Expert: Walter Hintz - 9/10/2009

Question
Of the elements hydrogen, carbon, oxygen, magnesioum, and copper, whuch have atromic weights that are not close to whole number values? explain please...

Answer
 Hi Heather
The mass number listed on the periodoc table is the average of all weights of the isotopes. And the mass is the mass of the protons plus the neutrons.  Carbon for example has 6 protons and 6 neutrons and the mass is 12. But Carbon has three isotopes C12,C13 and C14 so the average of these is 12.0112. Of the elements you listed Cu is the heavest. The atomic number of Cu is 29 so the mass of stable Cu is 58. But the periodic table list the mass as 63.546.
The heavier the element the greater the difference between the actual mass and the number calculated