Question

Why is the helium atom smaller than the hydrogen atom? Why is the hydride anion $H^{-}$ , larger than both atoms?

Answer 1

Hint: To answer the question we need to know the concept of atomic and ionic radius. Atomic radius is used to measure the size of an atom. It is the distance between the atomic nucleus and the outermost valence shell electron. For a neutral gaseous atom, atomic and ionic radii are the same. However when an atom gains or loses electrons it forms anions and cations respectively and their radius is called the ionic radius.

Complete answer:
Before answering the question let us discuss the basic trends of the atomic radii-
(A)Atomic radius decreases from left to right in a period of the periodic table. The reason is as we move from left to right the number of inner shell electrons increases and the nucleus attracts electrons more strongly and thus it’s effective nuclear charge increases which results in the decrease of atomic radius.
(B)Atomic radius increases from top to bottom in a group of the periodic table. The reason is as we move from top to bottom the number of atomic shells increases and thus the nucleus attracts the electron less so the effective nuclear charge decreases and thus increases atomic radius.
In the question, we are asked why helium is smaller than the hydrogen atom.
Reason-Hydrogen has 1 electron in its atomic shell while Helium has 2 electrons in its atomic shell. Helium belongs to the right in the periodic table and as we move from hydrogen to helium number of inner shell electrons increases and effective nuclear charge increases and thus its size is smaller than the hydrogen atom. Helium is the smallest element in the periodic table. Its size is 31pm while the size of hydrogen is 53pm.
In another part of the question, we are asked why the hydride ion is larger than both hydrogen and helium atoms.
Reason-Hydride $\left(H^{-}\right)$ is an anion of the hydrogen atom. It is formed when a hydrogen atom gains an electron. The number of electrons increases but the number of protons remains the same. Thus the attractive force decreases and the size increases.
The overall order of size of all these 3 species is as follows-
 $H^{-}>H>He$ .

Note:
Apart from nuclear charge and number of shells there is another factor that decides the size of an atom which is the shielding effect. As the number of shells increases, the attraction between the nucleus and the outermost electron decreases or we can say the inner electrons shield the nucleus from valence electrons. So overall attractive force decreases and size increases. So greater the shielding effect , greater will be the size of an atom.