Argon has 18 electrons, arranged 1s2 2s2 2p6 3s2 3p6. Written on the previous noble gas that is [Ne] 3s2 3p6, and the shells hold 2, 8, 8 electrons from the innermost outwards.
Every subshell is either full or empty, so the ground-state atom has no unpaired electron and is diamagnetic.
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