An atom of 135I has a mass of 134.910023 amu. Calculate the binding energy in MeV per atom. Enter your answer with 4 significant figures and no units. Use the masses: mass of 1H atom = 1.007825 amu mass of a neutron = 1.008665 amu 1 amu = 931.5 Me. On this definition, the value of 1 atomic mass unit = (1/12) (12 gram)/ (value of Avogadro Number). Using known value of Avogadro Number the value of one amu can be determined in gram. It comes not to be 1 amu = 1.661 × 10^.

Why is the atomic mass unit (amu), rather than the gram, used to express atomic mass?

1 Answer

Because atoms are ridiculously small.

And the #'amu'# is numerically equivalent to the #'g/mol'#. For instance, if I were to be so lucky as to isolate #'1 atom'# of #'N'#, it would have a mass of

#14.007 cancel'amu' xx (1.6605 xx 10^(-24) 'g')/(cancel'1 amu')#

1 amu = 1.660539040 (20)×10^−27 kilograms (Unified atomic mass unit - Wikipedia). There are 1000 grams in a kilogram. You can do the math. If not, check out the Khan Academy lesson on dimensional analysis here. 1.11% of the sample has a mass of 13.003355 amu (this isotope is 1.0836129 times as massive as 12 C) The average mass is simply a weighted average. Mass = 12.01 amu (Yes, the number 12.01 has the right number of significant figures, even though 1.11% only has 3 significant figures.).

#= ul(2.326 xx 10^(-23) 'g')#

which is immeasurably small. We don't care for masses that small because we physically can't see or measure it. Instead, we care for masses we can touch, like #'1.000 g'# or #'12.50 g'#.

And that involves:

#1.000 cancel'g N' xx cancel'1 mol N'/(14.007 cancel'g N') xx (6.022 xx 10^23)/(cancel'1 mol')#

#= ul(4.299 xx 10^22 'N atoms')#

#12.50 cancel'g N' xx cancel'1 mol N'/(14.007 cancel'g N') xx (6.022 xx 10^23)/(cancel'1 mol')#

#= ul(5.374 xx 10^23 'N atoms')#

You can clearly tell that this number of atoms is impossible to count. And so Avogadro's number, #6.022 xx 10^23 'mol'^(-1)#, was invented to describe this many particles...

#4.299 xx 10^22##'N atoms' xx ('1 mol')/(6.022 xx 10^23)#

#=##ul'0.0714 mols N'#

#5.374 xx 10^23##'N atoms' xx ('1 mol')/(6.022 xx 10^23)#

#=##ul'0.8924 mols N'#

And as you can see, these numbers look much nicer and more physically useful.

Value Of 1 Amu

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