(a) First, we perform non-inclusive steric number calculation and see that the atom of interest (oxygen) is sp 2 -hybridized (2 sigma bonds + 1 localized lone pair = 3, sp 2 ). The delocalized lone pair counts towards the number of pi electrons, and since 8 pi electrons (6 from the pi bonds and 2 from the delocalized lone pair on the oxygen) corresponds to a Huckel number the non-inclusive approach makes the compound antiaromatic. Next, we perform the inclusive steric number calculation which makes the oxygen sp -hybridized (2 sigma bonds + 2 lone pairs = 4, sp ) thus breaking the conjugated pi system and making the compound nonaromatic, which is more stable than an antiaromatic configuration. Therefore, we agree with the inclusive approach and consider the compound as nonaromatic. (b) Similar to the oxygen in compound (a), the non-inclusive approach makes the nitrogen sp 2 -hybridized (3 sigma bonds + 0 localized lone pairs = 3, sp 2 ) and the compound antiaromatic (8 pi electrons). The inclusive approach considers the nitrogen as an sp -hybridized (3 sigma bonds + 1 lone pair = 4, sp ) atom and, subsequently, the compound as nonaromatic. Since the nonaromatic configuration is more stable, we agree with the inclusive approach and consider the compound as nonaromatic. (c) The non-inclusive approach makes the sulfur and nitrogen atoms sp 2 -hybridized (2 sigma bonds + 1 localized lone pair = 3, sp 2 ). It is important to note that the nitrogen is sp 2 -hybridized in both the non-inclusive and inclusive approach (Non-inclusive; 2 sigma bonds + 1 localized lone pair = 3, sp . Inclusive; 2 sigma bonds + 1 lone pair = 3, sp 2 ). This is common among atoms involved in alkenes like the nitrogen in compound (c). Further evaluation of the compound under the non-inclusive approach makes the compound have 6 pi electrons since localized lone pairs only count towards the hybridization state of the atom of interest (nitrogen in this case) and not the Huckel number of pi electrons. 6 is a Huckel number which leads to the conclusion that the compound is aromatic, and since aromatic is the most stable configuration a compound can adopt and has already been determined through the non-inclusive approach, there is no need for evaluation under the inclusive method. (d) Each of the nitrogens are sp 2 -hybridized in both the non-inclusive and inclusive approach (Non-inclusive; 2 sigma bonds + 1 localized lone pair = 3, sp 2 . Inclusive; 2 sigma bonds + 1 lone pair = 3, sp 2 ). The lone pairs on the nitrogens are localized so they do not count towards the Huckel number of pi electrons in the compound, which makes the compound have 6 pi electrons (Huckel number) and thus aromatic.