Is ammonia a strong or weak ligand?

Hint: Ligand is a group or molecule capable of donating electrons to the central metal atom in coordination compounds. Ligands are neutral or negative charged species. In the presence of weak ligand, no pairing up of electrons takes place, whereas in presence of strong ligand, the electrons pair up takes place.

Complete answer:
Ammonia is a molecule consisting of one nitrogen atom and three hydrogen atoms.
The nitrogen atom in ammonia has one lone pair of electrons.
Ammonia can donate these lone pairs of electrons to the central metal atom, and thus can act as a ligand.
It is a neutral charged ligand. The charge on ammonia ligands is zero.
The series explained how the strength of ligands was discovered in \[1938\] .
They named it a spectrochemical series.
According to this series, the ligands were placed in the order of their capability of donating electrons.
\[{I^ - } < B{r^ - } < C{l^ - } < {F^ - } < ON{O^ - } < O{H^ - } < N{H_3} < N{O_2}^ - < PP{h_3} < C{N^ - } \approx CO\]
Ammonia is placed in the middle of the spectrochemical series.
It is a weak ligand but in presence of octahedral complexes containing a central metal atom belonging to \[3d\] series has a coordination number of \[3\] , ammonia acts as a strong ligand.
For example, in the complex \[{\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}\] , Cobalt belongs to \[3d\] series and has the oxidation number \[3\] , in this case ammonia acts as a strong ligand.

Note:
Thus, according to spectrochemical series, we can understand that ammonia is a weak ligand but, in some cases, it acts as a strong ligand. It is a neutrally charged ligand with a lone pair of electrons on nitrogen atoms.