Why is arsenic more chemically similar to phosphorus than is nitrogen? Why is arsenic more chemically similar to phosphorus than is nitrogen (to phosphorus)?

I thought that it may be because both phosphorus and arsenic have d orbitals (albeit one has one that is filled and the other has an unfilled one).

Another possibility I thought was that because nitrogen held the electrons very closely, and thus the difference between the second and third period may be more pronounced than the difference between the third and fourth periods.

I'd really appreciate it if anyone could help clarify/ explain. Thanks!
Every element which is in the first place of the group, is pretty much different from the other elements of the same group. The reason is because the first element has ALWAYS the smallest radius, which leads to some changings also in group properties. Due to the small radius they are more stable than the others and they require more energy for any specific reaction. With some research you can see that also other  first elements of other groups such as Li,O,C etc.  differ from the other elements of that specific group.