Chapter 38, Problem 18A

To determine

The reason for the small size of the Helium than the Hydrogen.

Explanation of Solution

Introduction:

The size of an atom depends on the electronic configuration of the atom. The distance of the electron in the outermost shell decides the size of the atom.

The atomic number of the Hydrogen is one, so it has one electron in the shell and one proton in the nucleus. The atomic number of the Helium is two and it has two electrons in the outer shell and one proton in the nucleus.

The attraction of the nucleus on the one electron of the Hydrogen atom is less than the attraction of the nucleus on the two electrons of Helium atom. The distance of the outer shell electrons from nucleus is less for the Helium atom than the for the Hydrogen atom due to strong attraction.

Conclusion:

The size of the Helium atom is less than the Hydrogen due to strong attractive force in the Helium atom than the Hydrogen atom.