The below table compares the number of electrons in two neutral atoms. Atom
Number of Electrons
Ne 10
Na 11
Compare the electron configuration and atomic radius of these two atoms and use that to explain the difference in their reactivity.

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The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.