Trends in Chemical Properties of Periodic Elements

Ionisation Energy:

  1. It is the amount of energy required to remove an electron from the outermost orbit of an isolated gaseous atom. (If given in terms of the amount of work done in removing an electron, the property is called ionization potential.)
  2. Units: kJ / mol (for ionization potential, units: eV/atom.)
  3. This is denoted by word I.
  4. An atom can have as many ionization energies as number of electrons it has. These are denoted as I1, I2, I3,…, for 1st electron, 2nd electron, 3rd electron and so on respectively.
  5. I1< I2 < I3 due to increased effective nuclear charge after removal of one electron from the neutral atom
  6. Moving down a group the IE decreases as an extra shell is added and the outer electrons are shielded by the inner electrons.
  7. Moving from left to right in a period, the IE increases as the electrons enter the same shell and the increase in nuclear charge outweighs the shielding effect.


  1. I1 of boron (Z = 5) < I1 of beryllium (Z = 4) because in the same principal quantum level, an s-electron is attracted to the nucleus more than a p-electron.
  2. In beryllium, the electron removed during the ionization is an s-electron whereas the electron removed during ionization of boron is a p-electron. Hence the 2p electron of boron is more shielded from the nucleus by the inner core of electrons than the 2s electrons of beryllium.

Factors Affecting Ionisation Enthalpy:

  1. IE ∝ 1/ size of atom
    e., smaller the atom / ion, larger will be the ionization energy.
  2. Atoms and ions having inert gas configuration have high ionization energy
  3. The order of energy required to remove an electron from different subshells (if rest of the conditions are same) is s > p > d > f .
    The s-subshell is nearest to the nucleus while f-subshell is farthest from the nucleus

Important Points:

  1. The first ionization enthalpy of elements of group 12 (IIB) is higher than their immediate neighbors. This is because of the stable configuration, i.e., (n−1) d10 ns2of these elements.
  2. Alkali metals have the lowest and inert gases have the highest IE in a period.
  3. He has the highest and Cshas the lowest ionization energy.
  4. The IE of the metalloids generally falls between those of metals and non-meta
  5. IE ∝ 1/ metallic or electropositive character
  6. IE ∝ 1/ reactivity
  7. IE ∝ 1/ basic character of hydroxides
  8. IE ∝ 1/ reducing power

The reducing power increases on going down a group because the IP value decreases.

Li > Cs > Rb > K > Na

Ionisation energy

metallic or electropositive character reactivity basic character of hydroxides

reducing power


decreases decreases increases


Electron Gain Enthalpy:

  1. The energy change involved when an extra electron is added to an isolated gaseous atom is called the electron gain enthalpy Δeg
  2. Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form anion.          X (g) + e = X(g)
  3. The process of adding an electron to the atom can be either endothermic or exothermic.
  4. Example: For group 17 elements i.e., halogens Δeg H is negative as they can gain an electron and attain the stable noble gas configuration
  5. For group 18 elements i.e., noble gases, energy is required to add an electron as it has to enter the next higher principal quantum level leading to a very unstable electronic configuration.

Factors Affecting Electron Gain Enthalpy:

  1. Magnitude of Δ e g H ∝ Zeff
  2. Magnitude of Δ e g H ∝ 1/atomic size
  3. Magnitude of Δ e g H ∝ penetrating power

Therefore, the order of Δ e g H is s > p > d > f.

Electron Affinity:

  1. The negative of the enthalpy change is defined as the electron affinity (Ae) of the atom under consideration.
  2. If energy is released when an electron is added to an atom, the electron affinity is taken as positive.
  3. If energy has to be supplied to add an electron to an atom, then the electron affinity of the atom is assigned a negative sign.
  4. Electron affinity is defined as absolute zero and, therefore at any other temperature (T) heat capacities of the reactants and the products have to be taken into account in
    Δeg H= –Ae – 5/2 RT.
  5. Units: eV/atom or kJ/g-atom.
  6. Electron affinity and electron gain enthalpy have same magnitude but opposite sign.
  7. Electron affinities are defined at absolute zero while electron gain enthalpies at 298 K.
  8. Electron affinity describes the tightness with which an atom binds the extra electron added to it i.e., greater the value of Ae(energy liberated) or Δeg H, greater is the binding force.

Electronegativity and Electronegative Character:

  1.  The tendency of an atom to attract the shared pair of electrons towards itself in a covalent bond is called electronegativity of that atom.
  2. It is different from electronegative character of an atom which is the tendency of an element to form the anion by gaining the electron.
  3. The first term is comparative while the second term is used for a single atom.

Factors Affecting the Magnitude of Electro negativity:

  1. Electro negativity ∝1/ atomic radius
  2. Electron negativity α Effective nuclear charge (Zeff)
  3. Electro negativity ∝oxidation state
  4. Electro negativity ∝s – character
    The electro negativity values of C-atom in various hybridization states are as under s-character

Hybridization states

sp3 sp2 sp


25 % 33.33 %

50 %

Electro negativity 2.48 2.75


  1. Electro negativity ∝1/ metallic properties.