Why Do Elements Have Variable Valencies and Other Chemistry Questions?

[rishch]

Here are a few doubts that I had while reading for an upcoming exam.While answering please keep in mind that I'm in 9th grade so simple answers would be better-

1)Why is the electronic configuration of Chromium - [Ar] 3d5 4s1 instead of [Ar] 3d4 4s2 ? Similarly why does Coppers electronic configuration not follow the pattern ?
2)Why do substances combine ? I was told they combine to become stable but what does 'stable' mean ?
3)My textbook says that substances combine so that they can attain octet (8 electrons in outer shell ) or duet (2 electons in outer shell ) configuration.I think that I read somewhere that an element cannot have a valency of more than 4 but Nickel which has an electronic configuration of 2,8,16,2 will have to gain 6 electrons to achieve octet configuration and 6 is more than 4.How is this possible ?
4)I also read that elements try to attain the nearest noble gas's electronic configuration.But the noble gas closest to Fe (2,8,14,2) is Argon (2,8,8).So Fe should lose 8 electrons.But the valency of Fe is 2 or 3.Why ?
5)Why do elements have variable valencies ?
6)Elements combine to become stable.That means that compounds should be stable.So in the reaction - 2 H2 + O2 → 2 H2O why do the compounds H2 and O2 combine if they are already stable ?
7)I think someone told me that ionic compounds dissociate in solution.For example NaCl.Why does this happen ?

 

[aroc91]

I never was very good with e- configs, so I'll skip #1

2. You answered this in #4. Elements are most stable when their valence shell is full, whether that's 2 or 8 electrons.

3. I don't think you read correctly. On the periodic table, from the noble gasses, which have 8 valence electrons, that number drops as you go left through the columns. Chlorine requires 1 electron to reach a valence of 8, while oxygen requires 2, so on and so forth. With the transition metals, it depends. I don't deal with inorganic chem, so I'm not too keen on those.

4. This is what I just found by doing a quick search on how to complete the valence of t-metals. It clarified it a bit for me, so I hope it helps-

The number of valence electrons in the transition metals is somewhat different than main group elements. As you go from left to right across the periodic table, the electrons added to the transition metals go into the d-orbitals. However, because the energy of the 4s orbital is lower than the 3d orbital (and the 5s is lower than the 4d, etc.), the 4s orbital fills first. Therefore the electron configuration of iron for instance is [Ar]4s23d6.

Because the valence electrons are defined as the electrons in the outermost or highest energy shell, for iron, that would be the 4th shell. So the 6 electrons in the 3d orbital don't count. Only the 2 electrons in the 4s orbital count since they are in the 4th shell. Most transition metals thus have 2 valence electrons (although some, such as chromium, only have one because of exceptions to the filling rules -- the configuration of chromium is [Ar]4s13d5).

Read more: http://wiki.answers.com/Q/How_do_you_count_valence_electrons_of_transition_metals#ixzz1kbpAtqjp

5. It depends on the oxidation state and that's all I really know.

6. "Stable" is a relative term. Water has a lower energy level than separate hydrogen and oxygen gasses, making it more stable. When you perform that reaction, it's exothermic, meaning the formation of those bonds has given off energy to the environment in the form of heat.

7. Ionic bonds aren't as strong as covalent bonds. The non-covalent interactions between ions and water are stronger than the ionic bonds, so they don't stay together.

I hope this helped. If you have any more questions, don't hesitate to ask.