Arsenic has 33 electrons, arranged 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3. Written on the previous noble gas that is [Ar] 3d10 4s2 4p3, and the shells hold 2, 8, 18, 5 electrons from the innermost outwards.
Hund's rule leaves 3 unpaired electrons in the ground state, which is what makes the free atom paramagnetic.
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