A chemical system is in dynamic equilibrium, if the forward
chemical reaction and its reverse reaction are proceeding at
equal rates, so that there is no net change in the chemical
composition of the system.
Electron probe microanalysis, EPM, is the method of analysing
a very small quantity (i.e. as little as 10^-13 gram) of a
substance. The method consists of directing a very finely focused
beam of electrons on to the sample to produce the
characteristic X-ray spectrum of the elements present in the
sample. It can be used quantitatively for elements with atomic
numbers in excess of 11.
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An electrolyte is a compound which conducts an electric current when
in solution or in the molten state, and is simultaneously decomposed
by the current. In contrast with metals where the current is
carried by electrons, in electrolytes the current is carried
by the ions migrating through the liquid.
Electrolytic corrosion of a metal occurs through electrochemical
reactions.
Electrolytic gas is the highly explosive gas formed by
the electrolysis of water. It consists of two parts
hydrogen and one part oxygen by volume.
The electromagnetic spectrum is the range of wavelengths over
which electromagnetic radiation extends. The spectrum includes
- radio waves
- (10 ^ 5 to 10 ^ -3 meters), the longest waves,
- infrared waves
- (10 ^ -3-10 ^ -6 meters), next longest,
- visible radiation
- 4 ( 10 ^ -7 to 7 ( 10 ^ -7 meters),
- ultraviolet waves
- (10 ^ -7- to 10 ^ -9 meters) and
- X-rays and gamma radiation
- (10 ^ -9 to 10 ^ -14 meters).
The electron is an atomic particle. It travels in the orbitals
about the nucleus, which is the central core of the atom. It has
a unit negative charge and its mass is only 1/1850 that of
the proton. It is characterised in terms of its particle
and wavelike properties.
The electron affinity of an element is the energy released when
an electron is added to a neutral gaseous atom of that element. It
is measured in electron volts. The values of
the electron
affinities are difficult to determine, but some have been obtained
by a method involving the Born-Haber
Cycle.
The fluorine and chlorine atoms can each readily acquire an
electron to form the fluoride and chloride ions, respectively. These
ions have the Electronic
Configuration
of the noble gases (i.e. a full outer valency shell), and are therefore stable ions. The
reason for this is that the fluorine and chlorine atoms can
accommodate an electron in their 2p and 3p orbitals respectively and
that the attraction of the positively charged nucleus is great
enough to overcome the repulsion from the other five
electrons. A list of values for the electron affinities of
fluorine, chlorine, bromine, iodine, hydrogen, oxygen and sulphur
is given below.
Table of Electron Affinity (eV)
===============================
Reaction Ion Formed Electron Affinity, eV
============ ========== =====================
F + e(-) F(-) - 3.62
Cl + e(-) Cl(-) - 3.79
Br + e(-) Br(-) - 3.56
I + e(-) I(-) - 3.28
H + e(-) H(-) - 0.77
O + 2 e(-) O(2-) + 7.28
S + 2 e(-) S(2-) + 3.44
The Electron Affinity for an element depends on
- the atomic radius,
- the nuclear charge, and
- the screening effect of inner layers of electrons.
The decrease in value of the Electron Affinity on going
from chlorine to iodine is reasonable, because the electron
added to the iodine goes into the fifth level and is less
tightly bound to the nucleus. Hence the addition of this
electron releases less energy.
For the elements oxygen and sulphur which form di-negative
ions, the electron affinity values are negative. Although energy
is released when one extra electron is added, much more
energy is required to force the second electron into an ionised atom.
The electron affinity for the alkali elements of
Group 1 are assumed to be zero.
An electron deficient compound is one in which there are fewer
electrons forming the chemical bonds than required in normal
electron-pair bonds.
The electron microscope is a form of microscope that uses
a beam of electrons instead of a beam of light (as in
optical microscope) to form a large image of a
very small object.
Electron pair repulsion is the divergent force between the
electron pairs in the sub-orbitals of the valence shell
about an atom. There are two types of electron pairs :
- Bond Pairs :
- where the electron pair are shared in a covalent bond,
- Lone Pairs :
- where the electron pair are not shared in any bond.
There are three types of interaction between these different
types of electron pairs :
- bond pair : bond pair repulsion (bp:bp)
- where both electron pairs are shared in covalent bonds,
- lone pair : lone pair repulsion (lp:lp)
- where both electron pairs are not shared in any bond, and
- lone pair : bonding pair repulsion (lp:bp)
- where one electron pairs are shared in a covalent bond,
and the other electron pair is not shared in any bond.
The magnitude of the repulsion force between electron pairs
depend on the proximity of each pair to the central atom. Lone
pairs lie closer to the central atom than bond pairs, since
lone pairs have no other nearby positive nucleus to attract
them away from the center. Bond pairs are attracted by a
second nucleus and so they are drawn further away from the
central atom.
Repulsion between electron pairs is inversely proportional
to the distance between them. The repulsion between electron
pairs influence the shape of simple covalent molecules, because
the presence of a lone pair in a molecule tends to cause
bond pairs to come closer together. This will result in smaller
bond angles in the molecule; since it is the angle between
the bond pairs which dictates the bond angle.
Electron pairs are the two electrons which have opposite spins
that occupy the same sub-orbital in the valence shell of an
atom, or in the single covalent bond between atoms.
An electron volt (eV) is the energy acquired by an electron
when it passes through a potential difference of one volt.
1 eV = 1.60 * 10 ^12 erg = 96 kj/mole.
Electronegativity is the measure of the relative attraction
that atoms in a molecule have for shared pairs of electrons.
Tables of the electronegativity values of the elements have
been prepared by Millikan
by correlating ionisation potential and electron affinity data :
EN = (IP + EA) / 2
Later, Pauling used bond energies to prepare a scale in which
he assigned fluorine, the most electronegative element, a value 4.
The electronegativity values increase across each row of
the periodic table because the nuclear positive charge is
increasing and the atomic radius decreasing. Hence the closer
the outer level of electrons is to the nucleus, the more
difficult to attract electrons from the atom.
The values decrease down each group, because the increasing atomic
radius means a looser hold on outer level electrons and the effect
of the increasing nuclear positive charge is weakened
by the screening effect of the extra inner shells.
The uses of electronegativity values are that they help
to predict the type of bonding to be expected in a
molecule, as well as the polarity of covalent molecules. The
greater the difference in electronegativity values between
the combining elements, the more likelihood of an ionic bond; the
closer the values, the more likelihood of a covalent bond.
For simple bonds, if the difference of the
electronegativities on the Pauling Scale of the
elements between which the bond is formed is
greater than 1.9, then the bond will likely be
ionic. Similarly, if the difference of the
electronegativities on the Pauling Scale of the
elements between which the bond is formed is equal
to or less than 1.9 the bond will be predominantly covalent.
The electronic configuration of an atom is the arrangement
of electrons in the atomic orbitals about the nucleus of
the atom. The electrons of an atom travel in orbitals
about the nucleus of the atom. Each orbitals is a volume
in space in which the electrons located in that orbital travel,
giving rise to an electron cloud. The orbitals about an atom
have different energy levels. The electrons associated with a given
atom fill these orbitals of that atom according to the
following rules.
- The Aufbau Principle
- Electrons occupy the lowest energy levels (i.e. sub-orbital)
available when the atom is in the ground state.
- Hund's Rule of Maximum Multiplicity
- Where more than one orbital of equal energy is available, the
electrons will occupy each orbital singly, before filling any orbital
in pairs.
- Pauli's Exclusion
Principle
- A pair of electrons in any sub-orbital must have opposite
spins. Hence, no two electrons in the same atom can have
the same four Quantum Numbers.
To construct the electronic configuration of an atom from
these rules it is necessary to know the energy levels of
the different orbitals in an atom, or at least the sequence
of increasing energy levels of the orbitals.
Electronic publishing is the method of preparing and delivering
text material for access and display on a computer system. This
hypertext document is an example of electronic publishing.
The electrons in the outer shells (i.e. in the outer atomic orbitals)
of the atom determine to a great extent the chemical properties of
the elements.
Electrophiles are atoms or radicals which are attracted to the
electron rich sites in a molecule. Frequently, when an
electrophile attacks a carbon atom in a molecule, it results
in the displacement of another atom, or group, from that
carbon, giving rise to a substitution reaction.
Electrophilic additions reactions occur when the electrophile,
adds to the molecule without displacing any atom or group of
atoms already present in the molecule.
Electrophilic substitution reactions are substitution reactions at
electron rich carbon sites, where the substituting reagent is an
electrophile and it displaces another atom, or
radicals, from an electron rich carbon atom during the
course of the reaction.
Common electrophiles include the positively charged
nitronium ion, NO2(+), the positively charged chloronium
ion, Cl(+), and the positively charged methyl ion, CH3(+).
Electroplating is the process of coating the surface of one
metal object with a thin layer of another metal. The applied
metal is usually more expensive than the base metal that is
coated. Electroplating is used for the protection of the
metal which is coated, or to give a shining surface.
Electrorefining is process of using electrolysis
for the purification of metals. For example, when copper rods are
used as electrodes in the hydrolysis of a solution of
copper sulphate, if impure copper is used as anode, it dissolves
and pure copper is deposited on the cathode, leaving the
impurities behind in solution.
The electrovalent bond is another name for the Ionic
Bond.
Chemists once defined an element as a substance which could not
be broken down chemically to give two or more simpler
substances. Thus, the elements are the simplest chemical
entities. However, with the discovery of
Radioactivity,
this definition was not strictly correct. For example, the
element radium was found to be constantly decomposing into
different substances.
Chemists once defined an element as a substance in
which all the atoms were alike. However, the discovery of
Isotopes
showed that not all atoms of an element are alike. A
new definition was thus proposed :
An element is defined as a substance in which all of the
atoms have the same atomic number (i.e. they contain the same
number of protons in their nuclei).
Elements cannot be further sub-divided by chemical means.
Each element is unique and is composed of atoms with a
definite fixed atomic composition. There are 92 naturally occurring Elements on Earth.
Further elements have been synthesised in
nuclear reactions. All known matter on Earth is composed of
different combination of these elements. The majority of
elements are classified as metals, the remainder being
non-metals.
- Metals
- Iron, Copper, Tin, Manganese, Potassium, Sodium
- Non-Metals
- Carbon, Hydrogen, Iodine, Phosphorus, Nitrogen, Sulphur
From the 18th Century, chemists began to discover new elements.
This discovery of elements continued until 92 elements had
been identified. Chemists have been successful in synthesising
new elements so that the total has now risen to 103.
Every element has its own symbol, which represents one atom or
sometimes one mole of that element. The symbols of the
elements which were known in earlier days were derived from
Latin names. Many of the initial letters of the elements; others
have a second letter where there is more than one
starting with the small letter (e.g. Hydrogen has the
symbol H and Helium has the symbol He).
Some elements can exist in different physical forms, which differ
in colour, hardness, melting point, etc.. These different
forms are called allotropes. For example, carbon can exist
in several different forms, including graphite and diamond (which are
pure forms of carbon) and charcoal, coke and lampblack (which
are impure forms of carbon). Sulphur has five different allotropes.
An elevation of the boiling point of a solvent is observed
when substances are dissolved in it. The amount by which the
boiling point is elevated is proportional to the number of molecules
of solute and independent of their nature.
The molecular elevation of boiling point is the elevation
of the boiling point produced when one gram-molecular weight of
the solute is dissolved in 100 gm. of solvent.
See also Raoult's Law
Empirical data are the results that are obtained by experiment
or observation rather than from theory.
The empirical formula of a compound is the number of atoms of each
element the are present in a molecule of the compound in their
simplest ratio.
End Point
The end-point of a titration occurs when the
reaction is complete, as shown by the change of colour of
an indicator or by other means.
An endothermic reaction is a chemical reaction in which
heat is absorbed.
The energy changes in chemical reactions is manifested in
the heat which is either absorbed or released during the
course of the reaction.
Energy is the ability to do work, and it is measured
in joules. Energy is conveniently classified into two forms.
- Potential Energy
- which is the energy stored in a body or system as
a consequence of its position, shape or state (this
includes gravitational energy, electrical energy, nuclear energy
and chemical energy), and
- Kinetic Energy
- which is energy of motion and is usually defined as
the work that will be done by the work that will be done
by the body possessing the energy when it is brought to rest.
The energy levels of the atomic orbitals about the nucleus of
an atom are described by four
Quantum Numbers, and describe
the energy associated with an electron in the orbital.
Enthalpy, H, is the thermodynamic property of a system defined by
the relationship
H = U + pV
where
H is the Enthalpy,
U is the Internal Energy of the system,
p its Pressure, and
V its Volume.
The entropy, S, is a measure of the fraction of the energy
in a system that is unavailable to do work.
In a closed system, an increase in entropy is accompanied
by a decrease in energy available to do work.
Enzymes are proteins that acts as a catalyst
in biochemical reactions. Each enzyme is specific to a particular reaction
or group of similar reactions. The names of most enzymes end
in "-ase", which is added to the names of the substrates
on which they act.
The equation of state is an equation that relates the
pressure, p, the volume, V, and the thermodynamic temperature, T,
and the amount of substance, n. The simplest equation
of state (which applies only to ideal gasses) is the
ideal gas law
pV = nRT
where R is the universal gas constant.
A more accurate equation of state is :
(p + k)(V - nb) = nRT
where k is a factor that reflects the decreased pressure on
the walls of the container as a result of the attractive forces
between particles, and nb is the volume occupied by the particles
themselves when the pressure is infinitely high.
In the Van der Waal's equation of
state:
k = n^2 * a / V^2,
where a is a constant. This equation more accurately reflects
the behaviour of real gases.
Equilibrium is a state in which a system has its energy
distributed in the statistically most probable manner, in which
the forces, influences, reactions, etc., balance each other
out so that there is no net change over time. A system at
equilibrium shows no tendency to alter over time. A
body is said to be in thermal equilibrium with its
surroundings if no net heat exchange is taking place within
it or between it and its surroundings.
The equilibrium constant of a chemical reaction is the product
of the molar concentrations of the products divided by the
product of the molar concentrations of the reactants at a
specific temperature.
The equivalent weight of an element or compound is the
mass that combines with or displaces one gram of hydrogen
(or eight grams of oxygen or 35.5 grams of chlorine)
in a chemical reaction. The equivalent weight represents the
combining power of the substance. For an element it is the
relative atomic mass divided by the valency. For a
compound it depends on the reaction considered.
The equivalence point in a titration
is the point at which reaction is complete, as shown by the colour change of the
indicator or by any other means.
Eriochrome Black T is the indicator used in titrations.
Esters, RCOOR', are organic
compounds
of carbon, hydrogen and oxygen, that are the products formed by the
condensation reaction
between organic acids and alcohols.
The ester functional group, -C=O.O-, is contained within the molecule.
Ethanal, CH3.CHO, is the second member of the
aldehyde
series of compounds.
Ethane, CH3CH3, is the second member of the
alkane
series of hydrocarbons.
Ethanolamines form soaps with higher fatty acids,
which are excellent emulsifying agents and are used
extensively in the manufacture of shaving creams and
lotions, insecticide sprays, furniture polishes, and
dry-cleaning preparation.
Eutrophication is the enrichment of bodies of water by
nutrients which lead to the enhanced growth of algae. High
levels of algae in natural waters give rise to undesirable
consequence, including increased turbidity due to the
suspended solids, reduced levels of dissolved oxygen by
night, and increased synthesis of toxic compounds associated with
algae. Further, the organic compounds synthesised by algae
give rise to chloroorganics when these waters are extracted
and disinfected with chlorine for use as drinking water.
Evaporation is the change of state of a liquid into a vapour at a temperature below the boiling
point of the liquid.
Evaporation occurs at the surface of a liquid, some of those molecules with the highest
kinetic energies
escaping into the gas phase. The result is a fall in the average kinetic energy of the
molecules of the liquid and consequently a fall in its temperature.
Excitatition is a process in which a nucleus, electrons, atom,
ion, or molecule acquires energy that raises it to
a higher quantum state (i.e. excited state) than
that of its ground state. The difference between the
energy in the ground state and that in the excited state
is called the excitation energy.
Exothermic reactions are chemical reaction in which heat is evolved.
An experiment is a procedure used to check the validity of
an assumption or hypothesis, or to measure some characteristic
of a substance.
Modern explosives are organic compounds of carbon, hydrogen, nitrogen
and oxygen, whose the molecules contain sufficient oxygen to
convert the carbon and hydrogen in the molecule to carbon dioxide and
water and to liberate the nitrogen as a gas. The decomposition of
explosives is extremely rapid and highly exothermic.
The extraction of oxygen from air involves the liquefaction of air by cooling and the subsequent
fractional distillation of the liquid air to
separate the oxygen and nitrogen.
There are significant differences between the methods used
to extract the different halogens from their naturally
occurring sources.
Fluorine
Chlorine
Bromine
Iodine
Astatine
Start of Hypertext ....
Elements ....
Compounds ....
Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.