Video Transcript
The following figure shows
information for some period two elements. Which of these elements is expected
to have the highest melting point? (A) Lithium, (B) beryllium, (C)
oxygen, (D) fluorine, (E) neon.
In the case of periods two and
three, there are some interesting trends in melting point as we move across each
period. As we go across periods two and
three from left to right, melting point first increases and then decreases. For the metals in each period, the
melting point increases as we go left to right. This is because as we move across
the period, we are adding more valence electrons to the element.
For metals, this increases the
total number of delocalized electrons, which increases the strength of the metallic
bonds that hold the metal together. With stronger interacting forces,
the melting point increases. Once we reach the nonmetal elements
in each period, melting point drops significantly.
Nonmetal elements are not held
together by metallic bonds. In the case of oxygen and fluorine,
their molecules are held together by weak dispersion forces. In the case of neon, the atoms are
held together by weak dispersion forces. The strength of these dispersion
forces is not influenced greatly by the addition of one or two extra electrons.
So in periods two and three, the
highest melting point elements tend to be the metal elements with the most
delocalized electrons. For the elements given, the metal
element that has the most delocalized electrons is beryllium.
So the correct answer is (B). Beryllium is the correct
answer.
