Elements of p-block , General properties of group 5A elements ( group 15 )
Group 5A ( group 15 ) consists of five elements which are nitrogen , phosphorus , arsenic , antimony & bismuth , Elements of this group are not abundant in nature , except nitrogen which represents about 80 % of atmospheric air ( 4/5 ) of air volume , We can not detect both nitrogen & phosphorus by flame test , because both of them are nonmetals .
Group 5A ( group 15 )
The forms of these elements which are found in nature :
- Phosphorus is the most abundant element of this group ( 5A ) in Earth’s crust , The form found in nature : Calcium phosphate Ca3(PO4)2 , Apatite CaF2.Ca3(PO4)2 ” the salt of calcium fluoride and calcium phosphate ” .
- Arsenic : The form found in nature is Arsenic sulphide As2S3 .
- Antimony : The form found in nature is Antimony sulphide Sb2S3 .
- Bismuth : The form found in nature is Bismuth sulphide Bi2S3 .
General properties of group 5A elements ( group 15 )
The properties of group 5A elements graduate regularly depending on the increase in atomic number and atomic size as illustrated in the following :
- Graduation of the metallic and nonmetallic properties .
- Different number of molecules atoms of each element .
- Several oxidation numbers .
- Oxides of group 5A elements .
- Hydrides of group 5A elements .
The graduation of the metallic and nonmetallic property
The metallic property increases by increasing the atomic number ( downward the group ) , The properties of this group elements tend to be those of the nonmetals , even bismuth – is a metal – but its ability to conduct electricity is weak .
Different number of molecules atoms of each element
The number of molecule atoms of each element of this group differs as the following :
- Nitrogen : The molecule contains two atoms , Symbol of molecule is N2 .
- Phosphorus : its vapour has molecule which contain four atoms , Symbol of molecule is P4 .
- Antimony : its vapour has molecule which contain four atoms , Symbol of molecule is As4 .
- Arsenic : its vapour has molecule which contain four atoms , Symbol of molecule is Sb4 .
- Bismuth : It forms a metallic crystal lattice & its vapor consists of diatomic molecules , Symbol of molecule is Bi2 .
Although bismuth is a metal , it has different properties than that of the other metals , because of its low electric conductivity relative to the other metals and it has a diatomic structure in its vapor state , whereas the other metals have mono-atomic structure .
Several oxidation numbers
Elements of this group are characterized by having several oxidation numbers in their different compounds from ( − 3 ) to ( + 5 ) , because through covalent sharing they may gain till 3 electrons or lose till 5 electrons , The oxidation numbers of nitrogen in some of its compounds :
The oxidation numbers of nitrogen in the oxygenated compounds are positive , while that in the hydrogenated compounds are negative , Because the electronegativity of nitrogen is less than of oxygen and higher than that of hydrogen .
Several allotropic forms
Allotropy is the presence of the element in more than one form which have different physical properties and similar chemical ones , The allotropic phenomenon appears in the solid nonmetals only , Due to the presence of the solid nonmetal element in different crystalline forms , each form differs in the number of atoms and in their arrangement .
- Phosphorus : Allotropic forms are white ( waxy ) , red , violet .
- Arsenic : Allotropic forms are black , grey , yellow ( waxy yellow ) .
- Antimony : Allotropic forms are yellow , black .
Allotropy exists in phosphorus , but not in nitrogen and bismuth , Because phosphorus is a solid nonmetal , while nitrogen is a gaseous nonmetal and bismuth is a metallic element , in which gases and metals do not have allotropic forms .
Oxides of group 5A elements
All elements of this group form oxides have the formula : X2O3 , X2O5 , Some of these oxides are acidic , others are amphoteric and others are basic , The basic property of these oxides increases ( the acidic property decreases ) with increasing the atomic number .
- Element is 7N : Its oxide is N2O3 , N2O5 , Type of oxide is Acidic oxide .
- Element is 15P : Its oxide is P2O3 , Type of oxide is Acidic oxide .
- Element is 33As : Its oxide is As2O3 , Type of oxide is Acidic oxide .
- Element is 51Sb : Its oxide is Sb2O3 , Type of oxide is Amphoteric oxide .
- Element is 83Bi : Its oxide is Bi2O3 , Bi2O5 , Type of oxide is Basic oxide .
Hydrides of group 5A elements
Most of the elements of this group react with hydrogen forming hydrides , the oxidation number of the element is −3 , as in Ammonia NH3 , Phosphine PH3 , Arsine AsH3 .
The central atom in these compounds still has a lone pair of electrons in its valence shell ( outer energy level ) , so , it can give this pair of electrons to other atoms or ions making coordinate bonds .
Based on that the basicity of the hydride depends on the ability of its central atom to lose one pair of electrons or gain a proton from another substance , ammonia is more basic than phosphine .
NH3 + H2O → NH+4 + OH−
PH3 + H2O → PH+4 + OH−
The properties of hydrides of the group 5A elements :
- Their thermal stability decreases , so , they decomposes even by gentle heating .
- The polarity of hydrogen compounds in this group decreases with increasing atomic number of the element attached to hydrogen , thus their solubility in water decreases .
The solubility degree of phosphine in water is less than that of ammonia , Because the polarity of phosphine is less than the polarity of ammonia .