The (s) in the chemical name of (s)-3-iodo-2-methylnonane indicates an S-configuration using the Cahn-Ingold-Prelog system of stereochemical nomenclature. The S-configuration means that an "imaginary" rotation from the highest priority substituent group to the lowest priority substituent group of the chiral center moves counterclockwise (to the left), provided that the lowest priority group is oriented "towards the back" (symbolized by dashed lines).
The highest priority group (iodine in this case) is the one with the highest atomic number and the lowest priority (hydrogen in this case) is one with the lowest atomic number.
If the atoms directly beside the chiral center have the same atomic number (Carbon-2 and Carbon-4 in this case), the atoms next to them will be evaluated until a point of difference is found. Carbon-2 is connected to 2 other carbon atoms and 1 hydrogen atom, while Carbon-4 is connected to only 1 carbon atom and 2 hydrogen atoms. Thus, Carbon-2 has a higher priority, with the point of difference being the carbon atom of the methyl group attached to Carbon-2. Both Carbon-2 and Carbon-4 are connected to one carbon atom from the main nonane chain, but the other atoms connected to Carbon-4 are hydrogen atoms only. Carbon-2 has an extra carbon connected to it and carbon has a higher atomic number than hydrogen.
If there is no point of difference, the central atom is not chiral and cannot be named using the Cahn-Ingold-Prelog system.
Thus, the structure of (s)-3-iodo-2-methylnonane is
