atomic structure question

1)
Electronic configuration of
Ne : 2,8
Al : 2,8,3
S(2-) : 2,8,8 ----> S is "2,8,6"; to form ion with a -2 charge, it gains 2 more valence electrons, so, it is 2,8,8
K(+) : 2,8,8 ----> K is "2,8,8,1"; to form ion with +1 charge, it loses 1 valence electron, so it is 2,8,8
Ar : 2,8,8

From wikipedia,
"Isoelectronic = molecular entity with same electronic configuration/same no. of electrons AND the same structure"

So, the isoelectronic chemical species are S(2-), K(+) and Ar

Among these three particles, Ar is the most inert, because it has an octet structure with zero charge.This can be proved by the fact that it rarely form bond with other atoms.
While for the other two particle, K(+) and S(2-). Although they attain an octet structure, they are charged, they will attract oppositely charged particles to form ionic compounds; or for K(+), a cation, it attracts electron, and returns to its ground state
(i.e. K(+) + e- ---> K). So, they are much more reactive than Ar.


2.
(i) C,
because along the same period, elements' atomic number increase(i.e. no. of proton increases);
As protons are positively charged, there will also be an increase in nuclear charge.
This cause the valence shell, which holds negatively charged electrons, contracts.
So, since C, N, O and F are in the same period, and C have the smalleset atomic number among four of them; it has the greatest atomic radius.
(ii)
Since the all have the same electronic configuration, "2,8", the electrons will occupy same no. of shells;
similar to the above case, the one with the most positively charged nucleus can contract the valence shell to the most extent.
Thus, the one with most protons, Mg(2+) will have the largest atomic radius.

I don't know at what level you are. So, the explanations in question (2) are basic, vague and general; concepts that are more complicated are not included in the answers.
(If you want them, you can ask; I'll try to elaborate a bit; although I'm not very clear about them, jajajajaja!)

Hope I can help you!!! =]

黑白灰_唯物主義者

2011-09-29 12:28:46 補充：
THX to 己式庚辛,
In 2(ii), Mg(2+) should have the SMALLEST stomic radius, the explanation is just the same.
i.e. F(-), with the least protons, will have the LARGEST atomic radius.

Mg2+ should be the smallest; largest is F- .
I guess it's just a typo.