Confused about amu
 

Hi,

First off, amu is just a convienent way of expressing the weight of atoms, subatomic particles, right? It is my understanding that the amu is defined as 1/12 the mass of a Carbon-12 atom (12 amu = 1 Carbon-12 atom). Is it not also true that both protons and neutrons have masses of over 1 amu? If this is so, Carbon-12, containing 6 neutrons and 6 protons, as well as 6 electrons, should have a mass of slightly over 12 amu, right?

As in: 6*1.0002 + 6*1.0003 + electron masses (I know those numbers are not correct, just an example)

Another thing: the Law of Multiple Proportions doesn't make any sense to me. Well it sort of does, I just don't grasp the importance of it. If someone could help me out with that (please use multiple examples) that would be great!

Thanks,
Cam

It is true that the mass of both protons and neutrons is greater than 1 amu - when treated as separate items.

However when they are combined together into a nucleus some of the mass is converted into energy according to Einsteins E=mc^2 - This is called the nuclear binding energy and the missing mass is called the mass defect.

It is the principle behind nuclear fusion energy. Small nuclei bind together and release energy - this energy is directly converted from the mass lost.

In a hydrogen bomb the small nuclei of the deuterium with one proton an one neutron are forced together to form one helium atomic nucleus. The mass lost is turned into energy and the reaction releases this energy as a sudden blast of radiation and heat.

These nuclear reactions are nothing to do with chemical reactions and are the only examples of the non-adherence to the law of conservation of matter.

In reality chemical reactions that release energy must also cause a mass difference but as this is so tiny as to be unrecordable then we say that mass is conserved.

To go back to carbon nuclei - if the nucleus is measured (=12amu) and the sum of 6p and 6n is added up, the difference is due to the nuclear binding energy. If you wished to separate the nucleus into its individual nucleons then this quantity of energy would have to be absorbed and would turn into the mass required for the total of 6p + 6n.

Thanks
 

Thanks for the reply,

That makes sense. So if we look at an atom of Nitrogen, the only reason it is not exactly 14 amu is because of the extra electron?

I'm confused about this question:

Gallium consists of two naturally ocouring isotopes with masses of 68.926 and 70.925 amu. How many protons and neutrons are in the nucleus of each isotope?

Well of course there is 31 protons in each, but how do you know for sure how many neutrons there are? Looking at the answers it would appear that you just round off to the nearest whole number and subtract the protons. This makes sense I guess but why the decimals?

Thanks,
Cam

Who mentioned electrons?!!!?

The electrons are ALMOST NEVER considered when calculating the atomic mass.

If you read my post you will see that there is something called BINDING ENERGY which subtracts from the sum of the protons and neutrons to give a formula (in the case of nitrogen) 7p + 7n does NOT equal mass of nitrogen.

We take the mass of protons and neutrons as determined by other means (when on their own) and we use a mass spectrometer to measure the mass of the nitrogen atom on its own. The difference between these two values is the mass defect which relates to the binding energy using Einstein's equation E=mc^2


you are given the decimals just to demonstrate that the mass values can be measured very accurately indeed. You are correct in assuming that they are not needed for this question.
You could read this as Ga-69 (31p, 38n) and Ga -71 (31p, 40n) without any problems.

Forget about the electrons, they have a mass of approx 0.00054 amu they are not going to affect any calculation at this level.

OK?

Thanks,

I wish the textbook would have provided information like this. So basically an atom of Nitrogen has differing amounts of nuclear binding energy (related to differing mass defect) from Carbon, therefore altering its atomic mass from what the untrained eye (such as mine) might expect?

Another question:

Alpha rays are streams of 2 protons, or similar to 2 protons? And Beta rays are streams of electrons?

Mass spectrometers...now those are confusing. I'll have to do some reading on those.

Thanks again,
Cam

EDIT: Could you help me out with the Law of Multiple Proportions?

alpha partices are helium nuclei 2p+2n ejected from the nucleus at high energy. This form of nuclear decay occurs in large mass nuclei.

beta particles are high energy electrons ejected from the nucleus. This decay occurs in either small or large nuclei. It reslts in no change in mass number but an increase in one atomic number (one neutron breaks down to give a proton and an electron, which is then ejected)

For info on the mass spectrometer with animationclick here

The decimals are also there to show that a standard sample of that element will contain different isotopes in certain standard ratios. For example, there are two isotopes of Cl which are present in nature, with masses (to the nearest whole number) of 35 and 37 in a 3:1 ratio. So, 0.75 x 35 + 0.25 x 37 = 26.25 + 9.25 = 35.5, which is the atomic mass of Cl as listed on periodic tables (to one decimal point).

Thanks for the help guys,

Anyone want to help me out with Multiple Proportions?

-Cam