Number of EDTA molecules required to make an octahedral complex with Ca2+

To determine the number of EDTA (ethylenediaminetetraacetic acid) molecules required to form an octahedral complex with Ca2+, we need to understand the coordination chemistry of EDTA and the electronic configuration of calcium.

Coordination Chemistry of EDTA
EDTA is a hexadentate ligand, which means it can form six coordinate bonds with a metal ion. Each EDTA molecule contains four carboxylate groups and two amine groups, each of which can form a coordinate bond with a metal ion.

Electronic Configuration of Calcium
The electronic configuration of calcium is [Ar]4s2, which means it has two valence electrons available for bonding. In order to achieve a stable octahedral structure, calcium needs to form six coordinate bonds.

Explanation
When calcium forms an octahedral complex with EDTA, it will utilize all six of its coordination sites. Each EDTA molecule can donate two electron pairs, which can form two coordinate bonds with the calcium ion. Therefore, only one EDTA molecule is required to form an octahedral complex with Ca2+.

The coordination bonds between EDTA and calcium are formed through the lone pairs of electrons on the oxygen atoms of the carboxylate groups and the nitrogen atoms of the amine groups. These bonds are formed due to the donation of electron pairs from the EDTA ligand to the calcium ion, resulting in the formation of a stable octahedral complex.

In summary, the answer to the question is one EDTA molecule is required to make an octahedral complex with Ca2+. This is because EDTA is a hexadentate ligand, capable of forming six coordinate bonds, and calcium requires six coordinate bonds to achieve an octahedral structure.