Chapter U1, Problem 10STP

Interpretation Introduction

Interpretation:

Thallium’s atomic number is 81 and the average atomic mass is 204.4 amu. There are two isotopes of thallium, thallium - 203 and thallium-205. The statement which is correct about thallium needs to be determined.

Concept introduction:

Thallium is a gray composition metal that is not in Free State. The average atomic mass of thallium is 204.38 amu.

Also,

Atomic mass of an atom = Atomic Number + Number of Neutrons

Answer to Problem 10STP

Option (A) A naturally occurring sample of thallium contains both isotopes.

Explanation of Solution

Reason for correct option:

Thallium is a metal that is not found in Free State and it always contain a mixture of thallium-203 (²⁰³TI₈₁) and thallium-205 (²⁰⁵TI₈₁) in its composition. Thus, thallium is the natural metal found with two isotopes that are thallium-203 (²⁰³TI₈₁) and thallium-205 (²⁰⁵TI₈₁).

Conclusion

Reasons for incorrect options:

(B) Thallium-203 isotope has 122 protons and Thallium-205 has 124 protons.

• This is wrong because the thallium-203 isotopes have atomic number 81 and mass number 203. So, it contains 81 protons, 81 electrons and 122 neutrons.
• Thallium-205 isotopes have atomic number 81 and mass number 205. So, it contains 81 electrons, protons and 124 neutrons.

(C) The most common atom of thallium has a mass of 204.48 amu.

• This statement is wrong because thallium has mass number 204.38 amu .

D) Thallium-203 and thallium-205 both have 81 neutrons.

• This statement is wrong because Thallium-203 and thallium-205 both have 81 protons and electrons. The number of neutrons is different.