Members may post their answers to questions asked in the announcement in this thread

ANSWER TO PHYSICS 1

The bug falls off when the normal force due to the sphere vanishes. This happens when the centripetal force (opposite to the normal force) is large enough (and equal to the normal force).

Using this condition and the conservation of mechanical energy, one gets the angle at which the bug falls off as sin-inverse of (2/3) (approx. 41.80 degrees). (I am measuring the angle from the centre of the sphere with respect to the negative x-axis in the clockwise direction).

ANSWER TO Chemistry 1: Does an orbital exist if there is not an electron in it?

I would say no. The orbital is not "a region where you can put electrons in" or a cloud or anything of that sort. It is a mathematical function (or more precisely, a state vector) that completely describes the state of the electron (excluding the spin). More specifically, an electron bound to an atom is allowed to be in a state of definite energy and angular momentum, and this state is what is known as the orbital. It corresponds to a state of the electron, and has no existence independent of the electron.

Since the state is described completely only when you include the spin, and since electrons can have either up or down (1/2) spin, and since electrons are fermions obeying Pauli's exclusion principle, an orbital can "hold" no more than 2 electrons. I've used incorrect terminology in the previous sentence; I should have said "No more than two electrons can be in the state described (incompletely) by a single orbital"

To sum up, orbitals are not pockets of an atom in which you can drop electrons in, they are the descriptions of the states of an electron in the atom. No electron - no orbital.

Answer is correct but you goofed in the above sentence Normal force disappears at the point of take off, it is the radial component of weight that is equal to the centripetal force.

This question belongs to philosophy rather than chemistry If existence requires possessing matter/energy then clearly the orbital does not exist. However, then numbers wouldn't exist either, in fact, thoughts wouldn't exist. As Syriac points out, an orbital is our description of what happens inside an atom, and thus is a thought and not a physical thing. But it exists as a thought, so those who would like to generalize the notion of existence can always claim that orbitals exists irrespective of electron.

Darn, this part confuses me too often. Doesn't the centripetal force point inwards in the same direction as the radial component of weight?

Every object that is in a circular orbit has to have a force exerted on it in the radial direction to provide the centripetal acceleration. The radial component of the weight of the object is initially maximum but the required centripetal acceleration is small, so a Normal force appears, to reduce the total inward radial force. As the object accelerates, the normal force diminishes, and at the point of take off, it would need to be negative to aid gravity. Thus, if you had a a particle moving between two concentric spheres, at the point of take-off the particle would jump out to the outer sphere and will move inside the surface as opposed to outside.

Prions are thought to cause a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt-Jakob disease (CJD) in people (not humans). All thus-far hypothesized prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal. In general usage, prion can refer to both the theoretical unit of infection or the specific protein (e.g. PrP) that is thought to be the infective agent, whether or not it is in an infective state.

Prions are hypothesized to infect and propagate by refolding abnormally into a structure which is able to convert normal molecules of the protein into the abnormally structured form. All known prions induce the formation of an amyloid fold, in which the protein polymerises into an aggregate consisting of tightly packed beta sheets. This altered structure is extremely stable and accumulates in infected tissue, causing cell death and tissue damage. This stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult.

Proteins showing prion-type behavior are also found in some fungi and this has been quite important in helping to understand mammalian prions. However, fungal prions do not appear to cause disease in their hosts and may even confer an evolutionary advantage through a form of protein-based inheritance.

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.

The prion protein PrP is a naturally occurring polypeptide that becomes transformed from a normal conformation to that of an aggregated form, characteristic of pathological states in fatal transmissible spongiform conditions such as Creutzfeld–Jacob Disease and Bovine Spongiform Encephalopathy. We report the crystal structure, at 2 Å resolution, of residues 123–230 of the C-terminal globular domain of the ARQ allele of sheep prion protein (PrP). The asymmetric unit contains a single molecule whose secondary structure and overall organisation correspond to those structures of PrPs from various mammalian species determined by NMR. The globular domain shows a close association of helix-1, the C-terminal portion of helix-2 and the N-terminal portion of helix-3, bounded by the intramolecular disulphide bond, 179–214. The loop 164–177, between β2 and helix-2 is relatively well structured compared to the human PrP NMR structure. Analysis of the sheep PrP structure identifies two possible loci for the initiation of β-sheet mediated polymerisation. One of these comprises the β-strand, residues 129–131 that forms an intra-molecular β-sheet with residues 161–163. This strand is involved in lattice contacts about a crystal dyad to generate a four-stranded intermolecular β-sheet between neighbouring molecules. The second locus involves the region 188–204, which modelling suggests is able to undergo a partial α→β switch within the monomer. These loci provide sites within the PrPc monomer that could readily give rise to early intermediate species on the pathway to the formation of aggregated PrPSc containing additional intermolecular β-structure.

I think that it should be cos inverse 2/3 instead of sin inverse 2/3.assuming that we measure angle the same way as Syriac.

If you measure from the pole it is arccos, if from the equator then it is arcsin. Syriac has not written it very clearly but I suspect that he is measuring what we call the latitude.

I have sorted out that with Syriac.........It is indeed the problem with latitude and longitude thing only and Tarun's explanation is very easy to grasp.

Gee, I thought that was clear. But yeah, I guess my x-axis need not be your x-axis; and the equator picture makes it easier



Thanks, that helped. (I know, I should be embarassed. I obviously need to pay more attention to inclined planes and pulleys.)

So... do I get a prize?

BIOLOGY ANSWER:

Prion is a proteinaceous structure.It si misfolded form of a protein PrPc
(c-common) which is a normal protein found in the cellular membrane.Its high affinity for binding to copper ions supports its proposed role as an effective anti-oxidant.
PrPc is mainly an alpha helical structure which,after misfolding,results in increased beta-sheet content in place of alpha helix.beta strand of these stacked beta sheets align perpendicular to axis of the fibril and polymerise to form insoluble fibrous protein aggregates known as amyloids.prions are resistant to proteases and any kind of denaturation which leads to their accumulation.
Aggregation of these plaques(amyloids) in the extracellular regions of CNS disrupt the normal histology. the disruption is due to spongy texture due to vacoule formation in neurons.This in turn leads to increased in number of astrocytes in response to abnormal architecture of nearby neurons.
All this results in neurodegeneration which may completely damage the brain and cause death.
The possibility of any genetic role is ruled out with the fact that there is no difference in amino acid sequence in mRNA of the two proteins.Moreover,ther is no covalent differences in PrPc and PrPsc.

i'm sorry i'm asking this quetion a little late.......if an orbital can't exist without an electron then how do u explain hybridisation in which we always say that the electron jumps to higher empty orbitals....and wat about LUMO

Hybridisation is best understood from the point of view of molecular orbital theory where the molecular orbitals are superpositions of atomic orbitals. (In the language I used, if there are several possible descriptions of an electron corresponding to its presence near specific atoms, then descriptions which are superpositions of those are also allowed, which would correspond to the electron being "delocalised" among several atoms.)

In my opinion, a major source of confusion comes from the use of the word "empty" or "unoccupied" (say, in LUMO) as a qualifier for orbitals. This seems to give the impression that orbitals are physical regions in space. The correct way to see it is that an empty orbital simply means that there is no electron with the state description corresponding to that orbital. So an empty orbital isn't really an empty orbital but an absence of an electron that is described by the state vector corresponding to the orbital. If electrons didn't obey the exclusion principle, it would be easier to imagine things this way since then you wouldn't think of orbitals as "small pockets in space where only two electrons can fit in"

This is all just a matter of language from a philosophical point of view and doesn't of course matter to practising chemists (and, I think, unfortunately to a majority of physicists). However, I think it is necessary (and more illuminating) to think of things this way because it helps rid us of classical notions and develop a more quantum intuition.