Hydroxy compounds, Rules for calculating the oxidation numbers

The oxygenated acids ( acids contain oxygen ) and bases are considered as hydroxy compounds , They can be represented by the general formula ( MOH ) , where M is the element atom , The hydroxy compounds can be ionized by either ways : As an acid , As a base .

Hydroxy compounds

As an acid : The compound will be ionized as an acid if : The ( M − O ) bond is stronger than the ( O − H ) bond , ( The attraction force between M⁺ and O^{− −}  is stronger than that between H⁺ and O^{− −} ) .

MOH ( oxygenated acid ) ⇔ MO⁻ + H⁺ ( Hydrogen ion )

As a base : The compound will be ionized as a base if : The ( O − H ) bond is stronger than the ( M − O ) bond , ( The attraction force between H⁺ and O^{− −} is bigger than that between M⁺ and O^{− −} ) .

MOH ( base ) ⇔ M⁺ + OH ⁻ ( hydroxide ion )

If the strength of ( M − O ) bond and the strength of  ( O − H ) bond are equal , the substance will be ionized as an acid or a base depending on the reaction medium , this means that it reacts as a base in the acidic medium and as an acid in the basic medium .

In general the attraction forces between each of ( O^{− −}, M⁺ ) and ( O^{− −}, H⁺ ) depend on the volume of atom M and the charge of M in the compound .

The basic property of sodium hydroxide compound

Sodium hydroxide is ionized as a base , where sodium atom has a big volume and its ion has only one positive charge , Accordingly the attraction between Na⁺ and O^{− −} decreases , The ( O − H ) bond is stronger than that of the ( Na − O ) bond , So , OH⁻ ion is produced .

NaOH ( sodium hydroxide ) ⇔ Na⁺ ( sodium ion ) + OH ⁻ ( hydroxide ion )

The hydroxy compounds of the nonmetallic elements as chlorine are ionized as an acid , Because the nonmetallic elements are characterized by the small atomic volume and the big charge which increase its attraction to O^{− −} ion and the ( Cl − O ) bond becomes stronger than the ( O − H ) bond , so , the positive hydrogen ion H⁺ is produced .

The strength of the oxygenated acids    

The oxygenated acids are represented by the following general formula : M O_n ( OH ) _m

Where : M is the central nonmetal atom , n is the number of nonbonded oxygen atoms with hydrogen , m is the number of bonded oxygen atoms with hydrogen .

The strength of the oxygenated acid increases as the number of nonbonded oxygen atoms ( O_n ) with hydrogen increases .

Summary of the elements graduation in the periodic table 

Trends and periodicity of properties in the periodic table :

Increasing the atomic number ( horizontal ) → in the periodic table  :

• Increasing : [ the ionization potential , the electronegativity , the nonmetallic property , The acidic property , The electron affinity ] .
• Decreasing : [ The atomic radius , The metallic property , The basic property ] .

Increasing the atomic number ( vertical ) , downwards ↓ in the periodic table :

• Increasing : [ The atomic radius , The metallic property ( in groups start by a metal ) , The acidic property ( in groups start by a nonmetal ) , The basic property ( in groups start by a metal ) ] .
• Decreasing : [ The ionization potential , The electronegativity , The electron affinity , The nonmetallic property ] .

Oxidation numbers

Oxidation number is a number that refers to the electric charge ( positive or negative ) that the atom or ion would have in the compound , whether it is an ionic or a covalent compound .

The advantage of using oxidation numbers is that they can help us in determining the type of chemical change occurring to an element during the chemical reaction .

The significance of oxidation numbers :

The significance of oxidation numbers differs in ionic compounds from that of the covalent compounds .

In ionic compounds : If the oxidation number is positive , it indicates that the number of electrons that the atom has lost to give positive ion ( cation ) , If the oxidation number is negative , it indicates that the number of electrons that the atom has gained to give negative ion ( anion ) .

In covalent compounds : If the oxidation number is positive , it indicates that the electronic shift in the chemical bond between the atoms is away from the less electronegative atom , If the oxidation number is negative , it indicates that the electronic shift in the chemical bond between the atoms is toward the most electronegative atom .

Rules for calculating the oxidation numbers

1. The oxidation number of the element atom in the molecule of similar atoms equals zero , whatever the multiplicity of the molecule atoms , because the electronic shift in the bonds between the atoms are equal .
2. The oxidation number for element ion equals the charge ( valence ) of the ion .
3. The oxidation number for polyatomic ions ( atomic groups ) equals the charge of group .
4. The oxidation number of any metal in Group 1A elements equals +1 , Group 2A elements equals +2 , Group 3A elements equals +3 .
5. The oxidation number for chlorine , bromine , iodine ( halogens ) in the most of their compounds equals ( −1 ) , however their other oxidation numbers can be calculated mathematically .
6. The oxidation number of oxygen in most of its compounds is −2 , while its oxidation number in : peroxides equals −1 , Superoxides equals −½ , Its compound with fluorine equals +2 .
7. The oxidation number of hydrogen in most of its compounds is +1 , except in binary compounds with active metals which are known as active metal hydrides , its oxidation number is −1 , Active metal hydrides are ionic compounds formed from the combination of an active metal with hydrogen in which hydrogen has an oxidation number −1 ( negative ion ) .
8. The algebraic sum of the oxidation numbers of the different atoms in the molecule equals zero , In sodium chloride molecule NaCl : The oxidation no. of Na ( + 1 ) + The oxidation no. of Cl ( −1 ) = zero .
9. The algebraic sum of the oxidation numbers of the atomic groups forming the molecule equals zero , In the molecule [ NH₄ ]⁺ [ NO₂ ]⁻ : The oxidation no. of ammonium group ( + 1 ) + The oxidation no. of nitrate ( −1 ) = zero .
10. The algebraic sum of the oxidation numbers of the different atoms in a polyatomic ion equals the charge of ion ( atomic group ) , In hydroxide group OH⁻ : The oxidation no. of oxygen ( −2 ) + The oxidation no. of hydrogen ( +1 ) = −1 .
11. Some elements especially the transition elements have several oxidation numbers which can be calculated by knowing the oxidation numbers of other known elements .

Hydrogen gas evolves at the anode ( positive ) in electrolysis of molten sodium hydride , while in electrolysis of acidified water , it evolves at the cathode ( negative ) , Because the oxidation number of hydrogen in NaH molecule equals −1 , while in H₂O molecule equals +1 .

How to assign the oxidation number of an unknown element in a given compound or atomic group 

Write the oxidation number for each known element above its atom symbol in the compound molecule or atomic group formula .

Multiply the oxidation number of each element by the number of its atoms in the molecule .

Assign the oxidation number of unknown element according to : The algebraic sum of the atoms of the different elements in the molecule equals zero , The algebraic sum of the atoms of the different elements in the atomic group equals the charge of the group .

Calculating the charge of the oxidation number in the oxidation reduction reaction ( redox reaction )

You already know the concepts of the oxidation and reduction , which are :

Oxidation is the process of losing electrons resulting in an increase of the positive charge ,  Reduction is the process of gaining electrons resulting in a decrease of the positive charge .

In the chemical reaction :

The metal loses one or more electrons , So , its oxidation number increases , It is oxidized ( oxidation process ) , The nonmetal gains one or more elements , So , its oxidation number decreases , It is reduced ( reduction process ) .

So , we can determine the changes happen to the elements during the oxidation-reduction reactions through the change in their oxidation no. before and after the reaction , where : the oxidizing agent is reduced , The reducing agent is oxidized .

Radius property , Ionization potential , Electron affinity & Electronegativity

Modern periodic table and classification of Elements

Metallic & nonmetallic property , Acidic & basic property in the periodic table

Chemical combination , Types of bonds ( Chemical bonds & Physical bonds )