Question Video: Identifying the Main Difference between the Arrangements of Mendeleev’s and Moseley’s Periodic Tables | Nagwa Question Video: Identifying the Main Difference between the Arrangements of Mendeleev’s and Moseley’s Periodic Tables | Nagwa

# Question Video: Identifying the Main Difference between the Arrangements of Mendeleev’s and Moseley’s Periodic Tables Science • Second Year of Preparatory School

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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. [D] Mendeleev arranged elements descendingly according to atomic masses, while Moseley arranged them descendingly according to atomic numbers.

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### 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.

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