Chemistry question on electron affinity

[leena19]

Homework Statement

under the same conditions which one of the following atoms will liberate the largest amount of energy when it gains an electron?
1.Na(g) 2.Ar(g) 3.Li(g) 4.N(g) 5.Mg(g)

Homework Equations

The Attempt at a Solution

I was told the answer's N 'cos it's the most electronegative atom out of the choices given but when we consider the electronic configurations of the above atoms Na & Li seems to attain a stable octet by gaining an electron so it would release a lot more energy than N which by gaining an electron becomes 2p4 which is far less stable than 2p³.

so I think its between Li & Na and since Li has a smaller covalent radius it would probably attract the electron more and so release more energy,am I right?

 

[epenguin]

Na, Li, attain stable octets by losing an electron, Mg by losing two.

 

[leena19]

I'm sorry but I don't get it .
Alkaline metals like Na and Li usually attain stable octets by losin an electron but, they may also attain stable octets by gaining an electron,no?

I got down the electron affinity values from http://hyperphysics.phy-astr.gsu.edu/HBASE/chemical/eleaff.html" [Broken]
but now I'm even more confused,

The EA values show that EA of Li > Na>N , also that group 2 and 8A elements don't form anions so the answer has to be Li. But,
what I don't understand is how these values are determined?
n why does N have an EAof 0 and the other group 5A elements have Electron Affinities which increase downwards ,
while in other groups, there seems to be a gradual decrease in EA values down their respective groups except, ofcourse, the irregularity seen between O , S in group 5 and F , Cl in group7,also which unsurprisingly I don't understand?