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 )

Group 5A Elements

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 :

1. Graduation of the metallic and nonmetallic properties .
2. Different number of molecules atoms of each element .
3. Several oxidation numbers .
4. Oxides of group 5A elements .
5. 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 :

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 :

1. Their thermal stability decreases , so , they decomposes even by gentle heating .
2. 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 .

The most famous elements in 5A group ( Nitrogen properties , Preparation & compounds )

Nitric acid properties , Passivity phenomenon & Economic importance of the 5A group elements