Chemical Bonds

So I live over in NZ and we have a stupid system for school called NCEA. Anyone I've been watching certain youtube videos, one is from this guy over in america and his stuff is quite detailed and its reliable and knowledgeable.

The other youtube channel I've been watching is a local youtuber, who does videos specifically for our NCEA exams. Anyone if there are any chemistry experts out there, it'd be great if you could help clear a few things up for me, just some stuff in this guys video doesn't seem right. For reference the video is :https://www.youtube.com/watch?v=VW6B1zjxoM0&t=22s

Anyway, some things he said were that convalent bonds are always non-polar and that the 4 types of bonding we have to learn are convalent, ionic, metallic & intermolecular. This seems to be the way it goes for our exams also, because I've been doing the practice tests.

The thing I don't understand is that ain't the main types of bonds Convalent, Ionic and Metalliac the main sort of bonds? and intermolecular is a force not bond? It would help if someone explained bonding and forces as simply as you can.

Ive attached a couple of images from our practice tests, please clarify if these answers are right?



[Edited on 10-7-2016 by NzForever]



[Edited on 10-7-2016 by NzForever]

Quote: Originally posted by NzForever

Anyway, some things he said were that convalent bonds are always non-polar

I would forget about the whole phrase "usually between metals and non-metals". I would phrase it as follows:
- Ionic bonds are between cations (positively charged) and anions (negatively charged).
- Frequently encountered cations are positively charged metal ions, but other types of cations do exist (e.g. ammonium ion).
- Frequently encountered anions are negatively charged non-metal ions, but other types of anions do exist. Many anions are composite entities, having multiple atoms, covalently bonded, with the entire entity having a negative charge (e.g. sulfate ion, nitrate ion).

I would say that there are following bonds:
- covalent: sharing of electrons by different atoms. Example: HCl, one electron donated by H atom and one electron donated by Cl-atom, the bond incorporates two electrons.
- ionic bond: two (or more) different charged species are combined in a single lattice and electrostatically remain together. Example: Na⁺Cl^-, usually simply written as NaCl. The charged entities themselves can be composite, with covalent bonds in the entities, the complete entities being charged, example: (NH₄⁺₂SO₄</sub>^2-, usually simply written as (NH₄₂SO₄</sub>
- metallic bond: a positively charged species (almost always a metal ion, but this need not be so), kept together by a counterbalance of freely moving electrons in a so-called 'electron sea'. The positive ions and the negatively charged electrons keep the whole thing together. We perceive such 'electron sea' as a shiny surface, a typical metallic appearance.
- dative bond: This in fact is not much different from a covalent bond, but with this dative bond, the shared electrons are all donated by one of the partners, involved in the bond. An example is the ammonium ion, in which H₃N: shares its two free electrons on the nitrogen with a H⁺ ion in order to make H₃N:H⁺, the common ammonium ion NH₄⁺. Other common examples are hydrated metal ions, e.g. Cr(:OH₂₆³⁺, in which two electrons of the water molecules on the oxygen atom are shared with the Cr³⁺ ion. Dative bonds can also exist between neutral species, e.g. in F₃B and :NH₃ forming the tightly bonded molecule F₃B:NH₃. Here :NH₃ is the donor of both electrons.

More types of bonds exist, but these are more exotic. An example is the so-called 3c-2e (three center, two electrons) bond in diborane, in which two electrons are donated by two atoms, while these electrons are shared by three atoms. Even more exotic bonds exist in ions like Te₄²⁺, but these kinds of bonds are not common and beyond the scope of standard introductory courses. These exotic bonds in fact also are covalent, but the bond is not between two atoms, sharing a pair of electrons, but between more than two atoms, sharing two or more electrons.

Finally, things like inter-molecule forces I consider as different from the above types of bonds. These forces do have a large influence on the physical properties of materials though.

The bond types, mentoned above, all are kind of 'extremes'. In practice, things may be not as simple as what is written above. A covalent bond may lead to asymmetric charge distribution over the molecule. As already pointed out, in e.g. HCl, the negative charge is concentrated somewhat more strongly around the Cl-atom, so the two shared electrons are not equally shared, the Cl-atom has them more 'around it' than the H-atom. In some covalent compounds the charge distribution may be so asymmetric that one hardly can speak of a covalent bond anymore and the bond is almost ionic. An example of this is a compound like anhydrous CuCl2 (a brown/yellow solid). Many people consider this a salt (having ionic bonds between copper(II) ions and chloride ions), but in fact it still is covalent, but with the negative charge almost completely drawn to the chlorine atoms. In many solvents, this compound breaks down in ions and you get a conducting solution. Such solutions tend to be blue, the common color of solvated copper(II) cations. In some solvents (e.g. acetone) this compound dissolves as molecules CuCl2 and such solutions are brown/yellow. The lesson to be learned from this example is that there is a gray area between bond types. Another borderline compound between ionic and covalent, but somewhat less pronounced, is HgCl2. Solutions of this in water still consist of HgCl2 molecules, but there also is quite some Hg²⁺ and Cl^- in solution.

There is also a gray area between bonds and intermolecular forces. An example is solvation of sodium ions in water. Water molecules have covalent bonds, but the charge is not evenly distributed. The oxygen atom has a somewhat negative charge and the hydrogens have a somewhat positive charge. The positively charged sodium ions attract water molecules on their oxygen-side. The oxygens have two free electrons and share these with the sodium ions. This bond, however, is quite labile and is easily broken. So, a certain water molecule is not really solid fixed to a certain sodium ion, but still, one can speak of a dative bond to some extent. Here we have an example of a gray area between dative bonds and intermolecular forces. If we replace the sodium ions with trivalent chromium ions, then the picture changes quite a lot. The chromium ions attract the oxygen side of the water molecules much more strongly than the sodium ions do and once a water molecule has shared its two free electrons on its oxygen atom with the chromium metal ion, then it is tightly bound and cannot escape anymore. The electron withdrawing power of the chromium ion is so strong that the negative charge is going deeply inside the complex ion and the hydrogens on the water molecule become almost completely positively charged. Some of these hydrogen ions actually split off as H⁺ and the solution becomes acidic.



[Edited on 7-10-16 by woelen]

Quote: Originally posted by j_sum1

The examination and the revision resource are from two different providers. (And the exam markers are different again

That sounds totally insane.

I guess it requires a whole army of publicly funded admin staff, which would explain the 'sense' behind it.

-----------------------------------------------------------------------

Happily, here at B&D Uni there are no admin staff, excellent Tutors and courses, plus well administered exams

B&D now operate as a fully Open and Transparent outfit.

(i stole all the firedoors, a few hundred roof tiles and eight wall panels last week. U2U while stocks last !)