Average atom mass

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

The average atomic mass shown in the periodic table is not equal to the mass number. 

Click Coached Problems for a self-study module on average atomic mass from isotopic abundance. 

Bromine is a red-orange liquid with an average atomic mass of 79.90 amu. Its name is derived from the Greek word bromos (fipofios), which means stench. It has two naturally occurring isotopes Br-79 (78.92 amu) and Br-81 (80.92 amu). What is the abundance of the heavier isotope ... 

STRATEGY First calculate the average atomic mass of the isotopes by adding together the individual masses, each multiplied by the fraction that represents its abundance. Then obtain the molar mass, the mass per mole of atoms, by multiplying the average atomic mass by Avogadro s constant. 

Two forms of the same element are called isotopes. The isotopes of an element have the same atomic number but have different atomic masses. Iron has several isotopes that, when weighted by their naturally occurring abundance, gives an average mass of 55.845 amu. A simple example would be an element with only two isotopes, one with a mass of 10 amu, the other of 12 amu. If the isotopes were equally common, then the average atomic mass for that element would be 11. If 90% of the element occurred naturally as the isotope with a mass of 10 amu, then the average atomic mass would be 10.2, as calculated below ... 

D The average atomic mass of one atom of each element in the molecule... 

Table 6.6 Some frequently encountered atoms with their monoisotopic and average atomic masses...

Most mass spectrometers will resolve ions with unit resolution up to at least 2000 Da, and so monoisotopic atomic masses are used in these cases. Above 2000 Da, the resolution should be checked and, if it is insufficient to resolve adjacent isotopes, then average atomic masses can be used in calculations. 

Naturally occurring boron consists of approximately 20% of 10B and 80% of UB, leading to an average atomic mass of 10.8 amu. Because 10B has a relatively large cross-section for absorption of slow (thermal) neutrons, it is used in control rods in nuclear reactors and in protective shields. In order to obtain a material that can be fabricated into appropriate shapes, boron carbide is combined with aluminum. 

A The average atomic mass of boron is 10.811, which is closer to 11.009305 than to 10.012937. Thus, boron-11 is the isotope present in greater abundance. 

The weighted-average atomic mass of the element iridium is just slightly more than 192 u. The mass of the first isotope is a bit less than 191 u. Hence, the mass of the second isotope must more than 192 u that isotope must be 193 Ir. 

Each of the isotopic masses is multiplied by its fractional abundance. The resulting products are summed to obtain the average atomic mass. 

To determine the average atomic mass, we use the following expression average atomic mass = (isotopic mass x fractional natural abundance)... 

We use the expression for determining the weighted-average atomic mass. 

Then the expression for the weighted-average atomic mass is used, with the percent abundances converted to fractional abundances by dividing by 100. The average atomic mass of potassium is 39.0983 u. 

Each of these calculations employs the average atomic mass as a conversion factor,... 

Here there are four measured frequencies with which to constrain three independent force constants, so the best-fitting force constants can be determined through an iterative least squares fit, minimizing S(v eas-Vcaic) - Assuming average atomic masses of 51.996 and 15.9994 for chromium and oxygen, respectively, the best-fit force constants are Ai = 495.2 Newtons/m,iT = 21.3 Newtons/m, andFjv = 44.7 Newtons/m. These force constants show the typical relationship K H,Fj j,. Calculated frequencies are ... 

Average atomic mass Common ion charge Other ion charges... 

Average atomic mass data in brackets indicate atomic mass of most stable isotope of the element. 

Carbon-12 is the basis for the average atomic mass units (amu) that is used to determine the atomic weights of the elements. Carbon is one of the few elements that can form covalent bonds with itself as well as with many metals and nonmetals. 

The average of the atomic masses of all the chemical element s isotopes (also known as atomic weight and average atomic mass)... 

What is the average atomic mass of a hypothetical sample of element X if it is found that 20% of the sample contains an isotope with mass of 100 50% of the sample contains an isotope of the element with mass of 102 and 30% of the sample contains an isotope of the element with a mass of 105 ... 

These three isotopes are why you see carbon s atomic mass on the periodic table written as 12.01. If you do a quick bit of deductive reasoning, you can probably determine that carbon-12 is far and away the most common of the three isotopes due to the average atomic mass being closest to 12. 

Precise measurements of the amounts of different isotopes can be important. You need to know the exact measurements if you re asked to figure out an element s atomic mass. To calculate an atomic mass, you need to know the masses of the isotopes and the percentage of the element that occurs as each isotope (this is called the relative abundance ). To calculate an average atomic mass, make a list of each isotope along with its mass and its percent relative abundance. Multiply the mass of each isotope by its relative abundance. Add the products. The resulting sum is the atomic mass. 

Certain elements, such as chlorine, occur in several very common isotopes, so their average atomic mass isn t close to a whole number. Other elements, such as carbon, occur in one very common isotope and several very rare ones, resulting in an average atomic mass that s very close to the whole-number mass of the most common isotope. 

Chlorine occurs in two common isotopes. It appears as j Cl 75.8% of the time and as jJCl 24.2% of the time. What is its average atomic mass ... 

Magnesium occurs in three fairly common isotopes, j Mg, Mg, and f Mg, which have percent abundances of 78.9%, 10.0%, and 11.1%, respectively. Calculate the average atomic mass of magnesium. 

The reverse isotope dilution technique can be applied for accurate determination of the Mg contents in a sample, sample, on applying equation 7, by measuring the isotope ratio of a selected pair of stable isotopes, in a weighed mixmre of the sample with an isotopically enriched CRM. The average atomic masses m and the isotopic ratios R of Mg in the enriched CRM and in nature are known. The method was applied for determination of Mg in plant material using a CRM isotopically enriched with Mg, measuring with an ICP/MS instruments. ... 

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