Isotopes

Definition of Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

For example, all carbon atoms have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Both are carbon isotopes.

Atomic and Mass Numbers

The atomic number of an atom is the number of protons in its nucleus. This defines the element.

The mass number is the total number of protons and neutrons in the nucleus.

Since isotopes have the same number of protons but different numbers of neutrons, they have the same atomic number but different mass numbers.

For example, chlorine has two common isotopes: chlorine-35 (17 protons, 18 neutrons) and chlorine-37 (17 protons, 20 neutrons).

The notation for isotopes is written as:

$$_{Z}^{A}X$$

where:

• $X$ is the chemical symbol of the element
• $Z$ is the atomic number (number of protons)
• $A$ is the mass number (protons + neutrons)

For example, carbon-14 is written as $_{6}^{14}C$.

Learning example: Calculate the number of neutrons in an isotope of sodium with mass number 23 and atomic number 11.

Number of neutrons = mass number − atomic number = $23 - 11 = 12$ neutrons.

Properties of Isotopes

Isotopes of the same element have very similar chemical properties because chemical reactions depend mainly on the number of protons and electrons, which are the same in isotopes.

However, isotopes have different physical properties because their mass numbers differ. This can affect properties like density and rate of diffusion.

Some isotopes are stable, while others are radioactive (unstable) and decay over time. Radioactive isotopes are used in various applications but their decay and radiation are covered in other topics.

Uses of Isotopes

Isotopes have important uses in science and medicine:

• Medical imaging: Radioactive isotopes can be used to trace processes inside the body, helping to diagnose diseases.
• Dating fossils: Certain isotopes like carbon-14 are used to estimate the age of ancient organic materials through radioactive dating.
• Tracing chemical pathways: Stable isotopes can be used as tracers to follow the movement of substances in chemical reactions or biological systems.

Additional Learning Examples

Example: Chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons. Calculate the number of neutrons in each isotope.

For chlorine-35: neutrons = 35 − 17 = 18

For chlorine-37: neutrons = 37 − 17 = 20

Example: An isotope of iodine is written as $_{53}^{131}I$. How many protons and neutrons does it have?

Protons = atomic number = 53

Neutrons = mass number − atomic number = 131 − 53 = 78