# Half an amu?

1. Oct 17, 2008

### MrPickle

I was looking at my periodic table in Chemistry today and noticed that Chlorine's relative mass was 35.5, how can it have half an amu when it has 17 electrons? That's not 0.5amu, is it?

2. Oct 17, 2008

### fedaykin

The periodic table will give the average amu of the element in bulk from the ratio of isotopes. Chlorine has two major stable isotopes, 35 amu and 37 amu in ratios of ~75.75% and ~24.25% respectively. This gives an average weight of ~35.5.

I'm not entirely certain, but I think binding energies also play a role in the apparent weight of an element in bulk. The effect may be to small to bother considering for most purposes.

3. Oct 17, 2008

### Staff: Mentor

Problem is not that the effect is too small, problem is it is masked by the fact that you have already mentiond - all elements are mixtures of isotopes.

4. Oct 18, 2008

### Mentallic

I was once confused by the periodic table's amu figures for those small elements. The fact that hydrogen is $$\approx 1.008$$ made me think the electron was playing the role of that extra amu. As fedaykin said, it has to do with the isotopes (amount of neutrons present) that affects the amu of the element.
As your teacher/professor would've told you, the mass of the electron is so small compared to the proton/neutron, that it can be considered negligible. Somewhere in the range of 1/2500 mass. So one of the larger known elements, say 100 atomic number, those electrons would only add 0.04 amu to the element.

5. Oct 18, 2008

### Staff: Mentor

More like 11836.