# Which mass number is shown on the periodic table?

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1. Mar 9, 2015

### Jewish_Vulcan

There are many different mass numbers for a given element known as isotopes. Are elements on the periodic table isotopes of their element? Why not use the average of all the mass numbers for an element?

2. Mar 9, 2015

### matteo137

I think it is usually the average weighted by the isotope abundance.

3. Mar 9, 2015

### Bystander

The average of the natural abundances of the naturally occurring isotopes is used. That's the "relative atomic mass" used for practical purposes.

4. Mar 9, 2015

### Quantum Defect

Carbon-12 is defined to be exactly 12.000000000000..... g/mole in the current version of the periodic table. It used to not always be this way, though. In the past Oxygen-16 was the yardstick to which all other masses would be measured.

You can tell that the peridic table lists the weighted averages of the molecular masses, because C has an atomic mass of 12.011 g/mole. The 0.011 comes from the small amount of Carbon-13 that is naurally there, as well as the teeny-tiny amount of Carbon-14 that is present in things.

What is the natural abundance? Well, it is some kind of average, that someone had to decide was reasonable. A lot of very interesting science can be done looking at small variations in the natural abundance of varous isotopes. Looking at the nitrogen isotopes in an organism's body can tell someone something about the diet. Looking at the isotopes of oxygen in ice cores can tell someone about the temperature of the climate when the ice was deposited. The relative decrease of carbon-14 in the atmospheric carbon dioxide tells us something about the impact of the combustion of fossil fuels on atmospheric sources of carbon dioxide. [Fossil fuels have very little carbon-14 left, as they are quite old...]

5. Mar 9, 2015

### Jewish_Vulcan

Great response! That answered part of my question about the atomic mass. I want to know why all the mass numbers(number protons+neutrons) are whole numbers on the periodic table. All mass numbers for every isotope of an element is a whole number but which mass number is used on the periodic table? If it was an average mass number than would there not be decimals to account for the average difference in the number of neutrons?

6. Mar 9, 2015

### Quantum Defect

Not all masses are whole numbers. Only Carbon 12 because it is defined to be that way. The neutron and proton masses are close to one, and when you combine these to make a nucleus, they almost add up to the atomic mass, but they don't because of the nuclear binding energy.

Look in a table of atomic masses (the CRC Handbook of Chemistry and Physics has these.) You can find many of these on wikipedia. You will see some interesting numbers for different nuclei.

The Wikipedia entry for iron lists many of the isotopes for iron, as well as their atomic masses. If you look at stability compared with the difference between the atomic mass and the atomic number, you may see something interesting. If you took a weighted average of all of these, weighted by their natural abundances, you should get the atomic mass listed in the periodic table.

http://en.wikipedia.org/wiki/Isotopes_of_iron

7. Mar 9, 2015

### Jewish_Vulcan

I do not think you understand what I am asking, maybe I should have explained better. I simply want to know why the mass numbers are all whole on the periodic table,
"Not all masses are whole numbers" That is true for atomic mass numbers( which is the same as atomic mass) but ALL MASS NUMBERS ARE WHOLE NUMBERS. There are many whole number mass numbers for a given element.

I believe you are explaining the atomic mass number which is measurement that actually uses the mass of the particles in u units. u units are now based off of the element carbon 6 12. a u unit is 1/12 the mass of a carbon 6 12 atom which is the most common isotope of carbon. U units are used to measure the average atomic mass number of every element. To clarify, atomic mass number is not mass number. atomic mass number is about equal to the atomic mass. Atomic mass number has units of U.
sorry ;___; for being confused.

8. Mar 9, 2015

### SteamKing

Staff Emeritus
If you are referring to the mass number = number of protons + number of neutrons in the atomic nucleus, then yes, these are whole numbers.

However, from a practical standpoint, in chemistry the weighted average atomic mass is used in calculations, especially the stoichiometric ones where you are trying to figure out how to make a certain quantity of chemical C from chemicals A and B. In nuclear physics, you may use actual numbers of nucleons. It depends on what you are doing with the information.

9. Mar 9, 2015

### Quantum Defect

Ok, I think I understand. The atomic number on the periodic table is the number of protons or electrons. Different isotopes have different mass number, i.e. Uranium's most famous isotopes are Uranium 235 and Uranium 238, but have the same atomic number. Both isotopes have atomic number 92. The mass numbers tell you the total number of (protons plus neutrons as @ SteamKing has noted) that went into making the nucleus. As these are countable, they are represented as whole numbers. Uranium 235 has 143 neutrons, and Uranium 238 has 146 neutrons. The atomic masses are close to the integers 235 and 238, respectively, but not quite due to the nuclear binding energy (E = mc^2).

10. Mar 9, 2015

### Jewish_Vulcan

Yes I was reffering to mass number=number protons + number neutrons. I believe I found my confusion, I thought the periodic table showed 3 things: mass number, atomic mass, and atomic number. I just looked at a periodic table and it there is no mass number on the periodic table. This clears a lot of things because it would not make sense to include a mass number in the periodic table. The atomic mass is the average atomic mass of all of the natural isotopes of the elements. I was reading some particle physics chapters in my physics book so I was mostly focused on mass number. Sorry for being stupid about this, I should have seen a periodic table before posting.OVERALL THE PERIODIC TABLE DOES NOT SHOW MASS NUMBER, ONLY ATOMIC NUMBER AND AVERAGE ATOMIC MASS OF NATURAL ISOTOPES OF THAT ELEMENT.

11. Mar 9, 2015

### Jewish_Vulcan

Of course. Thank you, it was my fault for thinking the periodic table included mass number. Thank you very much. Thank you.

12. Mar 9, 2015

### Quantum Defect

These are things that confuse many students.