Atomic Mass Unit Definition (AMU)

Atomic Mass Unit Definition (AMU) In chemistry, an atomic mass unit or AMUÂ  is a physical constant equal to one-twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses. When the mass is expressed in AMU, it roughly reflects the sum of the number of protons and neutrons in the atomic nucleus (electrons have so much less mass that they are assumed to have a negligible effect). The symbol for the unit is u (unified atomic mass unit) or Da (Dalton), although AMUÂ  may still be used. 1 u 1 Da 1 amu (in modern usage) 1 g/mol Also Known As:Â  unified atomic mass unit (u), Dalton (Da), universal mass unit, either amu or AMU is an acceptable acronym for atomic mass unit The unified atomic mass unit is a physical constant that is accepted for use in the SI measurement system. It replaces the atomic mass unit (without the unified part) and is the mass of one nucleon (either a proton or a neutron) of a neutral carbon-12 atom in its ground state. Technically, the amu is the unit that was based on oxygen-16 until 1961, when it was redefined based on carbon-12. Today, people use the phrase atomic mass unit, but what they mean is unified atomic mass unit. One unified atomic mass unit is equal to: 1.66 yoctograms1.66053904020 x 10-27 kg1.66053904020 x 10-24 g931.49409511 MeV/c21822.8839 me History of the Atomic Mass Unit John Dalton first suggested a means of expressing relative atomic mass in 1803. He proposed the use of hydrogen-1 (protium). Wilhelm Ostwald suggested that relative atomic mass would be better if expressed in terms of 1/16th the mass of oxygen. When the existence of isotopes was discovered in 1912 and isotopic oxygen in 1929, the definition based on oxygen became confusing. Some scientists used an AMU based on the natural abundance of oxygen, while others used an AMU based on the oxygen-16 isotope. So, in 1961 the decision was made to use carbon-12 as the basis for the unit (to avoid any confusion with an oxygen-defined unit). The new unit was given the symbol u to replace amu, plus some scientists called the new unit a Dalton. However, u and Da were not universally adopted. Many scientists kept using the amu, just recognizing it was now based on carbon rather than oxygen. At present, values expressed in u, AMU, amu, and Da all describe the exact same measure. Examples of Values Expressed in Atomic Mass Units A hydrogen-1 atom has a mass of 1.007 u (or Da or amu).A carbon-12 atom is defined as having a mass of 12 u.The largest known protein, titin, has a mass of 3 x 106 Da.AMU is used to differentiate between isotopes. An atom of U-235, for example, has a lower AMU than one of U-238, since they differ by the number of neutrons in the atom.

Where to Find Specific Types of Rocks and Minerals

Where to Find Specific Types of Rocks and Minerals Rocks and minerals are all around us. You can pretty much find interesting specimens in almost any natural environment but you have to know where to look and what to look for. If youre new to geology, there is no substitute for examining as many different rocks as possible to familiarize yourself with whats out there. This guide will give you a good idea of some of the best places to get started. Hunting Rocks: Beaches and Riverbeds Whether youre a kid or a grownup, one of the best hunting grounds for rocks is a beach. Ocean beaches boast a wide variety of specimens and since theyre spread out across large areas and renewed with every tide, youre pretty much assured of finding something interesting. Beaches are beginner-friendly. Just bring along some sunscreen, water, something to put your finds in, and youre basically good to go. Beach rocks tend to be of the harder rock varieties (igneous and metamorphic). They get a good grinding in the surf zone, so they tend to be fairly clean and smooth. However, since its not always possible to pinpoint their source of origin, beach rocks are known by geology fanciers as stones without context. A stone on the beach may have fallen from cliffs along the shore or have broken off of a submerged underwater outcrop; it may have even traveled downstream in a river from a great distance inland. River rocks are much more likely to originate near the riverbed and banks. River rocks tend to include more of the softer rock types, and the farther upstream you can go, the truer this is. If you plan to hunt river rocks, be sure to wear sturdy footwear and make sure youre not trespassing. Bedrock: Exposures and Outcrops While beaches and rivers are good places for beginners to launch their education in rock collecting, for a  more serious study of rocks, youll need to find exposed bedrock. Bedrock- or living rock- is an  intact formation that has not been broken away from its original body. A place of any kind where bedrock is lying out in the open ready for your hammer is called an exposure; a naturally occurring exposure is called an outcrop. Outcrops may be found at the beach or along a river bank. In fact, in many geographic regions, these are the only places to find them. For more, youll need to visit the hills or the mountains. If you take manmade sites into account, exposures are quite common. Building sites with their excavations are plentiful all over the country. Mines and quarries offer excellent exposures as well, and they have the advantage of being more permanent than excavation sites. The best bedrock exposures are generally found in road cuts, and amateurs and professionals alike rely on them heavily for their best finds. In civil engineering jargon, a cut or cutting  is the area from which soil and rock are removed to facilitate the building of a road. Road cuts have many good features: Theyre clean, especially when newTheyre easy to visit, alone or in a groupIf theyre on public property, hammering is generally not forbiddenThey expose rocks well, even soft rocksThey expose rocks in their context, including features and structures not visible in a hand specimen Hunting Minerals Minerals can generally be found wherever rocks are found. Thats a good starting point, but a mineral hunter needs to know more geology than the rock hunter. For instance, the mineral grains in rocks such as shale or basalt are too small to be viewed with a magnifier but even these rocks offer possibilities to those who know where to look and what to look for. Minerals grow in several main settings: Primary minerals form during the solidification of a melt.Evaporitic minerals form by precipitation out of concentrated solutions.Diagenetic minerals form at low and moderate temperatures during the consolidation of rock from sediment.Vein minerals form during injection of deep hot fluids.Metamorphic minerals form in solid rocks under prolonged heat and pressure. If you can recognize the signs of these settings, you can expect to find the typical minerals they give rise to. Even a plain-looking mudstone may have zones of alteration or contain veins or partings that reveal mineral nodules that formed during diagenesis. Rock Hunting Etiquette Unfortunately, many of the best places for rock and mineral hunting are on private property or in protected parks. Although many beaches are public parks, where collecting is prohibited, no one is likely to prosecute you for discreetly picking up a few pebbles- but use discretion. Road cuts are off limits wherever parking is not permitted, such as along a freeway. Railways are private property and should be avoided. Likewise, when visiting road cuts in a park- whether national or local- you should generally leave your hammer in the car. Most federal public lands, such as national forests, can be explored freely by amateurs, however, its forbidden for anyone to deface or remove any natural features- this includes rocks, and this includes you. For all other areas, the best rule of thumb is to leave the rocks looking no worse than you found them. Most excavation sites and rock quarries are on private property so youll need to get the owners permission before you start your collecting expedition. Due to liabilities, fear of property damage, and other concerns, the person who owns your hunting ground may have more reasons to say no than yes. Experienced, organized groups generally have the best shot at gaining admittance to private property, so if youre really serious, you might want to think about joining a club.