# Electron number

1. Oct 10, 2011

### lssam

Hi
l have this question in my mind l never found a good reference for the answer .

my question is to know how did they find the number of electrons for an element ?

l am not talking about how to read the table to know , but how that table was made ?
what process was used to figure out the number of electrons in an element?

Thanks

2. Oct 11, 2011

### mcbud

Read Henri Moseley's biography in Wikipedia and you will find out how he was able to do it.

3. Oct 11, 2011

### lssam

Thank you .

l did , lf l got it right he figured it out based on Bohr Model or theory of the atom and analyzed
the specturm of of the element emitting light .
different colors refers to energy levels so he could then specify what energy levels the atom
of that element has and by seeing carefully the number of streaks of every individual
color specturm he could also know how many electrons in that energy level.

so by summing he could then know the number of electrons .
is that correct ?

4. Oct 11, 2011

### mcbud

Not quite. As you can see, the X-ray spectrum of an element has many bands, belonging to different energy level transitions. He found a nice correlation between the frequency of the most intense short-wavelength band of the X-ray spectrum (K-alpha line) of an element and its atomic number Z. The result has his name (Moseley's Law): the square root of the frequency of that band is proportional to Z. The line results from the transition of the electron from the 2p to the 1s orbital. The equivalent transition in the hydrogen atom belongs to the Lyman series (the transition 2p → 1s will be the Lyman-alpha band to be exact), but the emission here will be in the UV side of the spectrum, instead of in the X-ray region.

By doing this, he was able to predict missing elements in the periodic table at the time, and reorder others.

5. Oct 11, 2011

### lssam

so in a brief words , using those formulas based on the theories , by measuring
the frequency of the light speculum plug into the equation we know the atomic number.

that also means any frequency of light spectrum must give the same atomic number ?

6. Oct 12, 2011

### mcbud

For the lines in the X-ray spectrum, yes. Moseley also correlated the L-alpha lines belonging to the L-series of the spectrum with atomic number and found good correlation. The L-alpha series represents the transition from a level 3 orbital to a level 2 orbital. The L-alpha lines are not as intense as the K-alpha lines.

Section 2.2 of the following paper explains very well how X-ray techniques were used to determine Z. It is a good read, and shows the correlation lines between the frequency and atomic number: http://www.spectroscopynow.com/FCKeditor/UserFiles/File/specNOW/Enc_Anal_Chem_6801.pdf [Broken]

BTW, this kind of X-ray radiation coming from the movement of electrons between energy levels is called "characteristic" radiation (because each element would have peaks "characteristic" to that element). There is another kind of X-ray radiation, called "continuous", "white radiation", or "bremsstrahlung" radiation. When electrons are fired to a metal target (they are fired by using a difference in voltage), they decelerate. This deceleration emits energy in the form of X-rays if the original source is of high energy. Hence, white radiation does not arise from electronic transitions between energy levels, but from the reduction in the speed of electrons as they hit the metal. Continuous radiation is not correlated to Z, only to the accelerating potential of the electrons. Hence in theory, two different elements (different Z) should have similar continuous radiation spectrum if their electrons were accelerated to the target using the same high voltage. Their characteristic spectrum, however, will be different.

Notice that Moseley did not derive his equation based on theory. He used the results of his experiments to find a relationship between frequency and Z. Once he had that, he went to the Bohr model (which was the existing atomic model accepted at the time) and attempted (successfully) to correlate this relationship with what was known (or theorized) about electronic transitions at the time. So Moseley's relationships are empirical, but the results could be explained by the Bohr model of the atom.

Unfortunately, introductory chemistry books don't explain this very well. In my opinion, physics books are much better at explaining all of this, even though this has lots of implications in chemistry.

I hope this helps.

Last edited by a moderator: May 5, 2017
7. Oct 12, 2011

### lssam

Thanks a lot for the link and the explanation , very valuable information .
that was really useful clearing my thoughts on that topic and better understanding of things stack on top each other over the time .

Best Regards