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The Atomic Mass listed in the periodic is found by taking the average of all the different isotopes of a given element found in nature, weighted for their natural abundance.

Note that the atomic mass is not the same as the atomic weight. The atomic mass is the weight of one specific isotope of one atom, and is expressed in "atomic mass units" or amu. The atomic weight is expressed in grams per mole (g/mol) and is the weighted average of all the isotopes weighted by their abundance.

See the Web Links for more information about atomic mass and atomic weight.

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15y ago
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9y ago

The main reason that the atomic mass of elements is not a whole number is due to the presence of different isotopes - these have the same number of protons in the nucleus but a different number of neutrons - so their masses are different even though chemically they are identical.

The two isotopes of Chlorine are 35Cl- (75.77%) and 37Cl (24.23%) giving an overall "average" mass of 35.4527.

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7y ago

The atomic weights of some elements are averages, this is because some elements have different isotopes (same number of protons but a different number of neutrons) For example, chlorine has an atomic mass of 35.5 this is because if you were to take a sample of chlorine approximately 75% of the chlorine atoms in that sample would have an atomic mass of 35 and 25% would have a mass of 37 this gives and average of 35.5 and explains why the atomic weights of elements aren't integers.

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14y ago

Some of the elements in the Periodic Table are not whole numbers because it is the weighted average mass of all the different isotopes of that particular element, e.g the stated atomic mass for carbon takes into account carbon-12 (about 99%), carbon-13 (about 1%), and carbon-14 (trace amounts) to come up with about 12.011. Also, except for carbon-12, the mass even for a specific isotope is not a whole number due to two factors: 1) neutrons are slightly more massive than protons; 2) nuclear binding energy throws the mass off slightly. (Carbon-12 is the standard and has a mass of exactly 12 by definition.)

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12y ago

Atomic masses of elements are usually not whole numbers because of isotopes.

Elements tend to exist as more than one isotope, so the atomic mass in the corner is a weighted average of all of the isotopes an element exists as.

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12y ago

The atomic mass of an element listed on the periodic table is a weighted average of all of the naturally-occurring isotopes, which have different numbers of neutrons and therefore different masses.

Some elements have isotopes, isotopes are the same element (just with a different mass, so the same number of protons but more/less neutrons)

Most isotopes are unstable so are not is taken into consideration when calculating the RAM.

Take silicon as an example only three isotopes are stable;

28Si - 92.23%

29Si - 4.67%

30Si - 3.1%

For the element Si with the naturally occurring isotopes 28Si, 29Si, 30Si, and with the respective abundances of A%, B%, C% etc,

r.a.m. = (A/100 x a) + (B/100 x b) + (C/100 x c)

r.a.m. = ((92.23/100)*28) + ((4.67/100)*29) + ((3.1/100)*30)

r.a.m. = 28.1087

(RAM given by my data book is 28.1)

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8y ago
The atomic weight of an element, more accurately termed the relative atomic mass, is abundance-weighted for all of the naturally occurring isotopes of a given element. For many elements, there is a single isotope representing the majority of atoms, but even a small fraction of a different weight (i.e. more neutrons or less neutrons) will cause the atomic weight to vary from a whole number.

As an example, most calcium is calcium-40 with 20 protons and 20 neutrons. But about 3% of naturally occurring calcium has more neutrons, as calcium-41 through 44, 46, and 48. The atomic weight is given as 40.078 on the periodic table.
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Q: Why are atomic masses of elements usually not whole numbers?
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Related questions

How are the atomic numbers and the atomic masses of the elements related to how the elements are arranged on peridoic table?

Elements are arranged in a periodic table by atomic number, lower on top and left. Atomic masses have no direct relationship to the arrangement of atoms, although generally atoms with higher atomic numbers will have higher atomic masses. (There are at least three exceptions for atoms with atomic numbers differing by 1.)


Why aren't atomic masses of elements not whole numbers?

because of its no. in table of elements


Why don27t elements in the periodic table have whole numbers for their atomic masses?

Because the masses of protons, neutrons and electrons are not whole numbers.


Why the atomic mass numbers listed for elements on the Periodic Table are not whole numbers.?

Why are atomic masses of elements not generally whole numbers? The atomic masses listed on the periodic table are a weighted AVERAGE of an element'sisotopes. ... An element's atomic number is the number of protons in its nucleus. Number of protons specifies atom type.


What did Dimitri Mendeleeve do?

Dmitri arranged the elements in order of increasing atomic masses. He arranged elements in rows and columns according to atomic masses.


How did Mendeleev arrange chemical elements?

He arranged the elements in the increasing order of their atomic masses and repeating periodic properties.


When masses combine to form elements with larger masses?

Atomic fusion occurs when masses combine to form elements with larger mass.


How do atomic numbers and atomic masses increase in the periodic table?

The atomic number and atomic masses increase as you move from left to right.


What elements have such small masses that they are not even considered when atomic masses are calculated?

Electrons


Do all elements have different atomic masses?

yes


Early attempts at organizing the element arranged them according to?

properties of the elements and atomic masses


Why does the atomic mass of some elements is shown as a decimal value?

Many atoms exist in the form of different isotopes. These have different numbers of neutrons and so their atomic masses are different. The atomic mass for the element is the average of these masses, weighted together according to their abundance.