Carbon 12 scale, atomic mass units

To set up a scale of atomic masses, it is necessary to establish a standard value for one particular species. The modem atomic mass scale is based on the most common isotope of carbon, 1 C. This isotope is assigned a mass of exactly 12 atomic mass units (amu) ... 

Relative isotopic, atomic and molecular masses are measured on a scale in which the mass of an atom of carbon-12 is exactly 12 atomic mass units (a.m.u.). 

Atomic masses are so small that an appropriate unit was developed to report them—an atomic mass unit (amu). 1 amu = 1.66 x 10 " g. The atomic mass of the lightest element, hydrogen, was originally taken to be 1 amu. The modern values of the atomic masses are based on the most common kind of carbon atom, called carbon-12 and written as the standard. The mass of is measured in the modern mass spectrometer, and is defined to have an atomic mass of exactly 12 amu. On this scale hydrogen has an atomic mass of 1.008 amu. 

Finally, in 1962, a conference of chemists and physicists agreed on a scale based on an isotope of carbon. Carbon is shown in Figure 24. Used by all scientists today, this scale defines the atomic mass unit as exactly one-twelfth of the mass of one carbon-12 atom. As a result, one atomic... 

In Chapter 4, you learned that the relative scale of atomic masses uses the isotope carbon-12 as the standard. Each atom of carbon-12 has a mass of 12 atomic mass units (amu). The atomic masses of all other elements are established relative to carbon-12. For example, an atom of hydrogen-1 has a mass of 1 amu. The mass of an atom of helium-4 is 4 amu. Therefore, the mass of one atom of hydrogen-1 is one-twelfth the mass of one atom of carbon-12. The mass of one atom of helium-4 is one-third the mass of one atom of carbon-12. 

The masses for the elements listed in the table inside the back cover of this text are relative masses in terms of atomic mass units (amu) or daltons. The atomic mass unit is based on a relative scale in which the reference is the C carbon isotope, which is assigned a mass of exactly 12 amu. Thus, the amu is by definition 1/12 of the mass of one neutral c atom. The molar mass of is then... 

An early observation was that carbon and hydrogen have relative atomic masses, also traditionally called atomic weights (AW), of approximately 12 and 1, respectively. Thousands of experiments on the compositions of compounds have resulted in the establishment of a scale of relative atomic weights based on the atomic mass unit (amu), which is defined as exactly 75 of the mass of an atom of a particular kind of carbon atom, called carbon-12. 

The isotope of carbon with mass number 12 has been chosen as the defining element for the scale of atomic masses. We define the atomic mass unit, symbol u, as exactly 1/12 of the mass of one atom of carbon-12. Then u = 1.6605655 x 10 kg. The relative atomic mass ofanatom, Ar,is defined by A = m/u, where mis the mass of the atom for example, AXiH) = 1.007825 AJ[ C) = 12 (exactly) = 15.99491. In any macroscopic... 

The modern atomic weight system is based on the mass of the most common form of the element carbon. The mass of this form of carbon is defined to be exactly 12 atomic mass units, abbreviated as amu. On this scale, for example, hydrogen has an atomic weight of 1.0078 amu. The atomic masses of all the elements appear in Appendix B as the average masses relative to carbon twelve. We will examine the precise meaning of average mass in Sec. 2.4. 

The atomic weight of an element is the average weight of all the isotopic masses of the element, calculated on the basis of their relative abundance in nature. The atomic weights are set on a carbon-12 scale. This is the standard weight scale that is used worldwide to express atomic weights. Exploring this further, we can say that 12 atomic mass units (amu) make up the mass of one... 

What is m in i/z The scales of atomic masses are based upon an agreed standard. The currently accepted standard (lUPAC) is the unified atomic mass unit (u), a dimensionless number, defined so that a single carbon atom (the carbon-12 isotope that contains 6 protons and 6 neutrons) has a mass of exactly 12 u (1 u = 1.66 x 10- kg.) The unit u is also called a dalton (Da). Dalton is used in preference to atomic mass unit (amu), with the latter considered archaic. The Da is convenient for expressing the masses of both atoms and molecules, e.g., argon (Ar) = 40 Da, methane (CH4) = 16 Da, and cholesterol (C27H46O) = 386 Da. 

Atomic mass units are based on something called the Carbon 12 scale, a worldwide standard that s been adopted for atomic weights. By international agreement, a carbon atom that contains 6 protons and 6 neutrons has an atomic weight of exactly 12 amu, so 1 amu is Yu of this carbon atom. I know, what do carbon atoms and the number 12 have to do with anything Just trust me. Because the mass in grams of protons and neutrons are almost exactly the same, both protons and neutrons are said to have a mass of 1 amu. Notice that the mass of an electron is much smaller than that of either a proton or neutron. It takes almost 2,000 electrons to equal the mass of a single proton. 

Atomic mass units provide a relative scale for the masses of the elements. But because atoms have such small masses, no usable scale can be devised to weigh them in calibrated units of atomic mass units. In any real situation, we deal with macroscopic samples containing enormous numbers of atoms. It is convenient, therefore, to have a special unit to describe a very large number of atoms. The idea of a unit to denote a particular number of objects is not new. For example, the pair (2 items), the dozen (12 items), and the gross (144 items) are all commonly used. Chemists measure atoms and molecules in moles. In the SI system the mole (mol) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 g (or 0.012 kg) of the carbon-12 isotope. The actual number of atoms in 12 g of carbon-12 is determined experimentally. This number is called Avogadro s number (Njf), in honor of the Itahan scientist Amedeo Avogadro. The currently accepted value is... 

Atomic Mass and Molar Mass The mass of an atom, which is extremely small, is based on the carbon-12 isotope scale. An atom of the carix)n-12 isotope is assigned a mass of cactly 12 atomic mass units (amu). To work with the more convenient scale of grams, chemists use the molar mass. The molar mass of caibon-12 is exactly 12 g and contains an Avogadio s number (6.022 X 10 ) of atoms. The molar masses of other elements are also expressed in grams and contain the same number of atoms. The molar mass of a molecule is the sum of the molar masses of its constituent atoms. 

The relative atomic mass expresses masses of atoms as relative values using the carbon-12 atomic mass scale. Relative atomic masses (symbol A ) are simply pure numbers and do not have units. 

The mass of the electron is 9.110 X 10 g, which is much less than either a proton or neutron. Because the masses of subatomic particles are so small, chemists use a unit called an atomic mass unit (amu). An amu is defined as one-twelfth of the mass of the carbon atom with 6 protons and 6 neutrons, a standard with which the mass of every other atom is compared. In biology, the atomic mass unit is called a Dalton (Da) in honor of John Dalton. On the amu scale, the proton and neutron each have a mass of about 1 amu. Because the electron mass is so small, it is usually ignored in atomic mass calculations. 

AMU n The atomic mass unit (amu), a unit of mass equal to 1/12 the mass of the carbon atom of mass number 12. On the atomic mass scale C=12. 

If we know the atomic weight of an element on the carbon-12 scale, we can use the mole concept and Avogadro s number to calculate the average mass of one atom of that element in grams (or any other mass unit we choose). 

The atomic weight or relative atomic mass of an element is the mass of one atom of that element relative to that of the most abundant form of carbon taken as 12 units. On this scale the atomic weight of hydrogen is 1, oxygen is 16, and copper is 63.54a.m.u. Table 1.1 lists the atomic weights of the first 18 elements of the Periodic Table. 

On this scale, carbon-12 has a relative atomic mass of 12.000 000, carbon has a relative atomic mass of 12.01111, and hydrogen has a relative atomic mass 1.007 97. Since relative atomic masses are ratios of two masses, they have no units. As this value for hydrogen is very close to one, the value of H = 1 is used in most calculations. A table of approximate relative atomic masses is given on page 293. The symbol for relative atomic mass is A,. 

The relative atomic mass is the weighted average mass of naturally occurring atoms of an element on a scale where an atom of carbon-12 has a mass of exactly 12 units. 

The relative molecular mass of a compound (M. ) is the relative mass of one molecule of the compound on a scale where the carbon-12 isotope has a mass of exactly 12 units. We find the relative molecular mass by adding up the relative atomic masses of all the atoms present in the molecule. 

Kelvin scale A fundamental scale of temperature on which the triple point of water lies at 273.16 K and the lowest attainable temperature is at 0. The unit on the Kelvin scale is the kelvin, K. ketone An organic compound containing a carbonyl group between two carbon atoms, having the form R—CO R. Example CH3—CO—CH2CH , butanone. kilogram (kg) The SI unit of mass. See also Appendix IB. 

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