Video Transcript
Although unknown at the time, what
was the problem that isotopes caused when Mendeleev created his table?
In 1869, Dmitri Mendeleev proposed
his first periodic table of the elements. Atoms were ordered by atomic
weight. The question asks what problem
isotopes caused when Mendeleev created his table.
Isotopes are atoms of the same
element that have a different atomic mass. As Mendeleev ordered the periodic
table by atomic weight and most elements have more than one stable isotope, then
each isotope might need to be written separately. This means that the same element
would be listed in the table multiple times.
The different isotopes can be
identified by the different mass numbers. But the existence of isotopes was
not known at this time. So, in essence, the isotopes would
be treated as different elements. It would also be difficult to group
elements according to chemical properties. So we now know that Mendeleev had
to treat isotopes as different elements. And this was due to differences in
atomic masses. Thus, the problem that isotopes
caused when Mendeleev created his table was that he had to treat them as different
elements due to differences in atomic masses.
Nowadays, we know about
isotopes. And the periodic table is ordered
in terms of atomic number instead of atomic mass. All isotopes are accounted for in
the relative atomic mass. So there is no need for elements to
be listed multiple times. The problem that Mendeleev faced
with isotopes has now been resolved.
