MoleculeSketch 4+‎

MoleculeSketch is a simple and easy to use drawing App for chemical formulas and reactions.

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)