Final answer:
Lone pairs on atoms like chlorine are often omitted from Lewis structures to simplify the molecule's representation, especially when they do not contribute to its central reactivity or when illustrating a molecule for clarity without needing every detail.
So option (1) is the correct answer.
Step-by-step explanation:
When drawing Lewis structures, lone pairs on atoms like chlorine (Cl) are sometimes omitted intentionally to simplify the representation of the molecule. In molecular structures, elements such as oxygen (O) are more commonly seen forming double bonds than chlorine. For this reason, when a Lewis structure has an insufficient number of electrons around a central atom like nitrogen (N), it is reasonable to use a lone pair of electrons from an oxygen atom to create a bonding pair, resulting in a double bond with nitrogen. This decision is based on the chemical tendencies and the most stable configuration for the molecule. For example, in a molecule such as ClNO, selecting a lone pair from O to bond with N would yield the structure Ö=N—Cl.
Lone pairs are often essential for understanding the three-dimensional shape of a molecule, which influences its reactivity and physical properties. However, when illustrating molecules for clarity, these electron pairs may be left out of less electronegative elements like Cl, especially when such details are not crucial for the discussion at hand. The presence of lone pairs on terminal atoms like Cl could be understood without always being explicitly drawn in every diagram.