## Builder.add_hydrogens()


Fill atoms up to their default valence (see [VALENCES](VALENCES.md#tetramol.VALENCES)) with hydrogens.


Usage

``` python
Builder.add_hydrogens(
    keys=None,
    *,
    avoid=None,
)
```


## Parameters


`keys: Collection[int] | None = None`  
Atoms to fill. Defaults to all heavy atoms.

`avoid: ArrayLike | None = None`  
When an atom needs fewer hydrogens than it has empty slots (e.g. an amine nitrogen), leave the slots closest to this direction empty. By default, the last empty slots are left.


## Returns


`list[int]`  
Indices of the new hydrogens.