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It's not just an element, it's a particular isotope of a particular element.
The currently accepted standard is carbon-12, which by definition has a mass of precisely 12 Atomic Mass units when at rest, isolated, and in its nuclear and electronic ground states.
Before 1961, the scale was based on oxygen instead, but since this standard came into being before isotopes were known, it was a bit confusing:
Before 1929, or before 1961 if you were a chemist, an atomic mass unit was 1/16 of the isotopically weighted average mass of an oxygen atom.
However, if you were a physicist, between 1929 and 1961 an atomic mass unit was 1/16 the mass of an oxygen-16 atom.
The two definitions differed by about 0.03% - not a lot, but enough to show up in high precision work, which is the reason for the 1961 redefinition.
(Originally, the mass of hydrogen was used as the standard. I can't find any information on precisely when the switch to oxygen took place, but it would almost certainly have been after 1875.)
What else can I help you with?
Why was basis of classification of element in the periodic table changed from atomic mass to atomic number?
Mendeleev's periodic table was based on the atomic masses of elements. However, this was not effective when isotopes were discovered. An isotope of an element is defined as the element having the same atomic number but varying mass numbers. So, mass numbers weren't constant and hence, a better characteristic was chosen -atomic number. Atomic number of any element was a characteristic of a particular element. Hence atomic numbers were taken as the basis of classification instead of atomic masses.
Can the atomic number of an particular element ever change?
No, the atomic number of an element can't change. Because, we can identify an element by its atomic number and atomic mass. and hence they have been arranged in the periodic table on the basis of their mass number and atomic mass so , it couldn't be changed. Every element has a fixed atomic number..
How is moseleys basis for arranging the elements different from mendelees?
Moseley's basis and Mendeleev's basis both involved experimental measurements. But Moseley's involved X-ray properties, and X-rays were not discovered when Mendeleev first stated the Periodic Law. Atomic weight and atomic number follow nearly the same sequence, but there are three cases of reversal of order: Argon element 18 atomic weight 40.0; potassium element 19 atomic weight 39.1. (Argon was not discovered until 1894; first periodic table 1869). Cobalt element 27 atomic weight 58.9; nickel element 28 atomic weight 58.7. (Very small difference; Mendeleev gave these two elements identical atomic weights) Tellurium element 52 atomic weight 127.6; iodine element 53 atomic weight 126.9 (Major problem for Mendeleev -- he insisted that the atomic weight of tellurium had to be 125, but careful re-measurements continued to show 127.5-128) Moseley's atomic number is definitely a better basis for the periodic law than Mendeleev's original suggestion of atomic weight.
Why is atomic number the proper basis for classification of elements?
Because Atomic number is the identity of an element , all the atoms of an element have the same atomic number while their mass numbers ma be different. for Example Argon(Ar) a noble gas and Calcium(Ca) an active metal have the same mass number 40.
How did Henry Moseley revise mendeleevs periodic table?
In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore rearranged the table by nuclear charge / atomic number rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
John Dalton's atomic theory stated that atoms of the same?
According to John Dalton, all the atoms of a single element will have the same mass. This is the basis for the modern atomic theory.
Why was basis of classification of element in the periodic table changed from atomic mass to atomic number?
Mendeleev's periodic table was based on the atomic masses of elements. However, this was not effective when isotopes were discovered. An isotope of an element is defined as the element having the same atomic number but varying mass numbers. So, mass numbers weren't constant and hence, a better characteristic was chosen -atomic number. Atomic number of any element was a characteristic of a particular element. Hence atomic numbers were taken as the basis of classification instead of atomic masses.
Can the atomic number of an particular element ever change?
No, the atomic number of an element can't change. Because, we can identify an element by its atomic number and atomic mass. and hence they have been arranged in the periodic table on the basis of their mass number and atomic mass so , it couldn't be changed. Every element has a fixed atomic number..
How is moseleys basis for arranging the elements different from mendelees?
Moseley's basis and Mendeleev's basis both involved experimental measurements. But Moseley's involved X-ray properties, and X-rays were not discovered when Mendeleev first stated the Periodic Law. Atomic weight and atomic number follow nearly the same sequence, but there are three cases of reversal of order: Argon element 18 atomic weight 40.0; potassium element 19 atomic weight 39.1. (Argon was not discovered until 1894; first periodic table 1869). Cobalt element 27 atomic weight 58.9; nickel element 28 atomic weight 58.7. (Very small difference; Mendeleev gave these two elements identical atomic weights) Tellurium element 52 atomic weight 127.6; iodine element 53 atomic weight 126.9 (Major problem for Mendeleev -- he insisted that the atomic weight of tellurium had to be 125, but careful re-measurements continued to show 127.5-128) Moseley's atomic number is definitely a better basis for the periodic law than Mendeleev's original suggestion of atomic weight.
Element used to make atomic clock?
One type of atomic clock uses Rubidium.
Is carbon an element?
Yes, it is. It's atomic number is six.Carbon is an element: number 6 on the periodic table.
Why is atomic number the proper basis for classification of elements?
Because Atomic number is the identity of an element , all the atoms of an element have the same atomic number while their mass numbers ma be different. for Example Argon(Ar) a noble gas and Calcium(Ca) an active metal have the same mass number 40.
How did Henry Moseley revise mendeleevs periodic table?
In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore rearranged the table by nuclear charge / atomic number rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
What discovery did Henry Moseley lead him to reorganize the periodic table?
In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore resequenced the table by nuclear charge rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
How did Henry Moseley affect the development of the periodic table?
He discovered that each element has its own atomic number.In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore resequenced the table by nuclear charge rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
Why is the periodic table arranged in order atomic weight?
The periodic table is arranged in order of atomic number, not atomic weight. This is because the properties of elements are determined by the number of protons in the nucleus, which is the atomic number. Atomic weight is not used because it can vary due to isotopes of an element.
What does the letter C stand for in the periodic table?
Carbon. Mass of 12.011. Atomic number 6.
