Video Transcript
Which of the following is the main
difference between Mendeleev’s and Mosley‘s arrangements of elements in the periodic
table? (A) Mendeleev arranged elements
ascendingly according to atomic numbers, while Moseley arranged them ascendingly
according to atomic masses. (B) Mendeleev arranged elements
ascendingly according to atomic masses, while Moseley arranged them ascendingly
according to atomic numbers. (C) Mendeleev arranged elements
descendingly according to atomic numbers, while Moseley arranged them descendingly
according to atomic masses. Or (D) Mendeleev arranged elements
descendingly according to atomic masses, while Moseley arranged them descendingly
according to atomic numbers.
In this question, we must correctly
identify the main difference between how Mendeleev and Moseley arranged the elements
in their proposed periodic tables. Let’s clear some space on screen to
look more closely at Mendeleev’s and Moseley’s arrangements of the elements.
Dmitri Mendeleev and Henry Moseley
both contributed important work to the field of chemistry. Mendeleev first proposed an
arrangement of elements in 1869, whereas Moseley’s contributions happened much later
in 1913.
Mendeleev first grouped the
elements by increasing atomic weight and into groups of elements with similar
properties. As he continued to refine his work,
he eventually arranged elements into what he called groups and periods. The groups were vertical columns on
his periodic table that contained elements of very similar properties. The periods were the horizontal
rows of his arrangement. During his work, he also cleverly
left gaps that predicted elements which, at that time, were not yet discovered. Mendeleev’s work was crucially
important to the development of the modern periodic table.
However, the arrangement of
elements by ascending atomic weight posed some problems. Some pairs of elements, like cobalt
with the chemical symbol Co and nickel with the chemical symbol Ni, did not quite
fit Mendeleev’s arrangement. If arranged by atomic weight,
cobalt would be placed after nickel. However, to fit into a group with
other elements of similar properties, cobalt would need to be arranged before
nickel.
Moseley’s work addressed this
issue. Moseley, through his work with
X-rays, found that the charge of the nucleus was exactly equal to the element’s
position in the periodic table. This charge of the nucleus was
called the atomic number, with the symbol Z. From this discovery, Moseley
proposed that elements be organized by increasing atomic number. This arrangement solved the issue
for elements such as cobalt and nickel.
Let’s have another look at our
answer choices. We can see that the answer choice
that correctly describes the arrangements is answer choice (B). Therefore, the main difference
between Mendeleev’s and Moseley’s arrangements of elements in the periodic table is
that Mendeleev arranged elements ascendingly according to atomic masses, while
Moseley arranged them ascendingly according to atomic numbers.
