Video Transcript
Which of the following is a
property of alkali metals? (A) They have high melting
points. (B) They are bad conductors of
heat. (C) They are bad conductors of
electricity. (D) They are unreactive. Or (E) they are soft.
In this question, we want to know
which of the listed properties is descriptive of the alkali metals, which are the
group one metals. The group one or alkali metals
consist of lithium, sodium, potassium, rubidium, cesium, and francium. All group one elements have one
valence electron, which means one electron in their valence or outer electron shell,
although their inner electron configurations are different. The diagram shows the shells of
lithium and potassium as an example.
The valence electron from each atom
can separate from the atom, and we then say it is delocalized as it is no longer
bound to the atom. And when many atoms pack together,
a sea of delocalized electrons forms. These delocalized electrons can
move through the metal carrying electric current or thermal energy. And thus, the alkali metals, and
all metals in fact, are good conductors of both heat and electricity. For this reason, we can eliminate
answer (B), they are bad conductors of heat, and answer (C), they are bad conductors
of electricity.
All alkali metals are reactive, and
so alkali metals must be stored under oil to prevent them from reacting with oxygen
and water in the air. Lithium, with a small mass number,
is the least reactive, and cesium, with higher mass number, is extremely
reactive. There is a trend of increasing
reactivity down the group. Not much is known about francium,
but it is expected that francium is even more reactive than cesium. We can rule out answer option (D):
they are unreactive.
Physical properties are also
influenced by the mass of the elements. Going down the group, as the mass
of the atoms gets bigger, so the density increases. The group one metals all tend to be
rather soft compared to other metals on the periodic table. And there is also a trend in this
softness. Going down the group, the softness
increases. Statement (E), they are soft, is
correct.
Interestingly, their melting points
decrease as the atoms get heavier going down the group. But, in general, they all tend to
have lower melting points than many metals on the periodic table. Many metals have high melting
points, but not the alkali metals. And so we can eliminate answer
option (A): they have high melting points.
Finally, which of the following is
a property of alkali metals? The answer is (E): they are
soft.
