Chemistry: Periodic Table and Periodicity in Properties: Electron Gain Enthalpy and Valence or Valency (For CBSE, ICSE, IAS, NET, NRA 2022)

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Electron Gain Enthalpy

The energy released for one mole of neutral atoms in a gaseous state on acceptance of electron by each atom.

Here X represents an atom,

  • Electron gain enthalpies becomes less in negative as we go down the group shows that the electropositive character of the atoms increases.
  • The electron gain enthalpy of chlorine is more in negative value as compared to that of fluorine. This is due to the small size of the F atom.


  • It is defined as the measure of the ability of an atom to attract the electron pair in a covalent bond to itself.
  • The electron pair of the covalent bond in each molecule experiences equal attraction by each atom in a homonuclear diatomic molecule such as hydrogen (H2) or fluorine (F2) .
  • Electro negativity of elements on Pauling scale
Electro Negativity of Elements on Pauling Scale
  • A large difference between electronegativities of the two atoms indicates highly ionic character of the bond between them, for example in
  • The zero difference in the electronegativities between the two atoms indicates that the percentage ionic character is zero. Therefore the molecule is purely covalent e. g. etc.
  • As we go down in any group the value of electronegativity decreases and increases from left to right in the period. Thus fluorine is the most electronegative and cesium is the least electronegative element.

Valence or Valency

  • The combining capacity of an atom in an element is determined by the number of valence electrons in the outermost or the valence shell. The electrons present in the outermost shell are called the valence electrons.
  • The valence of an element can be taken by the number of atoms of hydrogen with which one atom of the element can combine as in case of Univalent hydrogen atom. For example in and the valencies of oxygen, nitrogen and carbon are 2,3 and 4, respectively.
  • The main group elements can have a maximum of eight electrons in their valence shell known as Octet Rule. The valencies of the elements can be calculated from the electronic configuration by applying the octet rule.
  • If the number of valence electrons is four or less than the valency is equal to the number of the valence electrons.
  • In cases when the number of valence electrons is more than four then generally the valency is equal to 8 minus the number of valence electrons.

Valency = Number of valence electrons (for 4 or lesser valence electrons) .

Valency = 8 - Number of valence electrons (for more than 4 valence electrons) .

Electron Configurations and the Periodic Table

Electron Configurations and Periodic Table
  • An electron in a core outside the noble-gas or pseudo-noble-gas core is called a valence electron.
  • The electronic configuration of helium, neon, argon, and krypton
  • Helium
  • Neon
  • Argon
  • Krypton
  • The electrons are primarily involved in chemical reactions, and similarities among the configurations of valence electrons (the valence-shell configurations) account for similarities in the chemical properties among the groups of elements.

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