of magnesium silicate ) = 1 and can be scratched by fingernails , The hardness of diamond = 10 and can be scratched by another metal .