Classification of elements and periodicity in properties of element II

   
 IONISATION ENTHALPY:

•  The ionisation enthalpy is the molar enthalpy change accompanying the removal of an electron from a gaseous phase atom or ion in its ground state. Thus enthalpy change for the reaction; M_(g)→ M⁺_(g) + e^-

•   Is the ionisation enthalpy of the element M. Like ionisation energies for successive ionisation, the successive ionisation enthalpy may also be termed as 2^nd ionisation enthalpy (∆_rH₂), third ionisation enthalpy (∆_rH₃) etc. The term ionisation enthalpy is taken for the first ionisation enthalpy, (∆_rH₁) is   expressed in kg mol^- or in eV.

      Periodicity:

i)    Generally the ionisation enthalpies follow the order ( there are few exceptions):

(∆_rH₁)               <            (∆_rH₂) <          (∆_rH₃)

ii)    The ionisation enthalpy decreases on moving top to bottom in a group.

iii)  The ionisation enthalpy increases on moving from left to right in a period.

     ELECTRON GAIN ENTHALPY: 

• The electron gain enthalpy ((∆_egH) is the molar enthalpy change when an isolated gaseous atom or ion in its ground state adds an electron to form the corresponding anion thus the enthalpy change for the reaction; X_(g) + e^- → X^-_(g)

• Is called the electron gain enthalpy (∆_eg H) of the element X. The∆_eg H may be positive or negative.

•  The successive values for the addition of second, third etc. Electron, these are called second, third    etc. electron gain enthalpies. For example

X(g)	+	e-	→	X-(g)	∆H= ∆eg H1	is called first electron gain
enthalpy
X-(g)	+	e-	→	X2-(g)	∆H= ∆eg H2	is called second electron gain
enthalpy
X2-(g)	+	e-	→	X3-(g)	∆H= ∆eg H3	is called third electron gain

enthalpy

      Usually the term electron gain enthalpy (∆_egH) means the first electron gain enthalpy.

      Periodicity:

(i)     In period- The electron gain enthalpy increases from left to right in a period.

(ii)   In group- The electron gain enthalpy decreases from top to bottom in a group.

ELECTRONEGATIVITY: 

“The relative tendency of an atom in a molecule to attract the shared pair 
   of electrons towards itself is termed as its electro-negativity.”

     Periodicity:

(i) In period- The electro-negativity increases from left to right in a period.

(ii) In group- The electro-negativity decreases from top to bottom in a group.

VALENCE ELECTRONS: 

 The electrons present in outermost shell are called as valence electron. 

 Because the electrons in the outermost shell determine the valency of an element.

 VALENCY OF AN ELEMENT: The number of hydrogen or halogen atom or double the        number of oxygen atom, which combin with one atom of the element is taken as its valency.  According to the electronic concept of valency, “ the number of electrons which an atom loses or    gains or shares with other atom to attain the noble gas configuration is termed as its valency.”

   Periodicity:

    (i) In period- The valency first increases then decreases from left to right in a period.
    (ii) In group- The valency remains constant from top to bottom in a group.

 ELECTROPOSITIVE OR METALLIC CHARACTER: The tendency of an element to lose    electrons and forms positive ions (cations) is called electropositive or metallic character. The    elements having lower ionisation energies have higher tendency to lose electrons, thus they are  electropositive or metallic in their behaviour.

    Alkali metals are the most highly electropositive elements.

    

    Periodicity: 

• In period- The electropositive or metallic characters decreases from left to right in a period.
• In group- The electropositive or metallic characters increases from top to bottom in a group

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