Mars (pronounced /ˈmɑrz/) is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface.
Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the Solar System, and of Valles Marineris, the largest canyon. Furthermore, in June 2008 three articles published in Nature presented evidence of an enormous impact crater in Mars' northern hemisphere, 10,600 km long by 8,500 km wide, or roughly four times larger than the largest impact crater yet discovered, the South Pole-Aitken basin.[5][6] In addition to its geographical features, Mars’ rotational period and seasonal cycles are likewise similar to those of Earth.
Until the first flyby of Mars by Mariner 4 in 1965, many speculated that there might be liquid water on the planet's surface. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in the Solar System other than Earth, Mars is the most likely to harbor liquid water, and perhaps life.[7] Radar data from Mars Express and the Mars Reconnaissance Orbiter have revealed the presence of large quantities of water ice both at the poles (July 2005)[8] and at mid-latitudes (November 2008)[9]. The Phoenix Mars Lander directly sampled water ice in shallow martian soil on July 31, 2008.[10]
Selasa, 10 Februari 2009
Proton
Atomic number
In chemistry the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, while a negative Cl- ion has 17 protons and 18 electrons for a total charge of -1.
All atoms of a given element are not necessarily identical however, as the number of neutrons may vary to form different isotopes. Again for chlorine as an example, there are two stable isotopes - 35Cl with 35 nucleons which are 17 protons and 35-17 = 18 neutrons, and 37Cl with 17 protons and 37-17 = 20 neutrons. Other isotopes of chlorine are radioactive.
[edit] Hydrogen as proton
Since the atomic number of hydrogen is 1, a positive hydrogen ion (H+) has no electrons and corresponds to a bare nucleus with 1 proton (and 0 neutrons for the most abundant isotope 1H). In chemistry therefore, the word "proton" is commonly used as a synonym for hydrogen ion (H+) or hydrogen nucleus in several contexts:
1. The transfer of H+ in an acid-base reaction is referred to "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor.
2. The hydronium ion (H3O+) in aqueous solution corresponds to a hydrated hydrogen ion. Often the water molecule is ignored and the ion written as simply H+(aq) or just H+, and referred to as a "proton". This is the usual meaning in biochemistry, as in the term proton pump which refers to a protein or enzyme which controls the movement of H+ ions across cell membranes.
3. Proton NMR refers to the observation of hydrogen nuclei in (mostly organic) molecules by nuclear magnetic resonance. This uses the property of the proton to have spin one-half.
[edit] History
Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen , and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle.
See also: William Prout and Prout's hypothesis
Prior to Rutherford, Eugen Goldstein had observed canal rays, which were composed of positively charged ions. After the discovery of the electron by J.J. Thomson, Goldstein suggested that since the atom is electrically neutral there must be a positively charged particle in the atom and tried to discover it. He used the "canal rays" observed to be moving against the electron flow in cathode ray tubes. After the electron had been removed from particles inside the cathode ray tube they became positively charged and moved towards the cathode. Most of the charged particles passed through the cathode, it being perforated, and produced a glow on the glass. At this point, Goldstein believed that he had discovered the proton.[4] When he calculated the ratio of charge to mass of this new particle (which in case of the electron was found to be the same for every gas that was used in the cathode ray tube) was found to be different when the gases used were changed. The reason was simple. What Goldstein assumed to be a proton was actually an ion. He gave up his work there, but promised that "he would return." However, he was widely ignored.
It is named after the neuter singular of the Greek word for "first", πρῶτον.
In chemistry the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, while a negative Cl- ion has 17 protons and 18 electrons for a total charge of -1.
All atoms of a given element are not necessarily identical however, as the number of neutrons may vary to form different isotopes. Again for chlorine as an example, there are two stable isotopes - 35Cl with 35 nucleons which are 17 protons and 35-17 = 18 neutrons, and 37Cl with 17 protons and 37-17 = 20 neutrons. Other isotopes of chlorine are radioactive.
[edit] Hydrogen as proton
Since the atomic number of hydrogen is 1, a positive hydrogen ion (H+) has no electrons and corresponds to a bare nucleus with 1 proton (and 0 neutrons for the most abundant isotope 1H). In chemistry therefore, the word "proton" is commonly used as a synonym for hydrogen ion (H+) or hydrogen nucleus in several contexts:
1. The transfer of H+ in an acid-base reaction is referred to "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor.
2. The hydronium ion (H3O+) in aqueous solution corresponds to a hydrated hydrogen ion. Often the water molecule is ignored and the ion written as simply H+(aq) or just H+, and referred to as a "proton". This is the usual meaning in biochemistry, as in the term proton pump which refers to a protein or enzyme which controls the movement of H+ ions across cell membranes.
3. Proton NMR refers to the observation of hydrogen nuclei in (mostly organic) molecules by nuclear magnetic resonance. This uses the property of the proton to have spin one-half.
[edit] History
Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen , and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle.
See also: William Prout and Prout's hypothesis
Prior to Rutherford, Eugen Goldstein had observed canal rays, which were composed of positively charged ions. After the discovery of the electron by J.J. Thomson, Goldstein suggested that since the atom is electrically neutral there must be a positively charged particle in the atom and tried to discover it. He used the "canal rays" observed to be moving against the electron flow in cathode ray tubes. After the electron had been removed from particles inside the cathode ray tube they became positively charged and moved towards the cathode. Most of the charged particles passed through the cathode, it being perforated, and produced a glow on the glass. At this point, Goldstein believed that he had discovered the proton.[4] When he calculated the ratio of charge to mass of this new particle (which in case of the electron was found to be the same for every gas that was used in the cathode ray tube) was found to be different when the gases used were changed. The reason was simple. What Goldstein assumed to be a proton was actually an ion. He gave up his work there, but promised that "he would return." However, he was widely ignored.
It is named after the neuter singular of the Greek word for "first", πρῶτον.
Neutron
Uses
The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in neutron activation, inducing radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons. The fissioning of elements like uranium-235 and plutonium-239 is caused by their absorption of neutrons.
Cold, thermal and hot neutron radiation is commonly employed in neutron scattering facilities, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.[7][8][9]
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
[edit] Sources
Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from neutron sources by neutron transport. For access to intense neutron sources, researchers must go to specialist facilities, such as the ISIS facility in the UK, which is currently the world's most intense pulsed neutron and muon source.[citation needed]
Neutrons' lack of total electric charge prevents engineers or experimentalists from being able to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by electric or magnetic fields. However, these methods have no effect on neutrons except for a small effect of an inhomogeneous magnetic field because of the neutron's magnetic moment.
[edit] Protection
Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions which give rise to other forms of radiation (such as protons). The normal precautions of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. Some particular thought must be given to how to protect from neutron exposure, however. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density make for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, hydrogen rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. This often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. After slowing, neutrons may then be absorbed with an isotope which has high affinity for slow neutrons without causing secondary capture-radiation, such as lithium-6.
Hydrogen-rich ordinary water effects neutron absorption in nuclear fission reactors: usually neutrons are so strongly absorbed by normal water that fuel-enrichement with fissionable isotope, is required. The deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is therefore used in CANDU-type reactors, in order to slow ("moderate") neutron velocity, so that they are more effective at causing nuclear fission, without capturing them.
The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in neutron activation, inducing radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons. The fissioning of elements like uranium-235 and plutonium-239 is caused by their absorption of neutrons.
Cold, thermal and hot neutron radiation is commonly employed in neutron scattering facilities, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.[7][8][9]
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
[edit] Sources
Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from neutron sources by neutron transport. For access to intense neutron sources, researchers must go to specialist facilities, such as the ISIS facility in the UK, which is currently the world's most intense pulsed neutron and muon source.[citation needed]
Neutrons' lack of total electric charge prevents engineers or experimentalists from being able to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by electric or magnetic fields. However, these methods have no effect on neutrons except for a small effect of an inhomogeneous magnetic field because of the neutron's magnetic moment.
[edit] Protection
Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions which give rise to other forms of radiation (such as protons). The normal precautions of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. Some particular thought must be given to how to protect from neutron exposure, however. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density make for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, hydrogen rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. This often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. After slowing, neutrons may then be absorbed with an isotope which has high affinity for slow neutrons without causing secondary capture-radiation, such as lithium-6.
Hydrogen-rich ordinary water effects neutron absorption in nuclear fission reactors: usually neutrons are so strongly absorbed by normal water that fuel-enrichement with fissionable isotope, is required. The deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is therefore used in CANDU-type reactors, in order to slow ("moderate") neutron velocity, so that they are more effective at causing nuclear fission, without capturing them.
Langganan:
Postingan (Atom)