ScienceZ: The Science Dictionary

Pluto is a dwarf planet that until 2006 was classified as the ninth planet in our solar sytem, having a sidereal period of revolution about the sun of 248.5 years, 4.4 billion kilometers (2.8 billion miles) distant at perihelion and 7.4 billion kilometers (4.6 billion miles) at aphelion, and a diameter less than half that of Earth.

Pluto, the last planet to join the heavenly pantheon, became the first to leave it. The status of Pluto had been under discussion for some time but with the discovery of Eris, formerly known as Xena, the question became acute, for it seemingly had as much right as Pluto to be called a planet.

On August 24, 2006, the International Astronomical Union surprised the world by voting in a new definition of planet, one that would exclude Pluto and bring the total number down to eight. (There had previously been been strong speculation that the redefinition would bring the total up to 12 instead of down.)

Pluto was instead classified as a dwarf planet, along with Ceres and the aforementioned Xena. The main difference between a dwarf planet and the real thing is that the dwarf variety has not cleared the area of its orbital path.

This redefinition met with a wave of protests from those who wanted to see the ninth planet grandfathered in, including but not limited to supporters of the late Clyde Tombaugh, who discovered Pluto in 1930. His widow, however, said he would have been accepting of the IAU’s decision since “he was a scientist” and understood that astronomers had to take into account newly discovered objects in the Kuiper Belt (where Pluto is located).

But opponents of Pluto’s demotion remain unconsoled and have generated a thriving industry in T-shirts, mugs and other memorabilia. Among the many slogans of this movement was one which played on the mnemonic for the names of the erstwhile nine:

“My! Very educated morons just screwed up numerous planetariums.”

Definition: (j?l, joul) , abbr. J, unit of work or energy in the mks system of units, which is based on the metric system; it is the work done or energy expended by a force of 1 newton acting through a distance of 1 meter. The joule is named for James P. Joule.

One joule is the work done, or energy expended, by a force of one newton moving an object one metre along the direction of the force. This quantity is also denoted as a Newton-meter with the symbol N·m. Note that torque also has the same units as work, but the quantities are not identical. In elementary units:

One joule is also:

• The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
• The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt-hour), with the symbol W·s.

Its value was found by James Prescott Joule in experiments that showed the mechanical energy Joule’s equivalent, and represented by the symbol J. The term was first introduced by Dr. Mayer of Heilbronn.

One of two or more alternative forms of an element that have the same number of protons in their nucleus, but have different numbers of neutrons.

One member of a (chemical-element) family of atomic species which has two or more nuclides with the same number of protons (Z) but a different number of neutrons (N). Because the atomic mass is determined by the sum of the number of protons and neutrons contained in the nucleus, isotopes differ in mass. Since they contain the same number of protons (and hence electrons), isotopes have the same chemical properties. However, the nuclear and atomic properties of isotopes can be different. The electronic energy levels of an atom depend upon the nuclear mass. Thus, corresponding atomic levels of isotopes are slightly shifted relative to each other. A nucleus can have a magnetic moment which can interact with the magnetic field generated by the electrons and lead to a splitting of the electronic levels. The number of resulting states of nearly the same energy depends upon the spin of the nucleus and the characteristics of the specific electronic level.

Example: hydrogen has three isotopes, of atomic masses 1, 2, and 3, generally written as ¹H, ²H (deuterium), and ³H (tritium). ¹H is the most abundant isotope of hydrogen; ²H is stable, while ³H is radioactive. Radioactive isotopes are unstable, and decay to stable elements, emitting radiation in the process. This may be ?-radiation, ?-radiation (electrons), ?-radiation, or X-rays, depending on the isotope. The time taken for half the radioactivity to decay is the half-life of the isotope, and can vary from a fraction of a second, through several days to years (e.g. the half-life of ³H is 12½ years, that of ¹⁴C is 5200 years).

Stable isotopes can be detected only by their different atomic mass. Since they emit no radiation, they are safe for use in labelled compounds given to human beings. Examples of stable isotopes commonly used in nutrition research include ²H, ¹³C, ¹⁵N, and ¹⁸O.

An elementary particle which is the negatively charged constituent of ordinary matter. The electron is the lightest known particle which possesses an electric charge. Its rest mass is m_e ? 9.1 × 10^?28 g, about ¹/₁₈₃₆ of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. Discovered in 1895 by J. J. Thomson in the form of cathode rays, the electron was the first elementary particle to be identified.

The charge of the electron is ?e ? ?4.8 × 10^?10 esu = ?1.6 × 10^?19 coulomb. The sign of the electron’s charge is negative by convention, and that of the equally charged proton is positive. This is a somewhat unfortunate convention, because the flow of electrons in a conductor is thus opposite to the conventional direction of the current.

Electrons are emitted in radioactivity (as beta rays) and in many other decay processes; for instance, the ultimate decay products of all mesons are electrons, neutrinos, and photons, the meson’s charge being carried away by the electrons. The electron itself is completely stable. Electrons contribute the bulk to ordinary matter; the volume of an atom is nearly all occupied by the cloud of electrons surrounding the nucleus, which occupies only about 10^?13 of the atom’s volume. The chemical properties of ordinary matter are determined by the electron cloud.

The electron obeys the Fermi-Dirac statistics, and for this reason is often called a fermion. One of the primary attributes of matter, impenetrability, results from the fact that the electron, being a fermion, obeys the Pauli exclusion principle; the world would be completely different if the lightest charged particle were a boson, that is, a particle that obeys Bose-Einstein statistics.

A substance, usually used in small amounts relative to the reactants, that modifies and increases the rate of a reaction without being consumed in the process. For example, nitric oxide (NO) is a catalyst in the breakdown of ozone (O₃) in the upper atmosphere. An oxygen atom (O) will react with an ozone molecule form two oxygen molecules (O₂), but at a very slow rate. However, in the presence of nitric oxide, ozone is quickly broken down with the following series of steps:

Step 1 - An oxygen atom combines with a molecule of nitric oxide, forming a molecule of nitrogen dioxide (NO₂):

O + NO –> NO₂

Step 2 - A molecule of nitrogen dioxide combines with a molecule of ozone, forming two molecules of oxygen and a molecule of nitric oxide:

NO₂ + O₃ –> 2O₂ + NO

Although the molecule of nitric oxide participates in the reaction, it is not consumed by it and is available for additional reactions.

Catalysts may be gaseous, liquid, or solid; they may be inorganic compounds, organic compounds, or complex combinations. They tend to be highly specific, reacting with only one substance or a small set of substances.

Catalysts work by changing the activation energy for a reaction, i.e., the minimum energy needed for the reaction to occur. This is accomplished by providing a new mechanism or reaction path through which the reaction can proceed. When the new reaction path has a lower activation energy, the reaction rate is increased and the reaction is said to be catalyzed.

If the activation energy for the new path is higher, the reaction rate is decreased and the reaction is said to be inhibited. Inhibitors can provide an interesting challenge to the chemist. For example, because oxygen is an inhibitor of free-radical reactions, many of which are important in the synthesis of polymers, such reactions must be performed in an oxygen-free environment, e.g., under a blanket of nitrogen gas.

In some reactions one of the reaction products is a catalyst for the reaction; this phenomenon is called self-catalysis or autocatalysis. An example is the reaction of permanganate ion with oxalic acid to form carbon dioxide and manganous ion, in which the manganous ion acts as an autocatalyst. Such reactions are potentially dangerous, since the reaction rate may increase to the point of explosion.

Some substances that are not themselves catalysts increase the activity of a catalyst when added with it to some reaction; such substances are called promoters. Alumina is a promoter for iron when it is used to catalyze the reaction of hydrogen and nitrogen to form ammonia. Substances that react with catalysts to reduce or eliminate their effect are called poisons.