The Lewis structure for an atom of
helium is shown below. Which of these statements explains
why helium does not usually form chemical bonds? (A) Helium is too small to form
chemical bonds with atoms of other elements. (B) The first electron shell can
only hold two electrons; therefore, helium has a full valence shell. (C) Helium changes into hydrogen
when it loses one of its electrons. (D) The first electron shell can
hold eight electrons, but finding six electrons for a full valence shell is too
difficult. (E) Helium is a gas and therefore
cannot lose its two electrons.
The question asks us about
helium. Helium is one of, perhaps the most,
unreactive element on the periodic table. It is very small and lightweight
and is known as one of the noble gases. An atom of helium has two protons
in its nucleus and two electrons in its first electron shell.
Because the first electron shell
can hold a maximum of two electrons, we say helium’s valence shell is full. And this is why helium is so
unreactive. And it is why helium does not
usually form chemical bonds with atoms of other elements. We say “usually” because under
extreme conditions, helium may be induced to react, for example, in stars or in
special conditions in the laboratory. But in general, helium is
considered unreactive, again because it has a full valence shell.
Helium exhibits no need to accept
or donate electrons to other atoms in order to form a full valence shell. We could define this
relationship. A full valence shell makes an atom
unreactive. Other elements which do react do so
to obtain a full valence shell and therefore to become stable, just like the noble
gases. From this understanding, we can
deduce that the correct answer to this question is (B).
Why does helium not usually form
chemical bonds? The answer is (B). The first electron shell can only
hold two electrons; therefore, helium has a full valence shell.