198 12 · CHEMICAL FAMILIES
Atomic and ionic radii of the s-block elements
The atomic and ionic radii of the elements of Groups 1 and 2 are shown below:Element Atomic radius Ionic radius Element Atomic radius Ionic radius
/pm /pm /pm /pm
Li 145 060 Be 105 031
Na 180 095 Mg 150 065
K 220 133 Ca 180 099
Rb 235 148 Sr 200 113
Cs 266 169 Ba 215 135(i)iiFor each element, why is its ionic radius much smaller than its atomic radius?
(ii)iWhy do the atomic radii of the elements increase as the groups are descended?
(iii)Although they are in different groups, the chemistry of Li is quite similar to that of Mg. Why
do you think this is?Exercise 12F
BOX 12.1
Hardness of water
Hardness in water is caused by the presence of dissolved
calcium and magnesium compounds, such as calcium
hydrogencarbonate or magnesium sulfate. The hardness of
the water can be gauged by its ability to form a thick lather
with soap. Water that lathers easily is called soft, whereas
water that produces very little lather and a scum, is termed
hard. Soap (see page 175) is sodium stearate and scum
forms as a result of the precipitation of an insoluble stearate:Ca^2 (aq)2St(aq)CaSt 2 (s)where Ststearate (C 18 H 35 O 2 )Lather cannot form until all the calcium or magnesium ions
have been precipitated. Note that soapless detergents, which
are an ingredient of many washing-up liquids and detergents,
do not form a scum with hard water.Hard water is produced when rainwater, which is weakly acidic
because of the presence of dissolved carbon dioxide, falls
upon limestone or chalk which are both forms of calcium
carbonate. The acidic solution dissolves calcium carbonate
producing a solution of calcium hydrogencarbonate, or hard
water:H 2 O(l)CO 2 (g)CaCO 3 (s) Ca(HCO 3 ) 2 (aq)This type of hard water is called temporary hard water,
because it can be removed by boiling. Permanently hard water
contains calcium and/or magnesium salts which are not
decomposed when the water is boiled. Calcium
hydrogencarbonate solution decomposes on boiling to calcium
carbonate (scale), which can block pipes or fur kettles in hard
water districts:Ca(HCO 3 ) 2 (aq) H 2 O(l)CO 2 (g)CaCO 3 (s)Stalagtites and stalagmites are also found in hard water
districts. When drops of calcium hydrogencarbonate solution
collect on the roof of a cave, the solution decomposes, some
of the water evaporates, carbon dioxide is lost and a tiny
precipitate of calcium carbonate is left behind. Over many
years these specks of calcium carbonate accumulate to form
astalactite. Solution that drips on to the floor undergoes the
same decomposition, forming a stalagmite(Fig. 12.1).Temporary or permanent hard water are commonly softened by
ion exchangers, which remove the Group 2 metal ions and
replace them with sodium ions. Although hard water has
disadvantages, it does have a pleasant taste and the calcium
compounds it contains are good for bones and teeth.Fig. 12.1Formation of stalactites and stalagmites.