Chemiluminescence

Chemiluminescence is chemical reaction the emit energy as a light but without temperature chane. It is much less common for a chemical reaction to produce light without heat.While the light can, in principle, be emitted in the ultraviolet, visible or infrared region, those emitting visible light are the most common. They are also the most interesting and useful.

The light from such reactions is called cool light, because it is created without heat. Reactions that produce light without heat are called chemiluminescent reactions. Perhaps the most familiar chemiluminescent reactions are those that occur in living organisms. Fireflies produce light without heat by a chemiluminescent reaction. Chemiluminescent reactions that occur in living organisms are called bioluminescent reactions.

Chemiluminescent reactions can be grouped into three types:

1. Chemical reactions using synthetic compounds and usually involving a highly oxidized species such as a peroxide are commonly termed chemiluminescent reactions.
2. Light-emitting reactions arising from a living organism, such as the firefly or jellyfish, are commonly termed bioluminescent reactions.
3. Light-emitting reactions which take place by the use of electrical current are designated electrochemiluminescent reactions.

Chemiluminescent and bioluminescent reactions usually involve the cleavage or fragmentation of the O-O bond an organic peroxide compound. Peroxides, especially cyclic peroxides, are prevalent in light emitting reactions because the relatively weak peroxide bond is easily cleaved and the resulting molecular reorganization liberates a large amount of energy.

Phosphorescence

Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours.

In simpler terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged" by exposure to light. Unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and "store" it for a longer time as the subatomic reactions required to re-emit the light occur less often.

Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanoseconds. However, for light to be absorbed and emitted at these fast time scales, the energy of the photons involved (i.e. the wavelength of the light) must be carefully tuned according to the rules of quantum mechanics to match the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity (see term symbol), usually a triplet state. As a result, the energy can become trapped in the triplet state with only quantum mechanically "forbidden" transitions available to return to the lower energy state. These transistions, although "forbidden", will still occur but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called "glow-in-the-dark" materials.

Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s. The study of phosphorescent materials led to the discovery of radioactivity in 1896.

Taken from Wikipedia

Atom

In chemistry and physics, an atom (Greek átomos meaning "indivisible") is the smallest particle of a chemical element that retains its chemical properties. Whereas the word atom originally denoted a particle that cannot be cut into smaller particles, the atoms of modern parlance are composed of subatomic particles:

• electrons, which have a negative charge, a size which is so small as to be currently unmeasurable, and which are the least heavy (i.e., massive) of the three;
• protons, which have a positive charge, and are about 1836 times more massive than electrons; and
• neutrons, which have no charge, and are about 1838 times more massive than electrons.

Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called nucleons. The electrons form the much larger electron cloud surrounding the nucleus.

Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons (called the atomic number). Within a single element, the number of neutrons may vary, determining the isotope of that element. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons. The number of protons (and neutrons) in the atomic nucleus may also change, via nuclear fusion, nuclear fission or radioactive decay, in which case the atom is no longer the same element it was.

Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called ions. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals.

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions.

Taken from Wikipedia

Discovery of the Neutron (1932)

The story begins in 1932, with the discovery of the neutron by Sir James Chadwick, an English physicist.

Until 1932, the atom was known to consist of a positively charged nucleus surrounded by enough negatively charged electrons to make the atom electrically neutral. Most of the atom was empty space, with its mass concentrated in a tiny nucleus. The nucleus was thought to contain both protons and electrons because the proton (otherwise known as the hydrogen ion, H⁺) was the lightest known nucleus and because electrons were emitted by the nucleus in beta decay. In addition to the beta particles, certain radioactive nuclei emitted positively charged alpha particles and neutral gamma radiation. The symbols for these emissions are b ^- or _–1e⁰, a ²⁺ or ₂⁴He²⁺, and ₀^0g .

Twelve years earlier, Lord Ernest Rutherford, a pioneer in atomic structure, had postulated the existence of a neutral particle, with the approximate mass of a proton, that could result from the capture of an electron by a proton. This postulation stimulated a search for the particle. However, its electrical neutrality complicated the search because almost all experimental techniques of this period measured charged particles.

In 1928, a German physicist, Walter Bothe, and his student, Herbert Becker, took the initial step in the search. They bombarded beryllium with alpha particles emitted from polonium and found that it gave off a penetrating, electrically neutral radiation, which they interpreted to be high-energy gamma photons.

In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, decided to use their strong polonium alpha source to further investigate Bothe’s penetrating radiation. They found that this radiation ejected protons from a paraffin target. This discovery was amazing because photons have no mass. However, the Joliot-Curies interpreted the results as the action of photons on the hydrogen atoms in paraffin. They used the analogy of the Compton Effect, in which photons impinging on a metal surface eject electrons. The trouble was that the electron was 1,836 times lighter than the proton and, therefore, recoiled much more easily than the heavier proton after a collision with a gamma photon. We now know that gamma photons do not have enough energy to eject protons from paraffin.

Taken from Wikipedia