h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Do any of you now some chemistry

Yes I do, what do you want to know?

why do metal with 2 electrons in the outer shell have similar react to water

Ah right.

OK well are you familiar with why the periodic table is arranged in the way it is?

It is arranged so that the atomic nucleus increases by one proton (and so +1 charge) from left to right like reading a book.

When electrons fill up their orbitals around the nucleus they do so in an ordered manner. There are s orbitals that fill up with 2 electrons, p orbitals that have 6 electrons in all (3 p orbitals with 2 electrons each) and so on.
Each row of the periodic table represents the electrons begining to fill up a complete 'shell' of electrons so the elements on the left have just begun to

The elements in group 2 (beryllium, lithium, magnesium and so on) fill up so that there highest energy electrons are filling up an outer s orbital.

This electronic configuration is not stable though as atoms like to have complete shells of electrons and the lowest energy way of doing this in group 2 is for them to lose two electrons.

Hence when they react with other chemicals they lose two electrons to form +2 cations (cations are positively charged ions). All the elements in the group do the same because they have two unstable electrons in their outermost shell.

So when elemental magnesium reacts with water is is oxidised (loses electrons) to form Mg2+.
Of course this electron must go somewhere and it is to the water.
To counter the +2 charge of the magnesium ions water is reduced (opposite of oxidation - it gains electrons).
So H2O becomes OH- (hydroxide anion - anion being a minus charge ion) and a hydrogen atom. Two hydrogen atoms combine to form H2 gas and so the overall equation becomes:

Mg + 2H2O ----> Mg(OH)2 + H2

and as I said again, because all the elements in group 2 have a similar outer electronic configuration they all do this reaction.
As you go down the group, the two outer electrons become more easily lost (because the nucleus becomes shielded from the outer electrons by large numbers of inner electrons and so it loses the power of its grip on them. Therefore this reaction becomes more easily achievable as you go down the group and happens with increasing vigour.

You have to heat magnesium to get it to react with water but calcium (the next one down) will fizz spontaneously in water.


Hope you followed all that - if not just ask and I will try and further explain individual bits