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Group II
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GROUP II ELEMENTS
Beryllium to Barium
Introduction
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as s-block elements because their valence (bonding) electrons are in s orbitals.
\cline { 2 - 6 } \multicolumn{1}{c|}{} | Be | $M g$ | $C a$ | $S r$ | Ba
Atomic Number | 4 | 12 | 20 | 38 | 56
Electronic configuration | $1 \mathrm{~s}^{2} 2 \mathrm{~s}^{2}$ | {$[\mathrm{Ne}] 3 \mathrm{~s}^{2}$} | {$[\mathrm{Ar}] 4 \mathrm{~s}^{2}$} | {$[\mathrm{Kr}] 5 \mathrm{~s}^{2}$} | {$[\mathrm{Xe}] 6 \mathrm{~s}^{2}$}
PHYSICAL PROPERTIES
Atomic Radius
Increases down each group electrons are in shells further from the nucleus
\cline { 2 - 6 } \multicolumn{1}{c|}{} | Be | Mg | Ca | Sr | Ba
Atomic radius $/ n m$ | 0.106 | 0.140 | 0.174 | 0.191 | 0.198
Ionic Size
Increases down the group
The size of positive ions is less than the original atom because the nuclear charge exceeds the electronic charge.
\cline { 2 - 6 } \multicolumn{1}{c|}{} | $\mathrm{Be}^{2+}$ | $\mathrm{Mg}^{2+}$ | $\mathrm{Ca}^{2+}$ | $\mathrm{Sr}^{2+}$ | $\mathrm{Ba}^{2+}$
Ionic radius $/ \mathrm{nm}$ | 0.030 | 0.064 | 0.094 | 0.110 | 0.134
Melting Points
Decrease down each group metallic bonding gets weaker due to increased size Each atom contributes two electrons to the delocalised cloud. Melting points tend not to give a decent trend as different crystalline structures affect the melting point.
\cline { 2 - 6 } \multicolumn{1}{c|}{} | Be | Mg | Ca | Sr | Ba
Melting point $/{ }^{\circ} \mathrm{C}$ | 1283 | 650 | 850 | 770 | 710
Ionisation Energy
Decreases down the group atomic size increases
Values for Group I are low because the electron has just gone into a new level and is shielded by filled inner levels. This makes them reactive. Group II elements have higher values than their Group I equivalents due to the increased nuclear charge.
\cline { 2 - 6 } \multicolumn{1}{c|}{} | Be | Mg | Ca | Sr | Ba
Ist I.E. $/ \mathrm{kJ} \mathrm{mol}^{-1}$ | 899 | 738 | 590 | 550 | 500
2nd I.E. $/ \mathrm{kJ} \mathrm{mol}^{-1}$ | 1800 | 1500 | 1100 | 1100 | 1000
3rd I.E. $/ \mathrm{kJ} \mathrm{mol}^{-1}$ | 14849 | 7733 | 4912 | 4120 | 3390
There is a large increase for the 3rd I.E. as the electron is now being removed from a shell nearer the nucleus and there is less shielding.