|
|
| Home > Science > sci > chem-faq > |
Sci.chem FAQ - Part 5 of 7 |
Section 1 of 2 - Prev - Next
Archive-name: sci/chem-faq/part5
Posting-Frequency: monthly
Last-modified: 22 October 1999
Version: 1.17
18.6 What is the most bitter compound?
Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
aminium benzoate [3734-33-6]. It is added to toxic chemicals ( such as
methylated spirits ) as a deterrent to accidental ingestion.
18.7 What is the sweetest compound?
Most scales use sucrose as a sweetness of 1, and compare the relative
sweetness of other sweeteners to sucrose.
Name Relative Sweetness Category
D-Glucose 0.46 Natural Food Product
Lactose 0.68 " " "
D-Fructose 0.84 " " "
Sucrose 1 " " "
Cyclamate 30 EC Permitted, USA Prohibited
Aspartame 200 EC, USA Permitted.
Saccharin 300 EC Permitted, USA Prohibited
Sucralose 650 Au, Ca Permitted, trials elsewhere
Alitame 2,000 Undergoing trials
Thaumatin 3,000 EC permitted, US chewing gum only.
Carrelame 160,000 Guanidine sweetener
Bernardame 200,000 " "
Sucrononate 200,000 " "
Lugduname 220,000 " "
The guanidine sweeteners are not expected to be approved for food use.
There are several other important attributes of sweeteners, such as
low toxicity, no after-taste, whether metabolised or excreted, etc.,
that must also be considered.
The potency scale is fairly flexible, and differing publications can
assign different values. The August 1995 copy of the Journal of Chemical
Education contained several papers from a symposium on sweeteners [3,4],
and an article in Chemistry and Industry also discusses sweeteners from
both natural and artificial sources [5], and Kirk Othmer has a monograph
on sweeteners.
The sweetener used in "diet" beverages is usually Aspartame, and they
are usually required to display a warning for phenylketurics that the
product contains a source of phenylalanine. As Aspartame slowly degrades
in acid solutions, such products also have a "use-by" date.
Although banned by the FDA in 1970 ( because a mixture of saccharin and
cyclamate caused tumours in test animals ), saccharin has been still
marketed under extensions of approval, Ironically, subsequent work
implicated the saccharin, and the cyclamate was found not to be the
tumour-causing agent, but it is still banned.
18.8 What salts change the colour of flames?.
Both Vogel ( qualitative inorganic ) and the Rubber Handbook list details of
flame tests for elements. The spectra of the alkaline earth compounds are
relatively complex, so using filters to view the flame can change the colour
observed as dominant lines are filtered out. In general, except for copper,
any compound of an element can be used, however toxic salts ( such as
cyanides ) should not be used. Halogen salts are usually readily available,
and are reasonably volatile. In all cases, perform experiments in a
well-ventilated area - preferably a fume hood. The emission spectra in the
visible region is the sum of several emission lines, with dominant lines
masking others. The visible spectrum is approximately :-
Red 800 - 620 nm
Orange 620 - 600 nm
Yellow 600 - 585 nm
Green 585 - 505 nm
Blue 505 - 445 nm
Violet 445 - 400 nm
There are also the various bead tests employing borax ( sodium tetraborate
Na2B4O7.10H2O ), Microcosmic salt ( NaNH4HPO4 ), or sodium carbonate
(Na2CO3), using both oxidising and reducing flames. The bead test procedures
are detailed in Vogel ( qualitative inorganic ), and similar texts.
Element Colour Some of the contributing lines, and comments.
Arsenic Light Blue 449.4 nm, 450.7 nm.
( Arsenic is highly toxic - only perform in fume hood under supervision )
Barium Green-Yellow 553.6 nm, 539.1 nm, 536.1nm, 614.2 nm.
Blue (faint) 455.4 nm, 493.4 nm.
Cesium Red-Violet 852.1 nm.
Calcium Orange 618.2 nm, 620.3 nm.
Yellow-Green 530.7 nm, 559.5 nm.
Violet (faint) 422.7 nm.
Greenish with blue glass.
Copper Emerald Green 521.8 nm, 529.2 nm, 515.3 nm.
Not chloride, or in presence of HCl
Azure Blue 465.1 nm.
Copper chloride, or HCl present
Lead Light Blue 500.5 nm.
( Lead is highly toxic - only perform in fume hood under supervision )
Lithium Carmine Red 670.78 nm, 670.79 nm.
Orange (faint) 610.1 nm.
Violet with blue glass
Potassium Red 766.5 nm, 769.9 nm.
Violet 404.4 nm, 404.7 nm.
Purple-red with blue glass
Rubidium Violet 780.0 nm, 794.8 nm.
Sodium Yellow 589.0 nm, 589.6 nm.
Invisible when viewed with blue glass
Strontium Scarlet Red 640.8 nm, 650.4 nm, 687.8 nm, 707.0 nm.
Violet 460.7 nm, 421.5 nm, 407.8 nm.
Violet with blue glass
Tellurium Green 557.6 nm, 564.9 nm, 566.6 nm, 570.8 nm.
( Tellurium is highly toxic - only perform in fume hood under supervision )
Thallium Green 535.0 nm.
( Thallium is highly toxic - only perform in fume hood under supervision )
Zinc Whitish Green Large number of peaks between 468.0-775.8 nm.
( Zinc fumes are toxic - only perform in a fume hood under supervision )
Impressive coloured flames have been obtained using chlorides and a methanol
flame in a petri dish [6]. Even more spectacular results have been obtained
by nitrating cellulose filter paper, and impregnating it with salts prior
to ignition [7].
18.9 What chemicals change colour with heat, light, or pressure?.
Compounds that visibly and reversibly change colour when subjected to a
change in their environment are known as chromogenic materials. There are
four major categories - electrochromic, photochromic, piezochromic, and
thermochromic, all of which are extensively discussed in a recent, well
referenced, monograph in Kirk Othmer [8].
Electrochromic materials exhibit a change in light transmittance or
reflectance induced by direct current at potentials of approximately one
volt. The change usually is an oxidation-reduction reaction, using either
inorganic or organic compounds, and the colour change can occur at either
the anode or the cathode - which are usually thin films. There are two major
classes, the ion-insertion/extraction type - such as tungsten trioxide, and
the noninsertion group - such as the viologens, a family of halides of
quaternary bases derived from 4,4'-bipyridinium. One viologen example is
1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
clear to bluish purple. The most common application of viologens has been
the electrochromic interior rearview mirrors available for cars since 1988.
These utilise a substituted viologen as the cathode colouring material, with
a compound like phenylene diamine as the anode colouring electrochromic
material. The mechanism details, along with a description of the ingenious
control system, are described in a recent comprehensive review of
electrochromic materials [9].
Photochromic materials undergo a reversible change in light absorption that
is induced by electromagnetic radiation, however most common applications
involve reversible changes in colour or transparency on exposure to visible
or ultraviolet light. This is often seen as a change in the visible spectrum
( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
of photochromic materials, inorganic and organic.
Examples of the inorganic type are the silver halides, which are suspensions
of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
has been slowly cooled. An alternative technique involves diffusion of the
silver halide into the surface of the glass. The cuprous ion can catalyse
both the photochromic darking and thermal fading reactions, and the colour
can be shifted from grey to brown by the addition of gold or palladium -
which may be added to the glass in trace amounts. The most popular current
application for glass containing silver halide is for prescription eyewear.
The organic photochromic systems can be subdivided according to the type of
reaction. Geometric isomerism can result in different optical properties,
eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the
cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
Cycloaddition can produce photochromism, such as the reversible formation of
the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red
parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride
[76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole
[63245-02-3] to form a red-purple free radical ), may also produce
photochromism.
UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N
[226-36-8] ), to their triplet state, which has a different absorption
spectrum. Viologens may undergo redox reactions and exhibit photochromic
behaviour when crystalline and subjected to UV. The most popular photochromic
materials utilise reversible electrocyclic reactions, and are often indolino
spiropyrans and indolino spiroxazines, however the mechanism also covers
fulgide, stilbene, and dihydroindolizine examples. Details and structures
are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
Education has published descriptions and preparation techniques for both
inorganic [10] and organic photochromic compounds and sunglasses [11].
Piezochromic materials change colour as they are compressed. There are three
common types:- organic molecules ( such as N-salicylidene-2-chloroaniline
[3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates,
(Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
compounds like ethylene diamine. They are still being researched, and
interested readers should investigate the references in the Kirk Othmer
monograph [8].
Thermochromic materials reversibly change colour as their temperature is
changed. There are a very large number of systems, but one common example
of thermochromic transitions in metal complexes is the transition between
the blue tetrahedral and pink octahedral coordinations of cobalt (II) when
cobalt chloride is added to anhydrous ethanol and the temperature changed.
Examples of thermochromic transitions in inorganic compounds include
Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large
changes occurring in the infra-red range, and are being considered for IR
imaging applications.
There are thousands of organic thermochromic compounds, with well known
examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced
heterolytic bond cleavage resulting in ring opening), poly(xylylviologen
dibromide [38815-69-9] ( charge transfer interactions resulting in hydration-
dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
induced transitions in the unsaturated backbone resulting in rearranged side
groups ). Information on photochromism in organic and polymeric compounds is
available in published reviews [12,13].
------------------------------
Subject: 19. Physical properties of chemicals
19.1 Rheological properties and terminology
Contributed by Jim Oliver
RHEOLOGY
What is RHEOLOGY ?
RHEOLOGY describes the deformation of a material under the influence of
stresses. Materials in this context can be solids, liquids or gases. In this
FAQ we will be concerned only with the rheological properties of liquids.[1]
Perry discusses the some aspects of the behaviour of gases, and Ullmann
discusses elastic solids.
When liquids are subjected to stress they will deform irreversibly and flow.
The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
are very few, whereas non-ideal examples abound. Ideal liquids are : water
and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
mixed with a little water. [2]
What is VISCOSITY ?
VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
conditions used to measure the VISCOSITY must be given. This is due to the
fact that non-ideal liquids have different values of VISCOSITY for different
test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
curve reveals the particular type of VISCOSITY for the liquid being studied.
[3]
What is a NEWTONIAN LIQUID ?
NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
are mineral oil, water and molasses. (Isaac NEWTON first described the laws
of viscosity) [1] All the other types are NON NEWTONIAN.
What does NON NEWTONIAN mean ?
a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
the origin again and curving up and along but falling under the straight
line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
increasing shear. Examples are toothpaste and whipped cream.
b. DILATANT liquids give a curve which curves under then upward and higher
than the straight line NEWTONIAN curve. (Like a square law curve) Such
liquids display increasing viscosity with increasing shear. Examples are
wet sand, and mixtures of starch powder with small amounts of water. A car
may be driven at speed over wet sand, but don't park on it, as the car may
sink out of sight due to the lower shear forces (compared to driving over)
the wet sand.
There are other terms used which include :
THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
What this means is the curve runs straight up the y axis for a short way then
curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
This YIELD POINT is time dependant. Some water based paints left overnight
develop a FALSE BODY which only breaks down to become useable after rapid
stirring. Also: the curve describing a THIXOTROPIC liquid will be different
on the way up (increasing shear rate) to the way down (decreasing shear rate).
The area inside these two lines is a measure of it's degree of THIXOTROPY.
This property is extremely important in industrial products, e.g to prevent
settling of dispersed solids on storage. [3]
A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
viscosity with a constant shear rate over time. Again, time dependant but in
this case _increasing_ viscosity.
Why do some liquids become solid ?
A few special liquids (dispersions usually) display extraordinary DILATANT
properties. A stiff paste slurry of maize or cornflour in water can appear to
be quite liquid when swirled around in a cup. However on pouring some out
onto a hard surface and applying extreme shear forces (hitting with a hammer)
can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
VISCOSITY can become so high as to make it appear solid. The "liquid" then
becomes very stiff for an instant and can shatter just like a solid material.
It should be noted that the study of viscosity and flow behaviour is
extremely complex. Some liquids can display more than one of the above
properties dependant on temperature, time and heat history.
What are Electrorheological Fluids? ( added by Bruce Hamilton )
Electrorheological (ER) fluids change their flow properties when an electric
field is applied, and are usually dispersions of polarizable particles in an
insulating base fluid [5]. Their apparent viscosity can change by orders of
magnitude in milliseconds when a fews watts of electrical power are applied.
The shear stress versus shear rate properties of ER fluids vary as a function
of the applied electric field, When an electric field is applied, the fluid
switches from a liquid to semisolid. The particles are usually irregularly-
shaped 0.5-100um and present at concentrations of 10-40% by mass. ER fluids
are dielectric particles in an insulating medium ( such as silicone oil ),
along with additives ( such as surfactants, dispersants, and possibly a
polar activator ). ER fluid effectively function as leaky capacitors. The
electric field can be either AC, pulsed DC, or DC, with AC producing less
electrophoresis of particles to electrodes.
There are two categories of ER particulate materials, extrinsically
polarizable materials ( which require a polar activator ), and intrinsically
polarizable materials. Extrinsically polarizable materials can be polar
nonionic compounds ( such as silica, alumina, or polysaccharides ), or polar
ionic materials ( such as the lithium salt of polymethacrylic acid ),
Intrinsically polarizable materials provide simpler systems - because a polar
activator is not required, and they have a lower thermal coefficient of
conductance. The most common examples are the ferroelectrics like barium
titanate (BaTiO3 ) and polyvinylidene difluoride, however their performance
has been poor, as has been that of metal powders ( such as iron and
aluminium - even when coated with an insulating layer ), and research is
concentrating on conducting polymers ( such as polyanilines and pyrolysed
hydrocarbons ) [5,6].
The ability to utilise computer-based electrical switching to control ER
fluid properties has resulted in vehicle suspension and industrial vibration
control as major target applications for ER fluids. Demonstration systems
have been built, and they match performance predictions, however cost and
durability issues still have to be solved [7].
19.2 Flammability properties and terminology
There are several properties of flammable materials that are frequently
reported. It should be remembered that most discussions concerning
flammable liquids usually consider air as the oxidant, but oxygen and
fluorine can also be used as oxidants for combustion, and they will result
in very different values.
The flammability limits in air are usually reported as the upper and lower
limits ( in volume percent at a certain temperature, usually 25C ), and
represent the concentration region that the vapour ( liquid HCs can not burn )
must be within to support combustion. Hydrocarbons have a fairly narrow range,
( n-hexane = 1.2 to 7.4 ), whereas hydrogen has a wide range ( 4.0 to 75 ).
The minimum ignition energy is the amount of energy ( usually electrical )
required to ignite the flammable mixture. Some mixtures only require a very
small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ),
whereas others require more (eg methanol = 0.14mJ, n-hexane = 0.29mJ,
diethyl ether = 0.20mJ, acetone = 1.15mJ, dichloromethane = 133mJ @ 88C ),
and some require significant amounts, (eg ammonia = >1000mJ ).
The flash point is the most common measure of flammability today, especially
in transportation of chemicals, mainly because most regulations use the flash
point to define different classes of flammable liquids. The flash point of a
liquid is the temperature at which the liquid will emit sufficient vapours
to ignite when a flame is applied. The test consists of placing the liquid
in a cup and warming it at a prescribed rate, and every few degrees applying
a small flame to the air above the liquid until a "flash" is seen as the
vapours burn. Note that the flame is not applied continuously, but is
provided at prescribed intervals - thus allowing the vapour to accumulate.
There are a range of procedures outlined in the standard methods for
measuring flash point ( ASTM, ISO, IP ) and they have differing cup
dimensions, liquid quantity, headspace volume, rate of heating, stirring
speed, etc., but the most significant distinction is whether the space above
the liquid is enclosed or open. If the space is enclosed, the vapours will be
contained, and so the flash point is several degrees lower than if it is
open. Most regulations specify closed-cup methods, either Pensky-Martens
Closed Cup or Abel Closed Cup. It is important to remember that these methods
are only intended for pure chemicals, if there is water or any other volatile
non-flammable compounds present, their vapours can extinguish or mask the
flash. For used lubricants, this may be partially overcome by using the TAG
open cup procedure - which is slightly more tolerant of non-flammable
vapours. A material can be flammable, but may not have a flash point if other
non-flammable volatile compounds are present. For alkane hydrocarbons, flash
point increases with molecular weight.
There is an older measure, called the fire point, which is the temperature
at which the liquid emits sufficient vapours to sustain combustion. The fire
point is usually several degrees above the flash point for hydrocarbons.
The minimum autoignition temperature is the temperature at which a material
will autoignite when it contacts a surface at that temperature. The procedure
consists of heating a glass flask and squirting small quantities of sample
into it at various temperatures until the vapours autoignite. The only
source of ignition is the heat of the surface. For the smaller hydrocarbons
the autoignition temperature is inversely related to molecular weight, but it
also increases with carbon chain branching. Autoignition temperature also
correlates with gasoline octane ratings ( refer to Gasoline FAQ available in
rec.autos.tech, which lists octane ratings and autoignition temperatures for
a range of hydrocarbons.)
Flash Point Autoignition Flammable Limits
Temperature Lower Upper
( C ) ( C ) ( vol % at 25C)
methane -188 630 5.0 15.0
ethane -135 515 3.0 12.4
propane -104 450 2.1 9.5
n-butane -74 370 1.8 8.4
n-pentane -49 260 1.4 7.8
n-hexane -23 225 1.2 7.4
n-heptane -3 225 1.1 6.7
n-octane 14 220 0.95 6.5
n-nonane 31 205 0.85 -
n-decane 46 210 0.75 5.6
n-dodecane 74 204 0.60 -
n-tetradecane 99 200 0.50 -
19.3 Supercritical properties and terminology?
Supercritical fluids have some very unusual properties. When a compound is
subjected to conditions around the critical point ( which is defined as
the temperature at which the gas will not revert to a liquid regardless how
much pressure is applied ), the properties of the supercritical fluid become
very different to the liquid or the gas phases. In particular, the solubility
behaviour changes. The behaviour is neither that of the liquid or that of the
gas. The transition between liquid and gas can be completely smooth.
The pressure-dependant densities and corresponding Hildebrand solubility
parameters show no break on continuity as the supercritical boundary is
crossed. Physical properties fall between those of a liquid and a gas.
Diffusivities are approximately an order of magnitude higher than the
corresponding liquid, while viscosities are an order of magnitude lower.
These properties ( along with low surface tension ) allow SCFs to have
liquid-like solvating power with the mass transport characteristics of
a gas.
Potential Supercritical Fluids
Compound Critical Critical Density
Temperature Pressure
( C ) ( bar ) (g cm^-3)
Ammonia 132.4 112.8 0.235
Carbon dioxide 30.99 73.75 0.468
CFC-12 111.8 41.25 0.558
Dimethyl ether 126.9 52.7 0.271
Ethane 32.4 49.1 0.212
HCFC-22 96.15 49.90 0.524
HCFC-123 183.68 36.62 0.550
HFC-116 19.7 29.8 0.608
HFC-134a 101.03 40.57 0.508
Methanol 240.1 83.1
Nitrous oxide 36.4 72.54 0.453
Propane 96.8 42.66 0.225
Water 374.4 227.1
Xenon 16.6 58.38 1.105
Nitrous oxide is seldom used because early researchers reported explosions.
Note that using liquid CO2 at pressure ( as for the commercial extraction
of hops ) is still just liquid CO2 extraction, not supercritical CO2
extraction. There are several good general introductions to supercritical
fluids [8,9,10]
19.4 Formation of gaseous bubbles in liquids
Discussions about the behaviour of dissolved gases in liquids, especially
when discussing carbonated beverages, are usually more appropriate in
sci.physics and/or sci.mech.fluids, and there is a good text available [11].
Section 23.9 of this FAQ lists the change in solubility with temperature
for common atmospheric gases in water at near-ambient pressure. As the
temperature increases, the solubility decreases, creating a supersaturated
solution that can result in bubble formation. A similar effect occurs if the
pressure is reduced. The formation of bubbles can be understood in
thermodynamic terms using the Gibbs free energy of the bubble.
Gibbs free energy = -n * R * T ln(C/Cs) + gamma * A
A = Surface area of the bubble.
C = Concentration of gas in the liquid,
Cs = Concentration of gas in the liquid at saturation,
gamma = Interfacial tension between the gas and the liquid
n = Number of moles of gas in the bubble
= (P*V)/(R*T), where P = pressure, and V = volume of a sphere.
R = Gas Constant
T = Temperature
After inserting the expressions for the surface area of a sphere (r = radius)
and number of moles, and differentiating, then we obtain:-
r(mininum) = 2 * gamma / ( P * ln(C/Cs))
This describes the size of a bubble that would continue to grow under the
existing conditions, rather than redissolve. Of course, the expression
assumes homogeneous precipitation of the bubble, and in real life most
bubbles are created heterogeneously. Statistics and kinetics are also
required to determine the rate of formation of bubbles, and predict the
effect of changing parameters such as temperature. As the liquid is warmed,
bubbles may be created faster, as the higher temperatures overcome the
activation barrier - which is the difference between the Gibbs free energy
when r is less than r(minimum), and the Gibbs free energy at r(minimum).
The formation of a bubble also dramatically perturbs the system, even
causing secondary bubbles to form. Secondary bubble formation may be
implicated in the production of copious quantities of froth from shaken,
quickly-opened, carbonated drink containers. The sites for gaseous bubble
formation in supersaturated drinks are typically small particles, or minor
flaws on the smooth surface of the container.
19.5 Why is Mercury a liquid at room temperature?.
First, let's look at the melting points of some of the elements surrounding
mercury in the periodic table ( in degrees C ) :-
Period IB IIB IIIA
4s3d4p Cu 1083 Zn 419.5 Ga 29.8
5s4d5p Ag 960.8 Cd 320.9 In 157
6s(4f)5d6p Au 1063 Hg -38.4 Tl 304
The interesting comparison is between Hg and Au, as their properties differ
dramatically, although their electron structures are similar:-
14 10 1
Au(g) : Xe | 4f , 5d , 6s
79 54
14 10 2
Hg(g) : Xe | 4f , 5d , 6s
80 54
Very few chemistry textbooks discuss relativistic effects on chemical
properties, despite the availability of a comprehensive review by P.Pyykko
[12]. There several good introductory articles on the derivation and
calculation of various relativistic effects in molecules and atoms, so I'm
not going to include details [13,14,15]. Suffice to say, that whilst
smaller elements can treated simply, larger elements need treatment based
on the Dirac equation, which shows that the s electrons are approaching
the speed of light, consequently relativistic effects are important.
If we take the relativistic mass of mercury (m);-
Mo where
m = -------------------- c (speed of light) = ~137 atomic units
_____________ v = Z = 80
/ ( v ) 2 Mo = rest mass
/ 1 - ( - )
\/ ( c )
The masses of the 1s electrons are increased by approximately 20% over
their rest masses, which means that the radius is decreased by 20% - since
mass appears in the denominator of Bohr radius calculations. All the other
s shells also contract, with the 6s contracting ~14%, because their electron
speeds near the nucleus are comparable, and the contraction of the inner part
of the wave function also pulls in the outer tails. The p orbitals also
contract a similar amount, and these contractions also results in increases
the screening for d and f orbitals, which may then expand - about 3% for the
5d orbital of mercury.
In mercury, the relativistically-contracted 6s2 orbital is full, thus the
the two electrons do not contribute much to the metal-metal bond, which is
Section 1 of 2 - Prev - Next
| Back to category chem-faq - Use Smart Search |
| Home - Smart Search - About the project - Feedback |
© allanswers.org | Terms of use