Chapter U1.15, Problem 8E

Interpretation Introduction

Interpretation : Isotope name and symbol of new element formed due to alpha emission of given isotopes have to be determined and also their stability has to be predicted from isotope graph.

Concept introduction : Unstable elements can be changed to new elements by emitting subatomic particles. When alpha particles are emitted from nucleus of an atom, it is called alpha decay. The alpha particle consists of 2 protons and 2 neutrons.

Answer to Problem 8E

The following table shows isotope name and symbol of new element formed due to alpha emission.

Element	New isotope formed	symbol	stability
Platinum - 175	Osmium - 171	171 46Os
Gadolinium - 149	Samarium - 145	145 62 Sm
Americium - 241	Neptunium - 237	237 93 Np
Thorium - 232	Radium - 228	228 88 Ra

Explanation of Solution

Given information: The following isotopes of elements are given.

Platinum - 175, Gadolinium - 149, Americium - 241, Thorium - 232

From the periodic table we get to know the atomic number of elements.

1. Platinum - 175- Atomic number of platinum is 78. During alpha decay, an alpha particle is ejected from the nucleus of an atom. It consists of 2 protons and 2 neutrons. So the number of protons decrease by two and number of neutrons decrease by two in the nucleus. The atomic mass decrease by 4 and the atomic number also decreases by 2. So the atomic number of new atom becomes 76, and the mass number becomes 171. The new isotope with 76 as atomic number is osmium - 171. From the isotope graph we can see that it is not a stable atom as there is no square corresponding to 76 protons and 95 neutrons.
The equation for this is:

${}_{78}^{175}\text{Pt}{\to }_{76}^{171}\text{Os}{+}_2^4\text{He}$

2. Gadolinium - 149- Atomic number of gadolinium is 64. During alpha decay, an alpha particle is ejected from the nucleus of an atom. It consists of 2 protons and 2 neutrons. So the number of protons decrease by two and number of neutrons decrease by two in the nucleus. The atomic mass decrease by 4 and the atomic number also decreases by 2. So the atomic number of new atom becomes 62, and the mass number becomes 145. The new isotope with 62 as atomic number is samarium - 145. From the isotope graph we can see that it is not a stable atom as there is no square corresponding to 62 protons and 83 neutrons.
The equation for this is:

${}_{64}^{149}Gd{\to }_{62}^{145}Sm{+}_2^{4}He$

3. Americium-241- Atomic number of americium is 95. During alpha decay, an alpha particle is ejected from the nucleus of an atom. It consists of 2 protons and 2 neutrons. So the number of protons decrease by two and number of neutrons decrease by two in the nucleus. The atomic mass decrease by 4 and the atomic number also decreases by 2. So the atomic number of new atom becomes 93, and the mass number becomes 237. The new isotope with 93 as atomic number is neptunium. From the isotope graph we can see that it is not a stable atom.
The equation for this is:

${}_{95}^{241}Am{\to }_{93}^{237}Np{+}_2^{4}He$

4. Thorium - 232- Atomic number of thorium is 90. During alpha decay, an alpha particle is ejected from the nucleus of an atom. It consists of 2 protons and 2 neutrons. So the number of protons decrease by two and number of neutrons decrease by two in the nucleus. The atomic mass decrease by 4and the atomic number also decreases by 2. So the atomic number of new atom becomes 88, and the mass number becomes 228. The new isotope with 88 as atomic number is radium. From the isotope graph we can see that it is not a stable atom.
The equation for this is:

${}_{90}^{232}Th{\to }_{88}^{228}Ra{+}_2^{4}He$

Conclusion

Alpha decay causes the atomic number to decrease by two so one move two spaces backward in the periodic table. None of the new isotopes formed are stable atoms.