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8. Chemical Reactivity eLearn.Punjab

Therefore, they have the highest metallic character. For example, a comparison of sodium and
magnesium metals is given below for understanding.

Sodium Atom Magnesium Atom
3s1 electron configuration 3s2 electron configuration
having atomic size 186 pm, having atomic size 160pm,
and ionization energy 496 kjmol-1. and ionization energy 738 kJmol-1.

The 1st ionization energy of magnesium is high but the 2nd ionization energy of magnesium is
very high. It becomes very difficult to remove second electron from the Mg+ ion as nuclear charge
attracts the remaining electrons strongly. As a result of this attraction size of the ion decreases.
Similarly, all the elements of alkaline earth metals have high ionization energies as compared to
alkali metals as shown in table 8.1.

Table 8.1(a) Atomic Number, Electronic Configurations and Ionization Energies
(kJ/mole) of Alkali and Alkaline Earth Metals

Metal Atomic Electronic IE Metal Atomic Electronic IE1 IE2
No. Configuration No Configuration

Li 3 [He] 2 s1 520 Be 4 [He] 2 s2 899 1787

Na 11 [Ne] 3 s1 496 Mg 12 [Ne] 3 s2 738 1450

K 19 [Ar] 4 s1 419 Ca 20 [Ar] 4 s2 590 1145

Rb 37 [Kr] 5 s1 403 Sr 38 [Kr] 5 s2 549 1064

Cs 55 [Xe] 6 s1 377 Ba 56 [Xe] 6 s2 503 965

Low ionization energies of alkali metals make them more reactive than alkaline earth metals.

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