Of elements n, o, cl, na, and which two would likely have similar chemical properties and why

Oxygen and Nitrogen would be the most similar of the elements listed, because they are the closest in the periodic table. This isn't a very good reason for anything, but the two do have some similar properties. They are both non-metals, they are both highly electronegative, they are both diatmoic gasses in their natural states, they have a similar number of valence electrons, they are both generally oxidizing agents. Oxygen and Chlorine are also quite similar, but not quite as similar as Oxygen and Nitrogen.

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.