Final answer:
In Me3P, the phosphorus atom (P) is the nucleophilic atom as it can donate lone pairs of electrons to an electrophile.
Step-by-step explanation:
The nucleophilic atom in the molecule Me3P is phosphorus(P). Phosphorus in this molecule has lone pairs of electrons that can be donated to an electrophile, making it the nucleophile. The 'Me' represents methyl groups which are not nucleophilic in this context.
Hydrogen (H) is generally not a nucleophile unless it is in the form of a hydride ion (H-). Hence, in the given compound, Me3P, only the phosphorus atom acts as the nucleophile.