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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!

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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.

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    $\begingroup$ This answer has the right idea. This simple concept has many interesting and far-reaching implications across the periodic table. A deeper explanation of this effect and many of its consequences can be found in the article "The Role of Radial Nodes of Atomic Orbitals for Chemical Bonding and the Periodic Table". I've made a brief overview of it here. $\endgroup$
    – Nicolau Saker Neto
    Apr 25, 2015 at 15:51
  • $\begingroup$ @NicolauSakerNeto Indeed, the $4d$ transition metals are rather similar to the $5d$, but considerably different from the $3d$; and there are stark differences between the lanthanides ($4f$) and actinides ($5f$). Thank you for the reference, I will be reading it promptly! $\endgroup$
    – hBy2Py
    Sep 9, 2015 at 1:48

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