Isotopes Matter

Test Your Knowledge
Key Idea 5: Which Atomic Weight Values Should I Use?

24) You learned that elements in the pink category of the IUPAC Periodic Table of the Elements and Isotopes have been assigned conventional atomic weight values. Would there be a purpose to assigning conventional atomic weights to elements from the blue category? Why or why not?

Click here to show answer

The elements in the pink category have been assigned a standard atomic weight that is given as an interval, which conveys the variety of atomic weight values these elements can have. Elements of this category are assigned a “conventional atomic weight”, which is a single value to be used in calculations in which the isotopic composition of the sample of atoms of an element cannot be known. Contrary to elements in the pink category, elements in the blue category are assigned a standard atomic weight that is given as a single value. Since the atomic weight of elements in the blue category is in the format of a single value, there is no need for them to be assigned conventional atomic weights.

25) You are given the mass spectrum of a sample of boron atoms with an unknown atomic weight, and told that the sample was sourced from either sea water or igneous rock. By analyzing the mass spectrum, you discover that the relative abundance of boron-11 is approximately 0.808 and the relative abundance of boron-10 is 0.192. You consult the IUPAC Periodic Table of the Elements and Isotopes and note that the atomic mass of boron-10 is 10.012 937 and that of boron-11 is 11.009 305.

By consulting the table below, what can you conclude about the source of the boron sample?

Fig. 1: Variation in atomic weight with isotopic composition of
selected boron-bearing materials (modified from M. W. Wieser,
and Coplen, T.B. Pure Applied Chemistry. 83, 359 (2011)).

26) You are given a sample of nitrogen atoms and are told that 0.4% of them are nitrogen-15 atoms and the rest of them are nitrogen-14 atoms. Show, using the appropriate equation, how you would use this information to calculate the atomic weight of this sample of nitrogen. (The atomic mass of nitrogen-14 is 14.003 074 004 and that of nitrogen-15 is 15.000 108 899.)

Click here to show answer

(0.004)(15.000 108 899) + (0.996)(14.003 074 004)

27) Use the table below to help you answer the following question: you are asked to calculate the molecular weight of nitrous oxide (N₂O). However, you don’t know the isotopic composition of either of the elements, nor the source of the sample. Based only on the information given in this question and the table below, how would you proceed? What is your calculated value?

Element	Atomic Weight (Interval)	Conventional Atomic Weight
Nitrogen	[14.006 43, 14.007 28]	14.007
Oxygen	[15.999 03, 15.999 77]	15.999

28) You are a scientist advising a detective who is investigating a crime. The only evidence left at the scene is a chip of a fingernail from the suspected criminal. You are told that the suspect was from one of the 6 places listed below. You take a sample of the nail to your lab to analyze the hydrogen and oxygen isotopic composition of the substance, which reflect the isotopic composition of the water consumed by the person and determine that the relative abundance of oxygen-18 atoms is 0.00203 and the relative abundance of deuterium atoms is between 0.001425 and 0.000145. Where is the suspect most likely from? (Note: the terms "mole fraction" and "relative abundance" can be used interchangeably)

Fig. 2: Cross plot of mole fractions of ²H and ¹⁸O of human nail samples
from a variety of global sites. The hydrogen and oxygen isotopic compositions
reflect the oxygen and hydrogen isotopic compositions of water consumed,
and generally they decrease with increasing latitude, increasing elevation,
and distance inland from the ocean-continent boundary.
(From IUPAC Periodic Table of the Elements and Isotopes
for the Education Community (IUPAC Technical Report).)

Click here to show answer

Belgium