This is known as electro chemical series or E.C.S. Few important facts about the electrochemical series are summarized as : 

(i) Metals above hydrogen in ECS undergoes oxidation in the comparison cell 

i.e. they are anodes. Metals placed below hydrogen undergoes reduction i.e. they are cathodes. 

(ii) Whether or not the electrode reaction will occur spontaneously when electrode is connected to hydrogen electrode can be inferred from the sign of electrode potential. If it is positive, the reaction will occur as written and the electrode will act as anode. If the sign is negative, the reverse reaction will occur and it will be cathode and hydrogen electrode as anode. 

(iii) In the ECS, there is decreasing tendency from top to bottom to lose electrons (undergo oxidation) and increasing tendency to gain electrons (undergo reduction). In other words, the reducing strength decreases and the oxidizing strength increases from top to bottom. Thus lithium is the strongest reducing agent and F, the strongest oxidizing agent. 

(iv) In E.C.S, the metals are placed in the order of reactivity and shows the displacement order. Metals displace metals lying below them in the list from solution of their salts. Ali metals above hydrogen have negative electrode potentials. 

1.7 OXIDATION NUMBER (0.N.) Definition ; 

“The formal charge (i.e. not real) on the atom in the compound or ion under consideration is known as oxidation number or state.” 

In defining the oxidation number, the term ‘formal charge’ is used which means it is not the actual charge on the atom ina molecule orion. Whileusingthe . 

concept of O.N., ali the compounds (including all the covalent compounds) are imagined to be completely ionic, although no compound, is completely ionic. In covalent compounds, therefore, the bonding electrons (that are present in a bond) are considered to be “owned” by the more electronegative atom e.g. in H:0:H, oxygen is more electronegative than hydrogen, therefore two electrons, one from each hydrogen, are considered to be owned by oxygen giving rise to two negative charges on oxygen. Therefore, oxidation number of oxygen in water would be 2Each hydrogen is considered to have lost one electron giving rise to one positive charpe on each hydrogen. Therefore, oxidation number of hydrogen would be 1”. 

Guiding rules that have been established to determine the oxidation number