While learning Periodic Properties, I stumbled upon a doubt regarding reactivity of elements of period 2 and period 3.

(I am denoting reactivity, of any $X$ element as $R_{X}$.)

For reactivity of $s-block$ elements, I saw $R_{Li}<R_{Na}$ . Acording to me this is so because since the metallic character increases down the group, (Am i correct $?$)

But as we go towards right side, things become complicated. For boron group, I saw this: $R_{B}<R_{Al}$. I Similarly for other groups I saw following: $R_{N}<R_{P}$; $R_{O}>R_{S}$; $R_{F}>R_{Cl}$;

(According to me reactivity of third period elements should be greater than that of second period but i don't know why it doesn't follow for oxygen and halogen groups)

Please tell reasons why there is such unusual recativity trend for the elements I mentioned, and thanks in advance $!!$

(I saw on internet having divided opinions, which was not helpful).


$\ce{Li}$ vs $\ce{Na}$, $\ce{Be}$ vs $\ce{Mg}$, $\ce{B}$ vs $\ce{Al}$:

Rather, increasing metallic character and reactivity have the common cause, decreasing the ionization energy due nucleus charge shielding and greater electron distance from a nucleus.

$\ce{N}$ vs $\ce{P}$, $\ce{O}$ vs $\ce{S}$, $\ce{F}$ vs $\ce{Cl}$:

The same reason as above leads to decreasing of electron affinity of their atoms, what has the opposite effect - decreasing of reactivity.

Note that atomic nitrogen is more reactive than atomic phosphorus, so it follows the general trend. But molecular nitrogen is much less reactive than molecular phosphorus due its much stronger bond.


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