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Hydrogen Bomb; Nuclear Weapons

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Hydrogen BOMB!
An explosive weapon of enormous destructive power caused by the fusion of the nuclei of various hydrogen isotopes in the formation of helium nuclei.
A device in which an uncontrolled, self-sustaining, thermonuclear fusion reaction is carried out in heavy hydrogen (deuterium or tritium) to produce an explosion. In a fusion reaction, the collision of two energy-rich nuclei results in a mutual rearrangement of their protons and neutrons to produce two or more reaction products, together with a release of energy of amount E given by A. Einstein's formula E = mc², where m is the mass difference between the original and produced nuclei, and c is the velocity of light.

For the hydrogen bomb reaction to become self-sustaining, a so-called critical temperature of about 3.5 × 10⁷ K (6.3 × 10⁷°F) must be attained with the aid of the enormous temperature created by a fission explosive. Once this temperature is achieved, the energy released in the initial reaction maintains the temperature, and the chain proceeds either until the supply of fusionable material is exhausted or until sufficient expansion has taken place that the material is cooled below the critical temperature.

There are two ways to use fusion:
boosting of fission explosive yields or generating multistage thermonuclear reactions.

In a fusion-boosted warhead, when the sphere of fissile materials is compressed (imploded) by the chemical explosion, an uncontrolled fission chain reaction begins. If there is fusionable material inside the device, thermonuclear reactions will boost the fission yield. The fusion reactions do not directly contribute very much to the explosive energy, but instead enhance the fission rate, due to the release of a large number of additional neutrons.

If the massive casing is made mostly of uranium-238 (natural or depleted uranium), neutrons from the thermonuclear reactions will cause the uranium nuclei to undergo fission, giving off still more energy. A device of this sort can be regarded as a three-stage fission-fusion-fission bomb.

The yield, or total energy, of a hydrogen bomb is expressed in megatons (1 megaton equals 10¹⁵ calories or 4.18 × 10¹⁵ joules). Typical fusion-boosted weapons yield hundreds of kilotons (tenths of megatons), and typical multistage weapons yield megatons.

NUCLEAR WEAPONS

Nuclear weapons derive their explosive power from the fission (splitting) and fusion (combining) of atoms. Fusion devices need to be combined with a nuclear fission weapon to generate the intense heat necessary to begin the still more powerful process of fusion. Fusion weapons—the ‘H’ (hydrogen) bomb—can be a thousand times more powerful than fission weapons and these opened the horrific possibility of glob
al destruction through nuclear missile war. Many early ‘fusion’ weapons were in fact ‘boosted fission devices’, gaining most of their power from the fission explosion with a fusion component to enhance its efficiency. Military requirements have also led to enhanced radiation/reduced blast weapons, the so-called ‘neutron bomb’, in which the immediate radiation is multiplied in order to kill troops rather than destroy installations.

The idea of a source of enormous e
nergy for motive power or weapons featured in the work of 19th-century science-fiction writers including Jules Verne, George Earle Bulwer-Lytton, and H. G. Wells who talks of atomic bombs in The World Set Free. By 1914 the Newtonian view that the universe consisted of lumps of indestructible matte
r had given way to the realization that matter could be transformed into energy.

It was not until the eve of WW II that the practical possibility of a nuclear weapon was understood. On 2 August 1939, Albert Einstein signed a letter to Pres Franklin D. Roosevelt, saying that recent work in France and the USA had indicated the possibility of setting up a nuclear chain reaction in a large mass of uranium. This new phenomenon could ‘also lead to the construction of bombs and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port, together with some of the surrounding territory. However, such bombs might very well prove too heavy for transportation by air.’ He was wrong on the last point, and six years and four days later the US dropped the first atomic bomb over Hiroshima, the result of the Manhattan Project.

The first test bomb ever to be exploded, at Alamogordo in the New Mexico desert on 16 July 1945, ‘the Gadget’, was an ‘implosion’ device, with a hollow plutonium core weighing about 8.3 lb (3.8 kg), compressed to critical density by about 4, 866 lb (2, 270 kg) of high explosive. The ‘yield’—the size of the explosion—was 22 kilotons. Nuclear weapon yields are measured as kilotons (each 1, 000 tons of TNT) or megaton
s (one million tons of TNT).

The first bomb to be dropped on Japan on 6 August was of a different type—a ‘gun-assembly’ device called ‘Little Boy’. It was cruder than the Gadget—132.3 lb (60 kg) of highly enriched uranium in two pieces, one of which was fired at the other down a gun-type barrel, producing a far less efficient yield of 12 to 15 kilotons. The US B-29 bomber Enola Gay, named after the pilot's mother, carried the bomb to Hiroshima from its base at Guam, escorted by two other planes carrying observers and instruments. The bomb had been brought from the USA by the cruiserIndianapolis, which was to be sunk by a Japanese submarine on its return trip. The city was obliterated.

Although simple in principle, nuclear weapons are difficult to engineer in practice. The 'gun assembly' type is less efficient but more robust and favoured for artillery shells. The implosion type is more difficult to build and more fragile, but is more efficient and is essential to exploit the advantages of fusion
(Click to enlarge)

The second operational bomb, ‘Fat Man’, dropped on Nagasaki on 9 August, was of the same type as the Gadget. Implosion devices are more efficient than the gun type and do not require high-grade uranium, but they are more complex in their design and more delicate, requiring precise co-ordination of the timing for the detonation of the surrounding explosive charges, and are unsuitable to withstand the stress of being launched from a gun. For this reason, gun-assembly devices retain their importance as nuclear artillery shells, while implosion devices tend to be carried on rockets.

The detonation of a nuclear weapon produces an incandescent ‘fireball’ in which everything is vaporized. Being hot, the fireball climbs upwards, generating the characteristic ‘mushroom cloud’ as it pulls debris from the ground up with it. Nuclear weapons have five main effects. The first is heat, or ‘flash’, causing burns and eye damage far from the immediate source of the explosion. The second is blast, caused by a shock wave moving outwards from the fireball at about the speed of sound. Third is immediate radiation travelling at the speed of light-neutron radiation at shorter distances, and gamma radiation further out. Most of the latter is generated after the explosion, so a dive for cover may still be worth while. Fourth is residual radiation: the neutrons from the explosion make other material radioactive, which falls back to earth as fallout. Fifth is electromagnetic pulse (EMP), which burns out electronic apparatus. A multi-megaton weapon burst high above the earth, outside the atmosphere, would not have perceptible flash-and-blast effects but could knock out power grids and electrical equipment over a wide area.

After the explosion of the bombs over Japan, some thought the nuclear weapon would render traditional forms of war obsolete. Others were sceptical about the bomb's power, although some used this to try to discourage its development. As early as October 1945, Sir George Thompson, participating in the revision of the British Tizard report on future warfare, foresaw that the bomb's very potency might preclude its use. Whereas the tendency had been to wage war more and more unrestrictedly, it was possible that the nuclear weapon might reverse this trend. Therefore Britain should keep ‘pre-atomic defence forces’.

Here's a video of the reenactment of the atomic bomb that exploded in Hiroshima...

http://www.answers.com/topic/hydrogen-bomb

http://www.answers.com/topic/nuclear-weapon

Orbital Theories

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Molecular Orbital Theory

The goal of molecular orbital theory is to describe molecules in a similar way to how we describe atoms, that is, in terms of orbitals, orbital diagrams, and electron configurations. For example, to give you a glimpse at where we are headed, the following are orbital diagrams for O₂ and O.

O₂ O

Each line in the molecular orbital diagram represents a molecular orbital, which is the volume within which a high percentage of the negative charge generated by the electron is found. The molecular orbital volume encompasses the whole molecule. We assume that the electrons would fill the molecular orbitals of molecules like electrons fill atomic orbitals in atoms.

The molecular orbitals are filled in a way that yields the lowest potential energy for the molecule.
The maximum number of electrons in each molecular orbital is two. (We follow the Pauli exclusion principle.)
Orbitals of equal energy are half filled with parallel spin before they begin to pair up. (We follow Hund's Rule.)

Before we continue with a description of a model used to generate molecular orbital diagrams, lets get a review of light and electron waves and how two waves can interact. The wave description of light describes the effect that the light has on the space around it. This effect is to generate an oscillating electric and magnetic fields. These fields can vary in intensity, which is reflected in varying brightness of light.

electromagnetic wave

The wave description of the electron describes the variation in the intensity of negative charge generated by the electron.

1s waveform

Light waves can interact in-phase, which leads to an increase in the intensity of the light (brighter) and out-of-phase, which leads to a decrease in the intensity of the light (less bright). Electron waves can also interact in-phase and out-of-phase. In-phase interaction leads to an increase in the intensity of the negative charge. Out-of-phase interaction leads to a decrease in the intensity of the negative charge.

One common approximation that allows us to generate molecular orbital diagrams for some small diatomic molecules is called the Linear Combination of Atomic Orbitals (LCAO) approach. The following assumptions lie at the core of this model.

Molecular orbitals are formed from the overlap of atomic orbitals.
Only atomic orbitals of about the same energy interact to a significant degree.
When two atomic orbitals overlap, they interact in two extreme ways to form two molecular orbitals, a bonding molecular orbital and an antibonding molecular orbital.

For example, our model assumes that two 1s atomic orbitals can overlap in two extreme ways to form two molecular orbitals. One of the ways the atomic orbitals interact is in-phase, which leads to wave enhancement similar to the enhancement of two in-phase light waves. Where the atomic orbitals overlap, the in-phase interaction leads to an increase in the intensity of the negative charge in the region where they overlap. This creates an increase in negative charge between the nuclei and an increase in the plus-minus attraction between the electron charge and the nuclei for the atoms in the bond. The greater attraction leads to lower potential energy. Because electrons in the molecular orbital are lower potential energy than in separate atomic orbitals, energy would be required to shift the electrons back into the 1s orbitals of separate atoms. This keeps the atoms together in the molecule, so we call this orbital a bonding molecular orbital. The molecular orbital formed is symmetrical about the axis of the bond. Symmetrical molecular orbitals are called sigma, σ, molecular orbitals. The symbol σ_1s is used to describe the bonding molecular orbital formed from two 1s atomic orbitals.

The second way that two atomic orbitals interact is out-of-phase. Where the atomic orbitals overlap, the out-of-phase interaction leads to a decrease in the intensity of the negative charge. This creates a decrease in negative charge between the nuclei and a decrease in the plus-minus attraction between the electron charge and the nuclei for the atoms in the bond. The lesser attraction leads to higher potential energy. The electrons are more stable in the 1s atomic orbitals of separate atoms, so electrons in this type of molecular orbital destabilize the bond between atoms. We call molecular orbitals of this type antibonding molecular orbitals. The molecular orbital formed is symmetrical about the axis of the bond, so it is a sigma molecular orbital with a symbol of σ*_1s. The asterisk indicates an antibonding molecular orbital.

The following diagram shows the bonding and antibonding molecular orbitals formed from the interaction of two 1s atomic orbitals.

When two larger atoms atoms combine to form a diatomic molecule (like O₂, F₂, or Ne₂), more atomic orbitals interact. The LCAO approximation assumes that only the atomic orbitals of about the same energy interact. For O₂, F₂, or Ne₂, the orbital energies are different enough so only orbitals of the sameenergy interact to a significant degree.

Like for hydrogen, the 1s from one atom overlaps the 1s from the other atom to form a σ_1s bonding molecular orbital and a σ*_1s antibonding molecular orbital. The shapes would be similar to those formed from the 1s orbitals for hydrogen. The 2s atomic orbital from one atom overlaps the 2s from the other atom to form a σ_2s bonding molecular orbital and a σ*_2s antibonding molecular orbital. The shapes of these molecular orbitals would be similar to those for the σ_1s and σ*_1s molecular orbitals. Both σ_2s and σ*_2s molecular orbitals are higher energy and larger than the σ_1s and σ*_1s molecular orbitals.

The p atomic orbitals of the two atoms can interact in two different ways, parallel or end-on. The molecular orbitals are different for each type of interaction. The end-on interaction between two 2p_x atomic orbitals yields sigma molecular orbitals, which are symmetrical about the axis of the bond.

sigma MOs from 2p

The two 2p_y atomic orbitals overlap in parallel and form two pi molecular orbitals. Pi molecular orbitals are asymmetrical about the axis of the bond.

pi MOs from 2p

The 2p_z-2p_z overlap generates another pair of π_2p and π*_2p molecular orbitals. The 2p_z-2p_z overlap is similar to the The 2p_y-2p_y overlap. To visualize this overlap, picture all of the orbitals in the image above rotated 90 degrees so the axes that run through the atomic and molecular orbitals are perpendicular to the screen (paper). The molecular orbitals formed have the same potential energies as the molecular orbitals formed from the 2p_y-2p_yoverlap.

There is less overlap for the parallel atomic orbitals. When the interaction is in-phase, less overlap leads to less electron charge enhancement between the nuclei. This leads to less electron charge between the nuclei for the pi bonding molecular orbital than for the sigma bonding molecular orbital. Less electron character between the nuclei means less plus-minus attraction, less stabilization, and higher potential energy for the pi bonding molecular orbital compared to the sigma bonding molecular orbital.

When the interaction is out-of-phase, less overlap leads to less shift of electron charge from between the nuclei. This leads to more electron charge between the nuclei for the pi antibonding molecular orbital than for the sigma antibonding molecular orbital. More electron charge between the nuclei means more plus-minus attraction and lower potential energy for the pi antibonding molecular orbital compared to the sigma antibonding molecular orbital.

The expected molecular orbital diagram from the overlap of 1s, 2s and 2p atomic orbitals is as follows. We will use this diagram to describe O₂, F₂, Ne₂, CO, and NO.

We use the following procedure when drawing molecular orbital diagrams.

Determine the number of electrons in the molecule. We get the number of electrons per atom from their atomic number on the periodic table. (Remember to determine the total number of electrons, not just the valence electrons.)
Fill the molecular orbitals from bottom to top until all the electrons are added. Describe the electrons with arrows. Put two arrows in each molecular orbital, with the first arrow pointing up and the second pointing down.
Orbitals of equal energy are half filled with parallel spin before they begin to pair up.

We describe the stability of the molecule with bond order.

bond order = 1/2 (#e- in bonding MO's - #e- in antibonding MO's)

We use bond orders to predict the stability of molecules.

If the bond order for a molecule is equal to zero, the molecule is unstable.
A bond order of greater than zero suggests a stable molecule.
The higher the bond order is, the more stable the bond.

We can use the molecular orbital diagram to predict whether the molecule is paramagnetic or diamagnetic. If all the electrons are paired, the molecule is diamagnetic. If one or more electrons are unpaired, the molecule is paramagnetic.

EXAMPLES:

1. The molecular orbital diagram for a diatomic hydrogen molecule, H₂, is

The bond order is 1. Bond Order = 1/2(2 - 0) = 1
The bond order above zero suggests that H₂ is stable.
Because there are no unpaired electrons, H₂ is diamagnetic.

2. The molecular orbital diagram for a diatomic helium molecule, He₂, shows the following.

The bond order is 0 for He₂. Bond Order = 1/2(2 - 2) = 0
The zero bond order for He₂ suggests that He₂ is unstable.
If He₂ did form, it would be diamagnetic.

3. The molecular orbital diagram for a diatomic oxygen molecule, O₂, is

O₂ has a bond order of 2. Bond Order = 1/2(10 - 6) = 2

The bond order of two suggests that the oxygen molecule is stable.
The two unpaired electrons show that O₂ is paramagnetic.

4. The molecular orbital diagram for a diatomic fluorine molecule, F₂, is

F₂ has a bond order of 1. Bond Order = 1/2(10 - 8) = 1

The bond order of one suggests that the fluorine molecule is stable.
Because all of the electrons are paired, F₂ is diamagnetic.

5. The molecular orbital diagram for a diatomic neon molecule, Ne₂, is

Ne₂ has a bond order of 0. Bond Order = 1/2(10 - 10) = 0

The zero bond order for Ne₂ suggests that Ne₂ is unstable.
If Ne₂ did form, it would be diamagnetic.

We can describe diatomic molecules composed of atoms of different elements in a similar way. The bond between the carbon and oxygen in carbon monoxide is very strong despite what looks like a strange and perhaps unstable Lewis Structure.

The plus formal charge on the more electronegative oxygen and the minus formal charge on the less electronegative carbon would suggest instability. The molecular orbital diagram predicts CO to be very stable with a bond order of three.

We predict the nitrogen monoxide molecule to be unstable according to the Lewis approach to bonding.

The unpaired electron and the lack of an octet of electrons around nitrogen would suggest an unstable molecule. NO is actually quite stable. The molecular orbital diagram predicts this by showing the molecule to have a bond order of 2.5.

http://www.mpcfaculty.net/mark_bishop/molecular_orbital_theory.htm

ISOTOPES

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ELEMENTS OF THE PERIODIC TABLE...

The elements are substances that cannot be broken down into other matter by chemical means, and an atom is the fundamental particle in an element. As of 2001, there were 112 known elements, 88 of which occur in nature; the rest were created in laboratories.

What distinguishes one element from another is the number of protons, subatomic particles with a positive electric charge, in the nucleus, or center, of the atom. The number of protons, whatever it may be, is unique to an element. Thus if an atom has one proton, it is an atom of hydrogen, because hydrogen has an atomic number of 1, as shown on the periodic table of elements. If an atom has 33 protons, on the other hand, it is arsenic.

ISOTOPES???
Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes, atoms of the same element having different masses. The name comes from the Greek phrase isos topos, meaning "same place": because they have the same atomic number, isotopes of the same element occupy the same position on the periodic table.

Also called nuclides, isotopes

are represented symbolically as follows:

where

S is the symbol of the element

a is the atomic number

m is the mass number—the sum of protons and neutrons in the atom's nucleus.

For the stable silver isotope designated as for instance, Ag is the element

symbol; 47 its atomic number; and 93 the mass number. From this, it is easy to discern that this particular stable isotope has 46 neutrons in its nucleus.

Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number, thus: 93Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that this information can be obtained at a glance without having to do the arithmetic.

Stable and Unstable Isotopes

Radioactivity is a term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy emitted in radiation include alpha particles (positively charged helium nuclei); beta particles (either electrons or subatomic particles called positrons); or gamma rays, which occupy the highest energy level in the electromagnetic radiation emitted by the Sun.

Isotopes are either stable or unstable. The unstable variety, known as radioisotopes, are subject to radioactive decay, but in this context, "decay" does not mean what it usually does. A radioisotope does not "rot"; it decays by turning into another isotope of the same element—or even into another element entirely. (For example, uranium-238 decays by emitting alpha particles, ultimately becoming lead-206.)

A stable isotope, on the other hand, has already become what it is going to be, and will not experience further decay. Most elements have between two and six stable isotopes. On the other hand, a few elements—for example, technetium—have no stable isotopes.

Twenty elements, among them gold, fluorine, sodium, aluminum, and phosphorus, have only one stable isotope each.

The element with the most stable isotopes is easy to remember because its name is almost the same as its number of stable isotopes: tin, with 10.
As for unstable isotopes, there are over 1,000, some of which exist in nature, but most of which have been created synthetically in laboratories. This number is not fixed; in any case, it is not necessarily important, because many of these highly radioactive isotopes last only for fractions of a second before decaying to form a stable isotope. Yet radioisotopes in general have so many uses, in comparison to stable isotopes, that they are often referred to simply as "isotopes."

Understanding Isotopes

Stable nuclei with low atomic numbers (up to about 20) have approximately the same number of neutrons and protons. For example, the most stable and abundant form of carbon is carbon-12, with six protons and six neutrons. Beyond atomic number 20 or so, however, the number of neutrons begins to grow: in other words, the lowest mass number is increasingly high in comparison to the atomic number.

For example, uranium has an atomic number of 92, but the lowest mass number for a uranium isotope is not 184, or 92 multiplied by two; it is 218. The ratio of neutrons to protons necessary for a stable isotope creeps upward along the periodic table: tin, with an atomic number of 50, has a stable isotope with a mass number of 120, indicating a 1.4 to 1 ratio of neutrons to protons. For mercury-200, the ratio is 1.5 to 1.
The higher the atomic number, by definition, the greater the number of protons in the nucleus. This means that more neutrons are required to "bind" the nucleus together. In fact, all nuclei with 84 protons or more (i.e., starting at polonium and moving along the periodic table) are radioactive, for the simple reason that it is increasingly difficult for the neutrons to with stand the strain of keeping so many protons in place.

One can predict the mode of radioactive decay by noting whether the nucleus is neutron-rich or neutron-poor. Neutron-rich nuclei undergo beta emission, which decreases the numbers of protons in the nucleus. Neutron-poor nuclei typically undergo positron emission or electron capture, the first of these being more prevalent among the lighter nuclei. Elements with atomic numbers of 84 or greater generally undergo alpha emission, which decreases the numbers of protons and neutrons by two each.

Radioactivity
One of the scientists working on the Manhattan Project was Italian physicist Enrico Fermi (1901-1954), who used radium and beryllium powder to construct a neutron source for making new radioactive materials. Fermi and his associates succeeded in producing radioisotopes of sodium, iron, copper, gold, and numerous other elements. As a result of Fermi's work, for which he won the 1938 Nobel Prize in Physics, scientists have been able to develop radioactive versions of virtually all elements.

Interestingly, the ideas of radioactivity, fission reactions, and fusion reactions collectively represent the realization of a goal sought by the medieval alchemists: the transformation of one element into another. The alchemists, forerunners of chemists, believed they could transform ordinary metals into gold by using various potions—an impossible dream. Yet as noted in the preceding paragraph, among the radioisotopes generated by Fermi's neutron source was gold. The "catch," of course, is that this gold was unstable; furthermore, the amount of energy and human mental effort required to generate it far outweighed the monetary value of the gold itself.

Radioactivity is, in the modern imagination, typically associated with fallout from nuclear war, or with hazards resulting from nuclear power—hazards that, as it turns out, have been greatly exaggerated. Nor is radioactivity always harmful to humans. For instance, with its applications in medicine—as a means of diagnosing and treating thyroid problems, or as a treatment for cancer patients—it can actually save lives.

Hazards Associated With Radioactivity

It is a good thing that radiation, even the harmful variety known as ionizing radiation, is not fatal in small doses, because every person on Earth is exposed to small quantities of radiation every year. About 82% of this comes from natural sources, and 18% from manmade sources. Of course, some people are at much greater risk of radiation exposure than others: coal miners are exposed to higher levels of the radon-222 isotope present underground, while cigarette smokers ingest much higher levels of radiation than ordinary people, due to the polonium-210, lead-210, and radon-222 isotopes present in the nitrogen fertilizers used to grow

tobacco.

Nuclear weapons, as most people know, produce a great deal of radioactive pollution. However, atmospheric testing of nuclear armaments has long been banned, and though the isotopes released in such tests are expected to remain in the atmosphere for about a century, they do not constitute a significant health hazard to most Americans. (It should be noted that nations not inclined to abide by international protocols might still conduct atmospheric tests in defiance of the test bans.) Nuclear power plants, despite the great deal of attention they have received from the media and environmentalist groups, do not pose the hazard that has often been claimed: in fact, coal-and oil-burning power plants are responsible for far more radioactive pollution in the United States.

This is not to say that nuclear energy poses no dangers, as the disaster at Chernobyl in the former Soviet Union has shown. In April 1986, an accident at a nuclear reactor in what is now the Ukraine killed 31 workers immediately, and ultimately led to the deaths of some 10,000 people. The fact that the radiation was allowed to spread had much to do with the secretive tactics of the Communist government, which attempted to cover up the problem rather than evacuate the area.

Another danger associated with nuclear power plants is radioactive waste. Spent fuel rods and other waste products from these plants have to be dumped somewhere, but it cannot simply be buried in the ground because it will create a continuing health hazard through the water supply. No fully fail-safe storage system has been developed, and the problem of radioactive waste poses a continuing threat due to the extremely long half-lives of some of the isotopes involved.

http://www.answers.com/topic/isotope