Video Transcript
The image below shows the elements
in the alkali metal group. Which element is missing from the
group? (A) Magnesium, (B) hydrogen, (C)
calcium, (D) potassium, (E) aluminum.
We are given an image showing the
elements called the alkali metals. This group of elements is just one
column on the periodic table. Group one, which is column one of
the periodic table, is called the alkali metals. However, the alkali metals do not
include the top element of group one. This element is a nonmetal, not a
metal. This element is hydrogen, whose
chemical symbol is capital H.
The remaining five elements in
group one are the alkali metals. These elements are lithium,
chemical symbol capital L lowercase i. Then we have sodium, symbol Na,
then potassium, symbol K. Then we have Rb, which is the
element rubidium. And then we have Cs, which is
cesium. There is another element below
cesium in group one. It is called francium. Not much is known about francium
since it is so rare, but it is expected to behave like an alkali metal. Francium has the chemical symbol
Fr. We won’t consider francium in this
question.
These metals are listed in the same
group since they share many similar chemical as well as physical properties. All alkali metals tend to react in
the same way. For example, all alkali metals
react with water to form a metal hydroxide compound and hydrogen gas. A metal hydroxide is a metal
chemically bonded to oxygen and hydrogen. Another way that alkali metals are
similar is that they all have only one electron in their outer electron shell. This picture shows the electron
shell diagrams of lithium and sodium as examples. Notice they have very different
numbers of electrons. But both only have one valence
electron, or outer electron. These elements are also all quite
soft compared to other metals on the periodic table. And all tend to have low densities
compared to other metals.
We have already seen that the
missing element is potassium, symbol K. We know that hydrogen is not a
metal, but a nonmetal, so answer (B) is incorrect. Magnesium, symbol Mg, is a metal,
and so is calcium, symbol Ca. But they are not alkali metals. Magnesium and calcium are found in
group two of the periodic table. This group is called the alkaline
earth metals. And they all have similar
properties and so are placed in the same group. We can confirm that answers (A) and
(C) are incorrect. And aluminum, symbol Al, is in
group 13. We can also confirm that this is
not an alkali metal. And so we can eliminate answer (E)
aluminum.
Finally, the element that is an
alkali metal and which is missing from this group is answer (D) potassium.
