Respuesta :

Lets look at EC of each element one by one

#Argon

  • Z=18

Electronic Configuration given by

[tex]\\ \sf\longmapsto 1s^22s^22p^63s^23p^6[/tex]

  • Full filled orbital

#Hydrogen

  • Z=1

[tex]\\ \sf\longmapsto 1s^1[/tex]

  • it is half filled.

#Berylium

  • Z=4

[tex]\\ \sf\longmapsto 1s^22s^2[/tex]

  • Full filled orbital.

#Helium

  • Z=2

[tex]\\ \sf\longmapsto 2s^2[/tex]

  • Full filled

Hence our answer is hydrogen.