The increasing order of ionic character is:

Concept:​

Fajan's Rule:

• Coulombic attraction between the cations and anions in certain cases leads to the deformation of the ions.
• This deformation caused by one molecule to the other is called polarisation.
• The extent to which the molecule is able to polarise the other is called its polarisation power.
• The extent to which a molecule can get polarised is called its polarisability.
• The rise in deformity of ions may give rise to increased electron density between the ions and this leads to a considerable amount of covalent bonding.

Explanation:

Factors affecting the Covalent / Ionic character of bonds:

• The small size of cation:
  • The smaller the size of the cation, the greater is its covalent character.
• The larger size of the anion: 
  • The larger the size of the anions, the less tightly their electrons will be held by the nucleus and it will be polarised more easily and will thus have more covalent character.
• Larger charge on either of the two ions:
  • With the increase in charge on the ions, electrostatic attraction of the cation for the outer electrons of the anion also increases.
  • Consequently, the covalent character of the bond increases.
  • For example, the covalent character follows the order:  AlCl₃ > MgCl₂ > NaCl.
• ​Electronic configuration of the ions:
  • Out of two ions having the same size and charge, the ion with a pseudo noble gas configuration will have higher polarising power and more covalent character than a cation with a noble gas configuration (i.e., ion having 8 electrons in the outermost shell).
• Polarisability decreases along with a period and increases along with a group.
• Low positive charge and large size of cation and a small charge on anion and small size of anion favor the formation of ionic compounds.
• A high positive charge and small size of cation and a high charge on anion and large size of anion favor the formation of covalent compounds.
• Among the given compounds BeCl2, BaCl2, MgCl2, CaCl2, the cations have the same charge +2, and anions have a net charge of -2.
• All the cations Be⁺², Ba⁺², Mg⁺², Ca⁺² belong to the same group II of the periodic table.
• As we move from Be, Mg, Ca, to Ba down the group, the polarisability decreases as well as the covalent character. Hence, the ionic character increases down the group.
• The most ionic being BaCl₂ and the least ionic being BeCl₂.

Hence, the correct order of ionic character is: BeCl2 < MgCl2 < CaCl2 < BaCl2.