Chemistry in the News - Changes to Periodic Table

The periodic table is a tabular arrangement of chemical elements used in science classrooms across the world. And based on a recent change approved by the International Union of Pure and Applied Chemistry (IUPAC), the table just gained a little bit of weight. 19 elements have had a change in their atomic weight that will now be reflected in the table. Gold, arsenic, and aluminum are examples of some of the elements affected by the change. The changes mean that the elements on the periodic table now weigh 0.003640021 more than before the update.

How does this happen and what does this mean? Well, we first have to understand what atomic weight is and what it means. Atomic weight is the average mass of an element in atomic mass units (amu). To calculate this number, scientists take the average atomic weights of all the element's stable isotopes. If you recall from Chapter 1 of our studies, an element can exist in different forms based on the number of protons and neutrons it has in the nucleus. When the number of neutrons is different, isotopes are formed. Some elements have a plethora of isotopes, and this is one of the main reasons for the change in the official atomic weights.

Four of the 19 changes are due to a change in the calculations of isotopes which changed the atomic mass number. The remaining 15 elements were changed in minor ways based on improvements in measurements. An example of how minuscule the changes are: gold was changed from 196.966 569(4) amu to 196.966 569(5) amu. The number in parentheses (the one changed) represents the uncertainty in the last digit of the atomic weight.

To answer the question of the practical implications of this change, Juris Meija, the Secretary of the IUPAC Commission, stated "Knowledge of atomic masses is important to understanding the laws of physics. A good recent example of that was in 2005 when high-precision measurements of atomic masses allowed researchers to test the validity of the iconic 'E=mc^2.'" So scientists need the most accurate weights possible when conducting research, whether they are chemists or physicists. 

See below for a list of all the affected elements;

The elements that have gained weight

Aluminium (Al): from 26.981 5386 to 26.981 5385 (+0.0000001)

Beryllium (Be): from 9.012 182 to 9.012 1831 (+0.0000011)

Cadmium (Cd): from 112.411 to 112.414 (+0.003)

Caesium (Cs): from 132.905 4519 to 132.905 451 96 (+0.00000006)

Thulium (Tm): from 168.934 21 to 168.934 22 (+0.00001)

Holmium (Ho): from 164.930 32 to 164.930 33 (+0.00001)

Praseodymium (Pr): from 140.907 65 to 140.907 66 (+0.00001)

Selenium (Se): from 78.96 to 78.971 (+0.011)

The elements that have lost weight

Arsenic (As): from 74.921 60 to 74.921 595 (-0.000005)

Cobalt (Co): from 58.933 195 to 58.933 194 (-0.000001)

Fluorine (F): from 18.998 4032 to 18.998 403 163 (-0.000000037)

Manganese (Mn): from 54.938 045 to 54.938 044 (-0.000001)

Molybdenum (Mo): from 95.96 to 95.95 (-0.01)

Niobium (Nb): from 92.906 38 to 92.906 37 (-0.00001)

Phosphorus (P): from 30.973 762 to 30.973 761 998 (-0.000000002)

Scandium (Sc): from 44.955 912 to 44.955 908 (-0.000004)

Thorium (Th): from 232.038 06 to 232.0377 (-0.00036)

Yttrium (Y): from 88.905 85 to 88.905 84 (-0.00001)

Sources: http://www.livescience.com/39912-atomic-weight-changed-for-19-elements.html
http://www.telegraph.co.uk/science/science-news/10333341/The-Periodic-Table-puts-on-weight.html