Video Transcript
In this video, we will learn how to
describe the history and development of the periodic table, name the key people
involved, and outline their contribution.
Many scientists have attempted to
organize the known elements to make sense of their properties and relationships. In 1789, French scientist Antoine
Lavoisier published the first modern list of elements. Lavoisier categorized 33 elements
into gases, metals, nonmetals, and earths. In 1817, German physicist Johann
Wolfgang Döbereiner grouped together elements with similar properties. He called these groupings, which
often contained three elements, triads.
In 1864, British chemist John
Newlands organized the 62 known elements by increasing atomic weight. Newlands noticed that when the
elements were listed in order of atomic weight, every eighth element had similar
behavior. So, he arranged the elements into
rows and columns so that the elements were still in order of atomic weight but
elements with similar properties were grouped together. Five years later, a new periodic
table would be proposed that would create the framework for what would eventually
become the modern version of the periodic table.
In 1869, Russian chemist Dmitri
Mendeleev published his first periodic table. Like Newlands, Mendeleev organized
the elements into rows and columns. The elements increased in atomic
weight down a column. And elements that had similar
properties were grouped into the same row. When constructing this periodic
table, Mendeleev noted that tellurium had similar properties as the elements oxygen,
sulfur, and selenium. He also noted that iodine had
similar properties as the elements bromine, chlorine, and fluorine. As a result, he concluded that
tellurium and iodine should not be listed in order of increasing atomic weight so
that tellurium and iodine were in the same row as elements with similar
properties. Tellurium proved to be a constant
dilemma for Mendeleev as the atomic weight changed several times while he was
constructing his periodic tables.
Mendeleev also noticed when
ordering the elements that there appeared to be some elements missing. When there wasn’t an element with
properties that he expected, he left a gap for these yet undiscovered elements. Mendeleev’s first periodic table
wasn’t perfect, and there was still some uncertainty particularly as elements got
heavier. In 1871, Mendeleev revised his
periodic table. His revised table had 12 rows and 8
columns. He called each of the horizontal
rows a period and each of the vertical columns a group. Modern periodic tables still use
this distinction. The elements were still listed in
order of increasing atomic weight, this time from left to right across a period. And elements that shared similar
properties were organized into the same group. This revised periodic table still
had gaps for elements that were yet to be discovered.
Mendeleev listed the expected
atomic weights of these elements and even predicted their properties based on the
properties of elements near them in his periodic table. Three of the most notable elements
Mendeleev predicted were expected to have atomic weights of 44, 68, and 72. He called these elements eka-boron,
eka-aluminum, and eka-silicon. Each of these elements were later
discovered and were found to have atomic weights and properties similar to those
Mendeleev predicted. In addition to leaving gaps for
some undiscovered elements, Mendeleev also proposed new atomic weights for a few
elements, which didn’t seem to fit his table.
Beryllium’s accepted atomic weight
at the time was 14, the same as nitrogen. But beryllium had similar
properties as the elements magnesium and calcium. So, Mendeleev proposed an atomic
weight of 9.4 in order for these elements to be grouped together. Similarly, uranium had an accepted
atomic weight of 120. But Mendeleev proposed that uranium
should have an atomic weight of 240 in order to be grouped with the element
tungsten. Mendeleev’s proposed atomic weights
are close to the accepted values today. Beryllium has a relative atomic
mass of nine, and uranium has a relative atomic mass of 238.
Although several arrangements of
the elements came before Mendeleev’s periodic table, his table often receives the
most credit and is frequently referred to as the first periodic table. But Mendeleev’s table did have some
flaws. Mendeleev did leave gaps in his
periodic table for undiscovered elements. But he did not leave room for or
predict the existence of the inert gas elements. In addition, the elements copper,
silver, and gold shared some properties with the elements in group one and some
properties with the elements in group eight. So, Mendeleev listed each of these
elements twice.
Another potential issue had to do
with the difference in atomic weights of successive elements. The atomic weights of boron and
carbon differed by one unit. But the atomic weights of carbon
and nitrogen differed by two units. This made some scientists question
whether there were more undiscovered elements that belonged in between some of the
elements on Mendeleev’s periodic table.
More flaws became apparent as
further scientific discoveries were made. As more elements were discovered
and atomic weights were more accurately measured, more instances occurred like the
issue with tellurium and iodine. For each of these pairs, the
element with the higher atomic weight needed to be listed first in order for both
elements to be in the same group as elements with similar properties.
The existence of isotopes was yet
another complication. Isotopes are atoms of the same
element that have a different atomic mass. For example, some atoms of the
element hydrogen have an atomic mass of one, some have an atomic mass of two, and
others have an atomic mass of three. If the periodic table is organized
by atomic weight, then each isotope of an element might need to be listed
separately. Major discoveries in the early
1900s would help to solve many of these issues.
In 1911, following the results of
the gold foil experiment, Ernest Rutherford proposed a new model of the atom. In this model, negatively charged
electrons orbited a central nucleus. The nucleus was positively charged
and contained most of the mass of the atom. Shortly after Ernest Rutherford’s
proposal, Dutch physicist Antonius van den Broek suggested that the charge of the
nucleus was exactly equal to an element’s atomic number. The atomic number was the position
number of the element on the periodic table where the first element listed had an
atomic number of one, the second element had an atomic number of two, and so on.
Van den Broek’s theory would be
proven true in 1913 by a young British physicist named Henry Moseley. Henry Moseley designed an
experiment to observe how X-rays emitted by an element were diffracted. When the power supply was turned
on, the elemental sample would emit X-rays, which were diffracted onto a
photographic plate. Each element tested produced a
unique pattern of lines on the photographic plate. Through some calculations, Mosley
was able to directly relate the pattern of lines produced to an element’s atomic
number.
Moseley performed his experiment
with the elements aluminum through gold and found that the X-ray diffraction was
directly related to atomic number for all of the known elements. Based on his findings, Moseley
proposed that the elements should be organized by increasing atomic number not by
atomic weight. Here we’ve shown the elements
listed in order of increasing atomic number in three rows. But in Moseley’s original paper,
these were listed in a single column.
Organizing the elements by atomic
number solved the flaws with Mendeleev’s periodic table. This organization accounted for and
included the inert gas elements. And no element was repeated more
than once. Unlike with atomic weight, the
difference between the atomic numbers of successive elements was always one
unit. In addition, the pairs of elements
that had to be reversed when listed by atomic weight were in the correct order when
listed by atomic number. Finally, isotopes were no longer an
issue if the elements were listed by atomic number. This is because while isotopes of
an element have a different atomic weight, they have the same nuclear charge and
therefore the same atomic number.
In addition to solving the flaws
with Mendeleev’s periodic table, Moseley’s discovery was important for two more
reasons. Like Mendeleev, Moseley predicted
the existence of four undiscovered elements. All four of these elements would
later be discovered and named technetium, promethium, hafnium, and rhenium. In addition, Moseley’s experiments
provided a concrete way to determine if a substance was in fact a new element. Each element produced a unique
pattern of lines on the photographic plate. So, if the lines produced by a
substance were different from the known elements, then a new element had been
discovered. Henry Moseley’s work paved the way
for the modern periodic table. But even more discoveries about the
nature of the atom would lead to the periodic table that we know today.
In the same year that Moseley
suggested organizing the elements by atomic number, Niels Bohr proposed a new model
of the atom. Bohr suggested that electrons in
the atom orbited the nucleus in discrete regions called energy levels. Bohr’s model would be expanded upon
over the years to include energy sublevels and orbitals. Then, in 1920, Ernest Rutherford
published another discovery. He had discovered a new subatomic
particle called the proton. Protons are found in the nucleus of
an atom and have a positive charge. The number of protons in an atom of
an element is equal to the nuclear charge, which is equal to the atomic number.
The modern version of the periodic
table is arranged based on all of these discoveries. The modern periodic table is
organized into 18 columns and seven rows. Just like with Mendeleev’s periodic
table, the vertical columns are called groups and the horizontal rows are called
periods. Based on Henry Moseley’s work, the
elements are listed in order of increasing atomic number from left to right across a
period. We could also say that the elements
are listed in order of increasing number of protons from left to right across a
period.
The elements are also organized
according to how the electrons fill various energy levels and sublevels. Elements found in the same group
have similar properties. This is because the electrons in
atoms of elements in the same group have a similar electron arrangement. In addition, atoms of elements
found in the same period have the same number of electron shells or energy
levels.
Before we summarize all that we’ve
learned about the history of the periodic table, let’s take a look at a
question.
The symbols and information for
five elements are shown below. How would these elements be ordered
according to Moseley’s periodic table?
Henry Moseley was a British
scientist who studied how X-rays emitted by an element were diffracted. In his experiments, a photographic
plate was exposed to the X-rays. The X-rays produced lines on the
plate. The bands produced were unique for
each element. Through some calculations, Moseley
discovered that the lines produced were directly related to the element’s atomic
number. Moseley then suggested that the
elements on the periodic table should be ordered by increasing atomic number instead
of by increasing atomic weight.
So, to organize the five elements
according to Moseley’s periodic table, we must put them in order of increasing
atomic number. The atomic number is written above
the chemical symbol in each box. Looking at the answer choices, we
can see that the answer choice that correctly lists the elements by increasing
atomic number is answer choice (A). Therefore, the correct order of the
elements according to Moseley’s periodic table is answer choice (A).
Now let’s summarize what we’ve
learned. There were many early attempts to
organize the elements to make sense of their properties and relationships. Dmitri Mendeleev, a Russian
scientist, is often credited with creating the first periodic table. He organized the elements by
increasing atomic weight. He also grouped elements with
similar properties into the same column. He called the columns groups and
the rows periods, terms we still use with the modern periodic table.
Mendeleev’s periodic table was
special because he left gaps for undiscovered elements and proposed new atomic
weights for elements that didn’t fit his table. Mendeleev’s periodic table did have
several flaws including that it did not account for the inert gases or isotopes and
some pairs of elements needed to be written in reverse order so that they were
grouped with elements that had similar properties.
Henry Moseley was a British
scientist that discovered a direct relationship between the X-ray diffraction
pattern of an element and the element’s atomic number. He suggested that the elements
should be organized in order of increasing atomic number. Like Mendeleev, Moseley left gaps
for elements that would later be discovered.
The modern periodic table is
organized by increasing atomic number, which is the same as the number of protons in
all atoms of an element, and by how electrons fill various energy levels and
sublevels in an atom.
