We know that everything in the universe is made from the atoms. They are the basic building entities. Atoms are not solid entities but are made of dense nuclei surrounded by fuzzy cloud of electrons whizzing in the outer. Nucleus is made of protons and neutrons.
Things don’t not end here. Nature is very complex. Even the atoms of same elements can have different number of neutrons making them have different weights. Such atoms are called isotopes.
Atoms of same element or different elements can combine in umpteen ways resulting in diverse molecules and different substances from very simple molecules like water and methane to complex molecules containing thousands of atoms.
Many elements have more than one isotopes. One of these is the most abundant and is for all practical purposes considered as the representative. Other isotopes exist in very low abundance. All elements can be divided into different groups in which the chemical and physical properties vary in periodic manner.
Earlier, we remember the chemical composition of compounds was determined by chemical methods using classical techniques. From the elements proportions an empirical formula was derived. Methods gave numbers of constituent elements which have to be rounded off. This happened due to the inherent limitations of determinations. So generally the molecular weights were either whole numbers or at most rounded off to first decimal place.
Thanks to the extremely accurate measurements now available with advancement of science, we are measure the abundance of all isotopes very accurately. Ten elements namely hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium which have one or more isotopes have now been selected whose atomic weights shall be displayed in the periodic table as a range.
For example, the atomic weight of boron (atomic number 5) is currently written as 10.811. On the new periodic table, it will be given as an interval—from 10.806 to 10.821. This might not seem like a big change—and it is very small—but such a change can be critical to calculations in scientific research and for industrial applications. Also, chemistry teachers and students will have to learn how to use the new weight intervals.
Since now we the abundance and atomic weights of each isotope very accurately, the range will fix the upper and lower limits. By comparing the atomic weight of a particular element in a sample and mapping it on the range interval we can know the source of sample.
For example, oxygen atoms in the water samples contain isotopes. During evaporation fractionation occurs. Water molecules with lighter oxygen atoms evaporate faster leaving behind the water with heavier isotopes.
Similarly, the atomic weight of carbon is smaller in performance-enhancing drugs than in natural testosterone meaning natural testosterone contains higher abundance if heavier carbon atoms. This difference can be used to test whether athletes used these drugs to improve their performance.
The new atomic weight measurements not only account for the presence of isotopes but also consider their relative concentrations in the universe. Carbon 12 makes up 98.89% of all carbon, while carbon 13 is 1.11%, and the natural abundance of carbon 14 is 0.0000000001%.
So, the weight interval for carbon will lean more heavily toward carbon 12 and range from 12.0096 to 12.0116. This range will replace the average atomic weight for carbon listed in any chemistry textbook, which is 12.011.