MoleculeSketch iP 4+‎

MoleculeSketch is a simple and easy to use drawing App for chemical formulas and reactions.
MoleculeSketch iOS is the standalone version of MoleculeSketch for iPad.
The free version offers basic functionality. The full version can be unlocked with an in-app purchase.

It has following features:
+ add atoms and bonds nicely by position locking (can be disabled)
+ choose different bond types (double, stereochemical, delocalised, etc.)
+ add bonds with higher hapticity
+ type functional groups by double-clicking on symbol
+ show missing hydrogen atoms depending on free valences
+ "grow" and remove hydrogen atoms (*)
+ show non bonding electron pairs on heteroatoms
+ add reaction arrows and text fields
+ access drawing attributes from popover panels
+ calculate average mass and formula of molecules (*)
+ group and arrange objects (*)
+ change background color of drawing area and add a grid
+ copy/paste/cut molecules and drawings
+ paste drawings into other applications
+ import from mol/sdf files (*)
+ use gestures for zooming and rotation
+ import structures from mol- or sdf-files (*)

(*) some of these features and others are only available when using the full version (inApp purchase)