The periodic table lists the mass of an atom in amu, So, the periodic table provides us with a great deal ofinformation. The number of atoms I placed on the balance is know as a mole.įor many years the number of atoms in a mole remained unknown however, now it is know that a mole of atoms contains 6.02214 x10 23 atoms.

Now, I state with absolute certainty that I have placed the samenumber of atoms on each balance! How do I know? I know because therelative masses of the samples on the balance, are the same as therelative masses of the individual atoms. It is also possible to place 183.9 g W, or 12.01 g of C on abalance. What is the relative mass of 100 W atoms as compared to 100 Hatoms? W:H = (183.9/1.0079):1 =182:1 The point here? As long as the number of atoms remains the same the relative mass does not change.Ītoms are small, and it is possible to place 1.0079 g of H on abalance ( possible but not easy in the case ofhydrogen). What is the relative mass of 1 W atom as compared to 1 H atom? W:H = (183.9/1.0079):1 =182:1 What is the relative mass of 1 C atom as compared to 1 H atom? C:H = (12.01/1.0079):1 =11.92:1 If thenatural abundance of each isotope is 60.00 and 40.00 % respectivelywhat is the mass (in amu) of 71Ga. Gallium, atomic mass 69.72 amu, has two major isotopes, 69Ga, atomic mass 68.9257 amu, and 71Ga. Of course, a question like the one above could be turned aroundanother way. To the correct number of significant figures Since there are only two major isotopes of Cu we know that One equation and two unknowns.is there another equation? Ifthere is another equation we would have two equations and twounknowns, and a system of two equations and two unknowns issolvable. The atomic masses of 63Cuand 65Cu are 62.9296 and 64.9278 amu respectively what isthe natural abundance of each isotope? IsotopeĪnother kind of question could be asked.Ĭopper has two isotopes 63Cu and 65Cu. If we know the natural abundance(the natural abundance of an isotope of an element is the percent ofthat isotope as it occurs in a sample on earth) of all the isotopesand the mass of all the isotopes we can find the average atomic mass.The average atomic mass is simply a weighted average of the masses ofall the isotopes. (Yes, the number 12.01 has the right number of significantfigures, even though 1.11% only has 3 significant figures.) The average mass is simply a weighted average. The mass spectrometer reports that there are two isotopes ofcarbon, 98.99% of the sample has a mass of 12 amu ( not a surprise since this is the atom on which the scale is based).ġ.11% of the sample has a mass of 13.003355 amu ( this isotope is 1.0836129 times as massive as 12C) The average mass of a carbon iscalculated from the information the mass spectrometer collects. 14C dating is accomplished by measuring the radioactivity of a sample, not by actually counting the number of 14C atoms. The natural abundances of 14C, 10C and 11C are so low that most mass spectrometers cannot detect the effect these isotopes have on the average mass. If a sample of carbon was placed in a mass spectrometer thespectrometer would detect two different C atoms, 12C and 13C.

No single carbon atom has a mass of 12.01 amu, but in a handful of C atoms the average mass of the carbon atoms is 12.01 amu. This is the average atomic mass of carbon. On the periodic table the mass of carbon is reported as 12.01 amu. Since many elements have a number of isotopes, and since chemistsrarely work with one atom at a time, chemists use average atomicmass. Since there are a variety of carbon isotopes we must specify which C atom defines the scale.Īll the masses of the elements are determined relative to 12C.īy the way, the mass of an element is not equal tothe sum of the masses of the subatomic particles of which the elementis made! 12C has 6 neutrons, 13C has 7 neutrons, and 14C has 8 neutrons and so on. Each carbon atom has the same number of protons and electrons, 6. Carbon exists as two major isotopes, 12C, and 13C ( 14C exists and has a half life of 5730 y, 10C and 11C also exist their half lives are 19.45 min and 20.3 days respectively). Why do we specify 12C? We do not simply state the themass of a C atom is 12 amu because elements exist as a variety ofisotopes. Atomic mass is based on a relative scale and the mass of 12C (carbon twelve) is defined as 12 amu.

Periodic table molar mass'