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In chemistry, a donor atom is an atom in a molecule that can donate a pair of electrons to form a coordinate covalent bond with another atom or ion. Donor atoms are usually found in groups 15 (pnictogens) and 16 (chalcogens) of the periodic table, such as phosphorus, nitrogen, arsenic, and antimony. These atoms have lone pairs of electrons that can participate in bonding with electron-deficient species.

Among the given options:

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  Phosphorus (P) has a lone pair of electrons and can act as a donor atom in compounds like phosphines (PH₃).

  
  


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  Antimony (Sb) also has lone pairs and behaves similarly to phosphorus in coordination compounds.

  
  


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  Arsenic (As) is a known donor atom in compounds like arsines (AsH₃).

  
  

Aluminum (Al), however, is electron-deficient and does not have a lone pair of electrons to donate. Instead, it often acts as a Lewis acid, accepting electron pairs from donor atoms. For example, in aluminum chloride (AlCl₃), aluminum accepts electron pairs from chloride ions or solvents to complete its octet. This makes aluminum not a donor atom, unlike the other elements in the list.

Understanding donor atoms is essential in coordination chemistry, inorganic chemistry, and organometallic chemistry. It helps explain the formation of complexes, reaction mechanisms, and the behavior of Lewis acids and bases. Recognizing which elements can donate electrons also clarifies bonding patterns and the stability of molecules in chemical reactions.

Thus, the correct answer is Aluminum, as it cannot donate electrons and instead typically accepts them in chemical reactions.