[philonossis]

"How many subatomic particles are present in an atom whose isotopic mass is 36.96590 amu and whose electron configuration is 1s²2s²2p⁶3s²3p⁵?"

My method:
1. the superscript are added to get the number of electrons (17) and number of protons (17).
2. rounding off the isotopic mass, 36.96590 amu, gives 37.
3. 37 - 17 = 20. therefore there are 20 neutrons.
4. 17 + 17 + 20 = 54 subatomic particles total in the atom.

I feel a little wobbly on this because the amu supplied by the question is a weighted average. Is this the correct method?

[sjb]

"How many subatomic particles are present in an atom whose isotopic mass is 36.96590 amu and whose electron configuration is 1s²2s²2p⁶3s²3p⁵?"

My method:
1. the superscript are added to get the number of electrons (17) and number of protons (17).
2. rounding off the isotopic mass, 36.96590 amu, gives 37.
3. 37 - 17 = 20. therefore there are 20 neutrons.
4. 17 + 17 + 20 = 54 subatomic particles total in the atom.

I feel a little wobbly on this because the amu supplied by the question is a weighted average. Is this the correct method?

A weighted average of what?

[philonossis]

I feel a little wobbly on this because the amu supplied by the question is a weighted average. Is this the correct method?

A weighted average of what?
[/quote]

I will go back and review that section of the book.

[TheUnfocusedOne]

Here are some questions to ask yourself. 
What are the wieghts of protons, electrons and neutrons?
What is the relationship between the number of electrons and number of protons in an atom?

If you can answer those, you should be able to figure out the rest.

[philonossis]

A weighted average of what?

"The isotopic mass for each isotope, that is, the relative mass of each isotope on the ¹²₆C scale."  p. 127, Intro to Chemical Principles, by Stoker.

"A relative scale of atomic masses has been set up....The unit is called the atomic mass unit, abbreviated amu.  The arbitrary reference point, involves a particular isotope of carbon, ¹²₆C.   The mass of this isotope is set at 12.00000 amu.  the masses of all other atoms are then determined relative to that of ¹²₆C.

Since isotopes exists, the mass of an atom of a specific element can have one of several values.... Despite mass variances among isotopes, the atoms of an element are treated as if they all had a single common mass.  the common mass value used ia weight average mass[italics the author's], which takes into account the natural abundances and atomic masses of the isotopes of an element." pp. 126-127.

Therefore, the "isotopic mass" is a relative mass not a weighted average.  My mistake.