Video Transcript
In this video, we will learn about
the history of our understanding of the elements and how that understanding was
reflected in the organization and shape of the periodic table of elements. How could you tell if a substance
is fundamental or elemental?
Let’s take a simple cake as an
example. If you break down a cake, some of
the mass will be from the flour, some will be from the butter, and some from the
sugar, and so on. If we want to prove that sugar
isn’t fundamental or elemental, we have to try to break it up again. If we can’t, it’s probably an
element. But in the modern era, we know that
sugar is made up of carbon, hydrogen, and oxygen atoms.
We know about atoms and protons
inside the nucleus. And we know that carbon atoms have
six protons in their nuclei, oxygen atoms have eight, and hydrogen atoms have
one. We know these substances are
elements because no matter how hard we try, we would never get another substance
more elemental than carbon, hydrogen, and oxygen. It takes tremendous amounts of
energy to split atoms. And if we do that, we change the
elements entirely. As our technology and understanding
has improved, we’ve been able to prove that more and more substances either are or
are not elements.
Long before we understood what
atoms, ions, or nuclei were, there were substances in use that turned out to be
chemical elements. It’s not until much more recently
we were able to prove that. These were the elemental substances
in use before about 1000 CE: copper, lead, gold, iron, silver, carbon, tin, sulfur,
mercury, zinc, arsenic, antimony, and bismuth.
At room temperature and pressure,
all of these substances were solids, apart from mercury, which was a liquid. After bismuth, it took over 600
years for the next element to be discovered. And by the end of the 1700s,
another 21 substances we now know to be elements had been discovered. For the first time, elements that
form gases at room temperature were identified: hydrogen, oxygen, nitrogen, and
chlorine.
In 1789, the discoverer of hydrogen
and oxygen as distinct elements, Antoine Lavoisier, published a chemistry textbook
with one of the first formal groupings of the elements — gases, metals, nonmetals,
and earths — based on their behavior. Under gases, Lavoisier identified
oxygen, nitrogen, and hydrogen. But under gases, he also grouped
light and heat. It may seem strange now because
we’re so familiar with the definition of element. But he was trying to understand and
group all the phenomena that he’d come across.
Under metals, Lavoisier only
included substances we consider to be elements today. Under nonmetals, Lavoisier listed
sulfur, phosphorus, and carbon and three substances we wouldn’t call elements:
hydrochloric acid, hydrofluoric acid, and boric acid. All the substances Antoine put
under earths are not, as we consider them, elements. It would take the discovery of
electrochemistry in the coming decades to separate out these compounds.
The next revolution came in the
early 1800s. The idea of an atom, something so
fundamental it can never be cut in two, is much older than the 1800s. But the first scientific model of
the atom wasn’t proposed until 1808 by John Dalton. At this point, 46 elements had been
discovered. His simple model drew atoms of the
elements as circles. These could be combined in specific
ratios, giving distinct chemical compounds, something we take for granted today.
In 1814, Jöns Jacob Berzelius came
up with a system that used letters rather than circles to indicate the elements,
using words like “oxygen” to derive O and the Latin for iron, “ferrum,” to derive
Fe. A lot of the symbols he came up
with are in use today.
There were a few observations made
over the next few decades that led the way toward a better understanding of the
elements, such as the work of Johann Wolfgang Döbereiner in 1829 that identified
triplets of elements that have patterns in their properties, like chlorine, bromine,
and iodine. But the first hint of a periodic
table we could recognize came for Alexandre-Émile Béguyer de Chancourtois in
1863. By this point, there were reliable
estimates of the relative atomic masses for many of the elements. So we put the elements in order of
relative atomic mass. After this, he noticed periodic
patterns in the properties of the elements.
In 1864, John Newlands proposed the
law of octaves. When arranging the elements by
relative atomic mass, he saw patterns of behavior that repeated every eighth
element. Newlands put these related elements
in columns, forming rows of seven elements each. While the beginning of his table
resembles the modern version, there were lots of groupings that didn’t prove to be
true. The noble gases were yet to be
discovered. If they had been, we might have had
a law of nonaves instead.
Five years later, 200 years of
elemental discovery led to one of the most memorable episodes in chemistry
history. In 1869, Dmitri Ivanovich Mendeleev
published his first periodic table of elements. His table used some of the same
principles as Newlands’ table. It was arranged by relative atomic
mass and grouped by chemical behavior. But Mendeleev did something that
every good scientist should be willing to do. He didn’t force the data to fit his
theories. Instead, where there wasn’t an
element with properties he expected, he left a gap. This is his initial table, where
the elements are arranged vertically in order of relative atomic mass and
periodically left to right. This is rotated 90 degrees compared
to what we consider normal.
He used element symbols and an
equal sign to connect it to its relative atomic mass. His table included predictions of
elements yet to be discovered, with their estimated relative atomic masses. Some of the symbols in this table
will look unfamiliar because he used Pl for palladium and Ur for uranium. Mendeleev also consciously went
against the arrangement by relative atomic mass when it made more sense based on the
chemical character of the elements. So the positions of tellurium and
iodine were reversed. Tellurium behaved more like the
elements oxygen, sulfur, and selenium. And iodine behaved more like
bromine, chlorine, and fluorine.
However, the table wasn’t
perfect. There was still some uncertainty,
particularly as elements got heavier. Mendeleev released a revised
version in 1871, arranging elements horizontally by relative atomic mass and
periodically top to bottom, giving us the groups and periods we’re more familiar
with. This version of the table is
slightly simplified compared to the original print.
The unknown elements below boron,
aluminum, and silicon were labeled eka boron, eka aluminum, and eka silicon. Eka, meaning one in Sanskrit,
indicates the element one below the given element. So eka boron is one below
boron. And it would’ve been below in the
original layout.
Eka boron was discovered eight
years later and named scandium. Eka aluminum was discovered in 1875
and was named gallium. And eka silicon was discovered in
1886 and named germanium. Mendeleev’s predictions came
true. So we could admire the courage of
Mendeleev to go against the established science of the time and introduce gaps.
There was still a problem with the
table. There were certain elements that
were reversed with respect to their order by relative atomic mass, like tellurium
and iodine. Revisions to Mendeleev’s table
occurred as new elements were discovered. But a breaking point in the
fundamental principle was reached in 1913.
With the discovery of the noble
gases and more accurate measurements of the relative atomic masses of copper and
nickel, more swapped pairs had emerged. Tellurium and iodine had been
joined by argon and potassium and cobalt and nickel. The existence of the nucleus inside
atoms and protons inside the nucleus had only being demonstrated two years before by
Ernest Rutherford, Marsden, and Geiger.
With more accurate relative atomic
masses, more evidence of a problem, and awareness of the proton, Henry Moseley
overrode the fundamental organizing principle for Mendeleev’s periodic table of
elements. Instead of organizing elements by
relative atomic mass, Moseley proposed the template for the modern periodic table,
with elements arranged by atomic number, the number of protons in the nuclei of the
element.
Atomic number and relative atomic
mass are roughly proportional across the elements. So it’s understandable why relative
atomic mass worked so well. Under this system, elements like
tellurium and iodine, which in the old system seemed to be swapped, actually
followed the correct order, sitting in their right group and increasing in atomic
number in a natural fashion.
The relative atomic mass is roughly
equivalent to the number of protons and neutrons in an atom of an element, while the
atomic number is equal to the number of protons in an atom or ion of an element. The number of protons in an atom or
ion is what primarily determines its chemical behavior. But neutrons only add mass. Because of the range of isotopes
and isotopic abundances for an element, the relative atomic mass of an element is
not as conclusive a predictor of chemical behavior as atomic number is.
Now, it’s about time we had some
practice.
The periodic table is an example of
a model. It allows scientists to make
predictions by highlighting patterns in the properties of elements. The discovery of new elements
allowed scientists to fill gaps and correct mistakes in the original periodic
table. Which of the following words best
describes the model used to construct the original periodic table?
Arguably, the very first periodic
table came in 1863 from Alexandre-Émile Béguyer de Chancourtois, who put elements on
a spiral on a piece of paper. Elements were arranged on the
cylinder left to right by relative atomic mass and arranged vertically using the
spiral by chemical behavior. However, this version isn’t
generally considered a traditional table.
Traditionally, the original
periodic table is that of Dmitri Ivanovich Mendeleev in 1869, which arranged
elements bottom to top by relative atomic mass and left to right in periodic
chemical behavior. In 1871, Mendeleev produced a
revised version where periods went top to bottom and groups went left to right, more
like our modern periodic table. The key feature that distinguished
Mendeleev’s system from previous systems was that he left gaps using existing data
to predict the properties of unknown elements. This made Mendeleev’s table a very
good model because it allowed for accurate prediction. New elements likes scandium,
gallium, and germanium were discovered later and inserted naturally into the gaps,
fitting the predictions very closely.
Now, let’s have a look at the
question. We need to look at five words and
find the one that best describes the model used in the original periodic table. These three organizing principles
constitute the model used to make the table. Now, it would be perfectly accurate
to say that Mendeleev’s table was wrong. There were lots of things that have
since being changed.
But the question isn’t just asking
for any description. We’re looking for the best
description, one that does justice to the great work that it was. So the original table was wrong in
some respects. But it was also correct in many
respects. It would also be fair to say that
Mendeleev’s tables were fundamentally flawed because they use relative atomic mass
rather than atomic number as we use today.
However, based on the data of the
time, tellurium and iodine were the only pair of elements that seemed out of
sequence. Tellurium had a higher relative
atomic mass, but its chemical behavior meant it fit better if it was before iodine
rather than after. What would be unfair is to call
Mendeleev’s tables unscientific because they reflected insight into the data
available at the time.
The fact that Mendeleev left gaps
suggested by the data and the fact that he switched round tellurium and iodine
despite it not fitting the relative atomic mass principle suggests that he was
genuinely thinking about what he was doing. He didn’t want to just make the
data fit his theory.
The last word that we could apply
sensibly to Mendeleev’s tables is simply “incomplete.” It was made before we understood
atoms in any more detail and before we understood protons and their impact on
chemical behavior. Out of all the answers, this is the
fairest. While there were wrong and correct
and a flawed aspects to the table, it was a step in the right direction, a decisive
turn in our understanding of the elements.
As with many scientific models and
theories, development happens in stages. And we don’t necessarily need to
discard a model just because it’s not perfect. So of the five words we’ve been
given, the one that best describes the model used to construct the original periodic
table is “incomplete.”
Let’s have a look at the key
points. The pure chemical substances we now
identify as being elements were isolated at different times in history. After enough elements were
discovered, they were arranged by relative atomic mass. And when scientists looked at how
the elements behaved, they noticed periodic patterns in chemical behavior. Models, in the form of periodic
tables, were made to predict properties of undiscovered elements, particularly those
of Mendeleev around 1870.
After minor flaws in Mendeleev’s
tables, relative atomic mass was replaced by atomic number as the primary
organization principle of modern periodic tables. Over the last 100 years, although
many elements have since been discovered, the fundamental organizing principles of
our periodic table haven’t really changed. Over the last 350 years, the number
of identified elements has skyrocketed from 14 in the late 1600s to 118 in the
modern day.
In 1789, Antoine Lavoisier
identified four groups of substances that he considered elements: gas, metal,
nonmetal, and earths. The gases, metals, and nonmetals
contained a number of substances we consider elements today. In 1808, John Dalton proposed the
first scientific model of the atom. And in 1869, Dmitri Mendeleev laid
the foundations of our modern periodic table with his first draft. And lastly, in 1913, two years
after Rutherford demonstrated the existence of the proton, Henry Moseley rearranged
the periodic table according to atomic number, fixing minor issues with Mendeleev’s
original design. Since then, elements have been
added to the periodic table into the gaps of atomic numbers.
