trinitrotoluene can you write NO2 in all three?

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    $\begingroup$ You have to show connectivity, so you always have to write your molecular formulae based on what's connecting to what. In the case of the nitro group on the 6th carbon in the benzene ring, you would have to write it like that since there's a bond between carbon and nitrogen, not carbon and oxygen. $\endgroup$ – user60221 Apr 16 '18 at 18:22

The Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008), include recommendations for contracted labels in structure diagrams. In particular

GR-2.3.1 General guidelines
Atom labels representing more than one non-hydrogen atom—also sometimes known as “contracted” labels—rely on the fact that many elements have consistent and well-understood bonding patterns. (…)


GR-2.3.3 Orientation of symbols within contracted labels
Contracted atom labels attached to only one bond should be read outwards from that bond, usually from left to right if the bond is on the left of the label. If the bond is instead attached to the right of the label, the label will normally be read from right to left, but ambiguities can result. Accordingly, contracted labels with a bond on the right should be avoided except for simple cases, usually limited to relatively small labels containing four or fewer combined element symbols and abbreviations. (…)


GR-2.3.4 Interpretation of contracted labels
In general, contracted labels are interpreted to fill as many valences as possible, as quickly as possible. (…)

Contracted labels for nitro groups cannot be interpreted with a simple application of valence rules, but also need some implicit charges to be added. It is equally possible to avoid entirely the issue of depiction and instead display the functional group using the $\ce{NO2}$ abbreviation. According to Table III in the Graphical Representation Standards, the left-to-right form of the label is $\ce{-NO2}$ and the corresponding right-to-left form is $\ce{O2N-{}}$.


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