4. When Mendeleev\'s periodic table was published, scientists had already determ

Question

4. When Mendeleev's periodic table was published, scientists had already determined atomic mass of some of the elements, without knowing exacts about atomic structure. (a) Using a periodic table, what is the trend in atomic mass across a period? Give an example using three elements from the third period. (b) Using a periodic table, what is the trend in atomic mass down a group? Give an example using three elements from any group. (e) Mendeleev was convinced of the importance of arranging elements in the periodic table in accordance with trends that he questioned the accuracy of the atomic masses of several elements. This occurs in the fourth period; identify the elements involved, and explain why he questioned the atomic masses. (d) Many of the elements on modern periodic tables were not yet known. Use the periodic table to identify two other situations comparable to the one you identified in (c).

Explanation / Answer

a) Across a period means left to right the atomic mass of elements increases.

The three elements in the third period having atomic masses are Na(23),Mg(24) and Al(27) the masses are given in the brackets. so it also shows that it increase along a period.

b) As you go down a group, the atomic number increases. Take group 16 example the first three elements are Oxygen,sulphur and selenium and their atomic masses are 16,32 and 79 respectively.

c) Some elements with higher atomic mass were placed before lower atomic mass elements in order to maintain similar chemical nature of elements.

The elements involves 27Co59 and 28Ni58.7

d) It does not reflect the exact distribution of electrons among all the orbitals of the atoms of all the elements and the position of hydrogen is not fix till now.

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