Final answer:
The carbon atom often serves as the central atom in Lewis structures due to its lower electronegativity compared to oxygen. In organic compounds, for aldehydes and ketones, the carbonyl carbon is designated as C1. Oxidation or reduction of carbon atoms is determined by changes in their bonding with oxygen and hydrogen.
Step-by-step explanation:
The statement Because carbon is less electronegative than oxygen and hydrogen is normally terminal, C must be the central atom pertains to the rules for drawing Lewis structures for molecules. In such structures, the central atom is typically the one that is less electronegative, as it is more likely to share electrons, making carbon (C) a common central atom when present.
In the case where one bond of the carbonyl group is made to an H atom, the molecule is identified as an aldehyde. To denote the position of a substituent on an aldehyde, the carbonyl carbon atom is always designated as C1, meaning it isn't necessary to number this group. Similarly, for ketones, numbering should provide the carbonyl carbon atom with the lowest possible number, with the understanding that in cyclic ketones, the carbonyl carbon is also C1.
When examining molecules to determine oxidation or reduction changes, such as comparing the marked carbon atoms in different molecules to ethanol, it involves looking at the changes in bond relationships, particularly with oxygen and hydrogen, to determine if the oxidation state of carbon has increased (oxidation) or decreased (reduction).