Sodium atom has atomic number 11. Its electronic configuration is 2, 8, 1. It has one electron in its outermost shell. To achieve the noble gas configuration, sodium atom has to lose one electron. After losing one electron sodium atom changes into sodium ion with +1 charge. The electronic configuration of the sodium ion will be 2 and 8 which is the noble gas configuration of Neon, Neon being a noble gas. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.