Gain/Loss of Electrons: Explaining Element Affinity

[Aezi]

Hi! It's been awhile since I've taken chemistry, and I forgot why some elements preferentially lose or gain electrons. It's not like they grow a brain and think, "hey, I'd rather lose 3 electrons than gain 5 to fulfill my octet!" What's the explanation for this phenomena? Some people would just say that certain elements have higher or lower electron affinity than the element it bonds with and such and such, but that doesn't tell me much (or maybe I'm missing something). Thank you! I hope I made my question as clear as possible. I'll copy what I read that sparked this question as well below:

"What makes carbon so special? Why are there so many carbon-containing compounds? The answer lies in carbon's position in the periodic table. Carbon is in the center of the second row of elements. Atoms to the left of carbon have a tendency to give up electrons, whereas the atoms to the right have a tendency to accept electrons."

 

[Trave11er]

People call it affinity to make it simpler, what really happens is that electrons go to the state with the lowest energy (just as raw egg falls from your hand on bad morning due to gravity). As you know all electrons in atoms are placed in orbitals - what happens is that in case if orbitals on atom 1 are high in energy (e.g. 2s in Li, 3s in Na, 3p in Al etc.) they can be transferred to another atom which has orbitals that are lower energy (e.g. 2p in O and F, 3p in Cl etc.).
Now if you start thinking about which atoms have high and which have low energy orbitals there are two things to consider. Firstly, across one period from left to right nuclear charge increases and atoms on the right are more electronegative. On the other hand, if we go down the periodic table in every successive period valence electrons are raised in energy (1s<2s<3s etc. - this is purely quantum property of atoms) and this rise is more significant than previous lowering to due increasing nuclear charge - this is why after periodicity of properties in elements is observed. Beutiful isn't it? This is why we say that "elements become more metallic or less electronegative as we go down the periodic table".
Why carbon is so special? - well, it has quite high energy orbitals, so it doesn't fully withdraw electrons to form negative ions as fluorine in RbF and it also doesn't easily give up all electrons as say barium in BaSO4. Instead carbon usually forms strong covalent bonds in which electrons shared equally between atoms. The most significant consequence of this is that it can form 4 strong bonds (quite a lot) with other C atoms, silicon and others can't because bonding is weaker between bigger atoms, e.g. bond dissociation energy decreases in F2 down to I2.

 

[Borek]

Check out orbitals and electron configuration.

 

[Aezi]

Ah! Thanks! Now it's all coming back to me. This universe is practically all about naturally going from higher to lower energy states!

 

[Trave11er]

Well, honestly this isn't true either - systems do not go to the state with lowest energy what they really do is go to the state with maximum disorder (biggest entropy)

 

[Borek]

That's not true either. At 1 atm and 765 deg C system made of gaseous potassium and chlorine has much higher entropy than KCl crystal, yet you will see solid KCl.

 

[Trave11er]

Well that is because total entropy of the universe increased when solid KCl was formed.

 

[Dickfore]

Well that is because total entropy of the universe increased when solid KCl was formed.

That's a very practical approach. Do you often consider the whole Universe when you perform experiments in the lab?

 

[Trave11er]

No, if it is not required by the experiment. The statement was just a point in the discussion: all the processes that take place in the Universe lead to the increase in entropy and those that don't are forbidden. Solid KCl is not an exception. No exception was ever observed to the second law of thermodynamics.

 

[Dickfore]

No, if it is not required by the experiment. The statement was just a point in the discussion: all the processes that take place in the Universe lead to the increase in entropy and those that don't are forbidden. Solid KCl is not an exception. No exception was ever observed to the second law of thermodynamics.

Have you measured the entropy of the Universe to be so certain in your claims?