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.
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.
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.
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.)
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.
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)
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.)
The atomic number and atomic masses increase as you move from left to right.
when you have two different forms of an atom with different masses, it's called an isotope of the atom.
same slot
Isotopes (Pretty Sure)
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.)
because of its no. in table of elements
Because the masses of protons, neutrons and electrons 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.
Dmitri arranged the elements in order of increasing atomic masses. He arranged elements in rows and columns according to atomic masses.
He arranged the elements in the increasing order of their atomic masses and repeating periodic properties.
Atomic fusion occurs when masses combine to form elements with larger mass.
The atomic number and atomic masses increase as you move from left to right.
Electrons
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properties of the elements and atomic masses
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.