Atomic Structure and Isotopes: Question 4

Worked solution

Part (a): Defining isotopes

Isotopes are atoms of the same element, meaning they have the same proton number, that have different numbers of neutrons, and so different nucleon (mass) numbers.

Part (b): Counting neutrons

Neutrons are found using: $\text{neutrons} = \text{nucleon number} - \text{proton number}$.

Both isotopes of iodine have proton number $53$.

$\text{iodine-123: neutrons} = 123 - 53 = 70$

$\text{iodine-131: neutrons} = 131 - 53 = 78$

Part (c): Why the isotopes share chemical properties

Iodine-123 and iodine-131 both have proton number $53$. A neutral atom of each isotope must therefore also have $53$ electrons, arranged in exactly the same electronic configuration. The extra neutrons in iodine-131’s nucleus have no effect on the electron arrangement.

Chemical properties are controlled by the number and arrangement of electrons, particularly in the outer shell, not by the number of neutrons. Since both isotopes have identical electron arrangements, they undergo the same chemical reactions and are used, chemically, as the same element. Even though iodine-131 is more radioactive and is used differently in medicine because of this.

Final answers

• (a) Isotopes: same proton number, different numbers of neutrons.
• (b) Iodine-123 has $\boxed{70}$ neutrons; iodine-131 has $\boxed{78}$ neutrons.
• (c) Both have 53 electrons and identical electronic configuration, so they have the same chemical properties.