Re: What is the differences between the Reactivity Series and Electronegativity

I'll start at the bottom line, and work my way back up through the 
concepts.

Fluorine gas is definitely more reactive than chlorine gas. It is an 
extremely toxic and dangerous material, and so reactive that it is 
difficult to design a container for it. Fluorine reacts rapidly and 
spontaneously at room temperature with nearly all other simple substances 
(elements) and with the vast majority of compounds as well. The only simple 
substances it cannot be made to react with are helium and neon, the noble 
gases. The only ones that it takes a bit of pushing to get it to react with 
are argon, krypton, oxygen and nitrogen. Chlorine is also very reactive -- 
it even reacts with very unreactive metals like gold and platinum, but it 
often requires warming or sunlight to get its reactions to go. It does not 
react with argon or krypton at all, and although nitrogen trichloride is a 
known substance, I do not think it can be made by direct reaction between 
nitrogen and chlorine.

Now let's talk about electronegativity. That is one of those chemical ideas 
that is very useful but slightly vague. It is difficult to give a 
definition or to define a numerical scale. To be more precise, there are 
several competing definitions and numerical scales which are all 
qualitatively similar, but differ greatly in fine detail.

The general idea is that electronegativity is a measure of an atom's desire 
for electrons. It is a measure of how strongly it holds onto the ones it 
has, and how greedy it is to pick up extra ones. Very electronegative 
elements often form negative ions readily in chemical interactions, and 
tend to grab an unfairly large share of electrons from a less 
electronegative atom in covalent bonding. Very electropositive elements 
readily lose their electrons to form positive ions, and are not too keen to 
form covalent bonds by grabbing a share of an electron from somewhere else.

At both extremes of the electronegativity scale, elements are very 
reactive. The most electronegative elements, fluorine, oxygen, and 
chlorine, are all very reactive as simple substances, as are the most 
electropositive elements, the alkali metals caesium, rubidium, and 
potassium.

But in between, there is a huge variety of reactivity that does not 
correlate with electronegativity. Nitrogen and bromine have almost 
identical electronegavity. What this means is that in a compound with an N-Br 
bond, the electrons associated with this bond will be very evenly shared. 
But bromine liquid is a very reactive simple substance, while nitrogen gas 
is very unreactive. Moreover bromine readily picks up an extra electron to 
form bromide ions, whereas nitrogen does not form simple negative ions in 
normal chemical conditions. To take two other elements with almost 
identical electronegativity: the light metal, aluminium is much more 
reactive than the heavy metal tungsten, both as a simple substance, and in 
the general behaviour of its compounds.

The rule that you give about reactivity in the periodic table simply does 
not apply. I think the confusion may come because the electrochemical 
series of reducing agents is sometimes loosely called 'the reactivity 
series of metals'. (Note that insofar as it does apply to reactivity, it 
only does so for metallic elements!)

The electrochemical series is precisely defined and quantified. It has to 
do with electrical potentials measured in standard electrochemical cells. 
It is a direct parallel with electronegativity. But it differs because it 
takes into account an environment and a context. The electrochemical series 
is normally quoted for aqueous solutions at room temperature. This is 
important, because many light metals form ions that bond strongly to water, 
and this tips electrochemical processes in their favour. In terms of 
electronegativity we have

Cs