Chapter 3, Problem 3.1P

To determine

To Find:

The number of times the arrangement of elements on the basis of atomic number and atomic mass are different.

Answer to Problem 3.1P

There are four cases in which orders of the atomic number and atomic mass are different.

Explanation of Solution

Introduction:

The periodic table is the arrangement of the chemical elements in the tabular form based on chemical properties, atomic number and electron configuration. The table consists of seven rows known as periods and eighteen columns known as a group.

Initially, the concept of atomic number was unknown therefore, scientists arranged the elements based on their atomic masses. Till now, 90 elements had been found that occur naturally on the Earth and all the matter found on Earth made of these elements. Furthermore, there are approximately two dozen elements which can be created artificially by nuclear reaction. But all the artificial elements are unstable and disintegrate having half-lives much less than the age of Earth.

From the modern periodic table, there are few exceptions given below based on the difference between atomic mass arrangement and atomic number arrangement.

Element	Thorium (Th), Protactinium|(Pa)	Tellurian (Te), Iodine (I)	Cobalt (Co), Nickel (Ni)	Argon (Ar), Potassium (k)
Atomic number	90, 91	52, 53	28, 29	18, 19
Atomic mass	232.0, 231.0	127.60, 126.9	58.9, 58.7	39.95, 39.10

Conclusion:

The modern periodic table is based on the atomic number, the concept which is completely unknown earlier. Due to which previously periodic table was based on atomic mass.