The analysis of water shows that a water molecule contains 11.19% of hydrogen and 88.89% of oxygen. So the ratio of both components masses is 11.19:88.89 or 1:8. Or we can say that the ratio of both hydrogen and oxygen is 2:1. Mass of oxygen is 16 times the mass of an atom of hydrogen. Therefore relative mass of an atom of oxygen is 16 units if we take mass of an atom of hydrogen as one unit.
Similarly relative atomic mass of other elements is determined by taking hydrogen atom mass as standard. But that is old approach. Then it was preferred to take oxygen atomic mass as standard. But the international union of chemist selected the most stable isotope of carbon and that is C-12 isotope. This is taken as standard for comparison of atomic mass of various elements.
The mass of C-12 isotope was taken as 12 atomic mass units.
So the definition of atomic mass of any element can be defined as the average relative atomic mass of an atom of element as compared with the mass of an atom of carbon isotope C-12 as 12 unit. A scale is used to express the relative atomic mass unit. So the Atomic mass unit definition can be stated as the unit which is used to express the relative atomic mass is known as atomic mass unit. It is denoted as a.m.u. It can also defined as atomic mass unit or as unified mass as 1/12th of the actual mass of an atom of carbon isotope C-12. The atomic mass of any element shows that how many times as atom of the element is heavier than 1/12 of an atom of carbon isotope C-12.
So the Atomic Mass Formula can be written as;
Atomic mass= mass of an atom of an element / 1/12 x mass of an atom of carbon isotope C-12
This formula is useful in calculating Atomic Mass. For example, atomic mass of magnesium is 24 u. This shows that an atom of magnesium is 24 times heaviour than 1/12 of an atom of carbon isotope C-12.
But there are many cases when different atoms of the same elements possess different relative masses. Such atoms of the same elements are called isotopes. So in these cases, atomic mass of the elements is calculated as average atomic mass.
The Average atomic mass formula = Σ (mass of isotope × relative abundance)
For example, chlorine has two isotopes having relative masses 35u and 37u with relative abundance in nature is 3:1. Thus the atomic masses chlorine is the average of these different relative masses as mentioned below;
Atomic mass of chlorine = 35x 3 + 37x1 / 4 = 35.5 u.
Similarly relative atomic mass of other elements is determined by taking hydrogen atom mass as standard. But that is old approach. Then it was preferred to take oxygen atomic mass as standard. But the international union of chemist selected the most stable isotope of carbon and that is C-12 isotope. This is taken as standard for comparison of atomic mass of various elements.
The mass of C-12 isotope was taken as 12 atomic mass units.
So the definition of atomic mass of any element can be defined as the average relative atomic mass of an atom of element as compared with the mass of an atom of carbon isotope C-12 as 12 unit. A scale is used to express the relative atomic mass unit. So the Atomic mass unit definition can be stated as the unit which is used to express the relative atomic mass is known as atomic mass unit. It is denoted as a.m.u. It can also defined as atomic mass unit or as unified mass as 1/12th of the actual mass of an atom of carbon isotope C-12. The atomic mass of any element shows that how many times as atom of the element is heavier than 1/12 of an atom of carbon isotope C-12.
So the Atomic Mass Formula can be written as;
Atomic mass= mass of an atom of an element / 1/12 x mass of an atom of carbon isotope C-12
This formula is useful in calculating Atomic Mass. For example, atomic mass of magnesium is 24 u. This shows that an atom of magnesium is 24 times heaviour than 1/12 of an atom of carbon isotope C-12.
But there are many cases when different atoms of the same elements possess different relative masses. Such atoms of the same elements are called isotopes. So in these cases, atomic mass of the elements is calculated as average atomic mass.
The Average atomic mass formula = Σ (mass of isotope × relative abundance)
For example, chlorine has two isotopes having relative masses 35u and 37u with relative abundance in nature is 3:1. Thus the atomic masses chlorine is the average of these different relative masses as mentioned below;
Atomic mass of chlorine = 35x 3 + 37x1 / 4 = 35.5 u.
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