Answer
The atomic number of an element is by definition the number of protons in the nucleus of one of its atoms. This number is usually represented by the letter "Z". The carbon-12 isotope of the element carbon has six protons and six neutrons in its nucleus.. Therefore, the atomic number of carbon is six(6). The atomic number defines the element, so any chemical element that has 6 protons is an isotope of the element carbon. Similarly, the element oxygen has an atomic number of eight (8) because it has eight protons in each atomic nucleus.
The isotope carbon-12 of the element carbon has has the same number of neutrons as the number of protons--six protons and six neutrons. This is not always the case ; the number of neutrons may differ from the number of protons.. In any case, the sum of the number of protons and the number of neutrons is the atomic mass number( represented by the letter A ) . Since carbon-12 has six protons and six neutrons, then the atomic mass number of Carbon-12 . By a similar reasoning the atomic mass number of the most common isotope of oxygen (O-16) is 16 because here are eight protons and eight neutrons in each of its atomic nuclei. (8P+8N=16 nucleons).
Mass is the amount of matter in an object, and should not be confused with weight which is the force or the pull of gravity on an object. The current standard definition of atomic mass derives from the definition of the unit of mass by the International Union of Pure and Applied Chemistry. This definition states that the unit of mass --the unified atomic mass unit (u), also called the Dalton(Da)-- is the mass of 1/12th of a standard standard neutral atom of carbon 12 at rest . The masses of all other atoms or elements are to be compared with this standard . The atomic mas of an element can be computed using the Dalton. The Da or unified atomic mas unit is approximately 1.660x10^{-27) kg i.e 1.660x10x^{-24} gm.
For O-16) therefore the the atomic mass is 16. Using this value the value for the gram atomic mass of O-16 is 16x1.66x10^{-24}gm. This is approximately 2.65x^{-24}g.
Work Step by Step
The atomic mass of any element can be found in the Periodic Table under the symbol for the element Thus under C (for Carbon) we find the number 12.01; under O ( for Oxygen) there is 15.999; under N for Nitrogen) there is 14.006 and Cl for Chlorine there is 35.45. These are not he atomic masses of any one atom or isotope, but are averages of the atomic masses of the stable isotopes of the relevant elements, weighted for relative natural abundance of each isotope . For an example, the calculation of one of the relative atomic mass of the element Chlorine has the following factors:
By mass spectrometry measurements, chlorine has many isotopes (more than 30) but the two stable ones are Cl-35 and CL-37. All Chlorine atoms have 17 protons, but the atoms of CL -35 have 18 neutrons and the atoms of CL-37 have 20 neutrons. These two isotopes also differ in natural abundance: CL-37 is the most abundant accounting for 75.76% of stable chlorine element and CL-37 responsible for 24.25%.
The number in the Periodic Table is a mixed number because the isotopes of the element do not have the same abundance naturally, nor do they have the same number of nucleons; both factors are important in the calculation of the relative atomic mass
Example of Calculation;
Relative atomic mass of Chlorine:
Mass spectrometry measurements:
Mass of Cl-35 is 34.969 Da
% abundance of CL-35 is 75.77%(=0.7576)
Mass of Cl-37 is 36.965 Da
% abundance of CL-37 is 24.24%(=0.2424)
Mass of Chlorine
34.6 x 0.7576=24.494
36.97 x 0.2424=8.9624
Total 26.496+8.960=35.456
Relative atomic mass in Daltons
