Ethylenediaminetetraacetic acid (EDTA) is a hexadentate ligand, meaning it has six donor atoms that can coordinate to a central metal ion. Its correct structure is derived from its name and its common function as a chelating agent.

Step-by-Step Explanation:

Step 1: Break Down the Name

The name provides the molecular blueprint:

Ethylene: Indicates a two-carbon chain, -CH₂-CH₂-.
Diamine: Means two amine (-NH₂) groups are attached to this ethylene chain. This forms the central core: H₂N-CH₂-CH₂-NH₂.
Tetraacetic acid: Means four acetic acid groups (-CH₂COOH) are attached to this diamine core.

Step 2: Determine the Attachment Points

The four acetic acid groups are attached to the six donor atoms: the two nitrogen atoms of the amines and the four oxygen atoms of the carboxylate groups. A correct structure must have the four -CH₂COOH groups bonded to the two nitrogen atoms.

Step 3: Analyze the Given Options

We must identify the structure where the central ethylene diamine backbone (H₂N-CH₂-CH₂-NH₂) has each nitrogen atom bonded to two acetic acid groups (-CH₂COOH). The correct structure is represented by the third image, which shows this arrangement clearly.

Final Answer: The correct structure is the third option.

Related Topics:

Coordination Chemistry: The study of compounds formed between metal ions and surrounding molecules or ions (ligands).
Chelation: A process where a multidentate ligand bonds to a metal ion using more than one donor atom, forming a very stable, ring-like structure called a chelate complex.
Ligands: Ions or molecules that donate a pair of electrons to a metal atom or ion to form a coordinate covalent bond. They are classified by their denticity (number of donor atoms).

Key Formulae and Theory:

The general structural formula for EDTA is often written as:

(HOOC - {CH}_{2}) 2_{} N - {CH}_{2} - {CH}_{2} - N ({CH}_{2} - COOH) 2_{}

Key Theory: EDTA is a classic example of a hexadentate ligand. Its two nitrogen and four oxygen atoms can simultaneously coordinate to a metal ion, wrapping around it and forming an extremely stable complex. This property makes it invaluable in analytical chemistry for titrations (complexometric titrations), in medicine as an antidote for metal poisoning, and in industry to sequester metal ions.