Isotopes and Their Properties
- You're comparing coins of the same currency.
- Some are slightly heavier than others because they contain more metal, yet they belong to the same denomination and have the same face value.
This is similar to how isotopes work in chemistry.
What Are Isotopes?
- Atoms are defined by their number of protons, known as the atomic number (Z).
- However, atoms of the same element can have different numbers of neutrons, which changes their mass number (A).
- These different versions of the same element are called isotopes.
Definition of Isotopes
Definition
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
This means isotopes share:
- The same atomic number (Z), since the number of protons determines the element.
- Different mass numbers (A), because the mass number is the sum of protons and neutrons.
Chlorine isotopes
- Chlorine-35 ($^{35}\text{Cl}$) has 17 protons and 18 neutrons.
- Chlorine-37 ($^{37}\text{Cl}$) has 17 protons and 20 neutrons.
The chemical properties of isotopes are nearly identical because chemical behavior depends on the number of electrons, which remains the same for all isotopes of an element.
Relative Atomic Mass and Isotopic Abundance
- Take a look at the periodic table in the data booklet.
- You’ll notice that atomic masses of elements are rarely whole numbers.
- For instance, chlorine’s relative atomic mass ($A_r$) is 35.45, even though its isotopes have whole-number mass numbers (35 and 37).
- You can ask, why is this?
Definition
Relative atomic mass
The relative atomic mass of an element is a weighted average of the masses of its isotopes, based on their natural abundances, compared to 1/12th the mass of a carbon-12 atom. Since it is a ratio, $A_r$ has no units.
Definition
Relative abundance
Relative abundance refers to the percentage of a specific isotope of an element present in a naturally occurring sample.
