(a)

(i) Mendeleev arranged the elements in increasing atomic weight. He arranged elements in vertical groups of similar chemical properties and he left gaps for undiscovered elements.

(ii) Mendeleev placed tellurium and iodine in groups of similar chemical properties and not according to their atomic weight.

(iii) Noble gases were not discovered at that time.

(b)

(i) Atomic number is the number of protons in the nucleus of an atom.

The relative atomic mass is the average mass of isotopes of an element as they occur naturally in abundance relative to 1/12 of a carbon-12 isotope.

(ii) It arranges the elements in terms of their chemical properties.

(c)

(i) Noble gases in Group 18 have full outer energy levels so they are stable and unreactive.

(ii) As you move down Group 17 the number of shells increases causing screening. Therefore, the atomic radius increases; hence, reactivity decreases as there is less attraction between incoming electrons and protons in the nucleus.

(d)

(i) Francium is an alkali metal and when placed in water it will float, spin and go on fire as its atoms lose one electron.

(ii) Products: Francium hydroxide and hydrogen gas.