Why is atomic radius of hydrogen smaller than of helium?
The radius of Helium is not more than of Hydrogen. On the contrary it is approximately 31 pm. and the radius of hydrogen is greater. Although Helium has two electrons, both of them are in the same orbital as the one electron of hydrogen i.e. the 1s orbital. So the radius does not increase due to electrostatic repulsion. The nucleus has two protons which considerably increases the force of attraction. Thus the overall radius reduces.
Atomic Size (Atomic Radius) The atomic size of an atom, also called the atomic radius, refers to the distance between an atom’s nucleus and its valence electrons. Remember, the closer an electron is to the nucleus, the lower its energy and the more tightly it is held. Moving Across a Period Moving from left to right across a period, the atomic radius decreases. The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons. True, electrons are also added as the elements move from left to right across a period, but these electrons reside in the same energy shell and do not offer increased shielding. Moving Down a Group The atomic radius increases moving down a group. Once again protons are added moving down a group, but so are new energy shells of electrons. The new energy shells provide shielding, allowing the valence electrons to experience only a minimal amount of the
