Exercise 2 solution

(a) Using the group pages read for this chapter:
  chlorine  - group 17, halogens (Halogen article lists chlorine)
  potassium - group 1, alkali metals (Alkali metal article lists potassium)
  argon     - group 18, noble gases (Noble gas article gives the group number; that argon
              is one of them is shown by its boiling point in that article's table)

(b) The Metalloid article names six: boron, silicon, germanium, arsenic, antimony
  and tellurium. From the list: boron, silicon and germanium are metalloids.
  Sodium (an alkali metal) and bromine (a halogen) are not on that list.

(c) Alkali metals: "the heavier alkali metals reacting more vigorously than the lighter
  ones" and reactivity increases down the group. Order down the group is lithium,
  sodium, potassium, so potassium is expected to be the most reactive with water.
  Metallic character: the Periodic table article says it "increases going down a
  group", so potassium is again the most metallic of the three.