is sodium atom greater in size than helium or not? why?
Yes, a sodium atom is greater in size than a helium atom. The atomic radius of sodium is 180 pm, while the atomic radius of helium is 30 pm. Generally, those atoms have greater size, which have more electron shells and/or in which the electrons of outermost shells are attracted weakly to the nucleus. In given case, sodium has three electron shells, while helium has only one. However, it is not the main cause of greater size of sodium atom compared to helium. Even when comparing sodium and argon (the inert gas with the same number of electron shells) the atomic radius is greater for sodium. The way the atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity. The shells are generally filled in order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus. The greater charge results in the greater attraction force. As the atomic number increases along each row of the periodic table (e.g. from Na to Cs), the additional electrons come into the same outermost shell. As the result, the atomic size gradually lowers within a period due to the nuclear charge increase. The outermost electron of sodium is attracted weaker than two electrons of helium resulting in greater atomic radius of sodium compared to helium.