Isotopes: same element, different mass
Definition
Isotope
Atoms of the same element with the same number of protons but different numbers of neutrons.
- The atomic number of an element is its number of protons, and it never changes for a given element.
- The mass number is the total number of protons and neutrons in an atom.
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
- Because they have extra or fewer neutrons, isotopes have different mass numbers.
- Chlorine exists mainly as two isotopes: chlorine-35 and chlorine-37.
- Isotopes have the same chemical reactions, because they have the same number of electrons arranged the same way.
Note
Neutrons change the mass of an atom but not its chemistry.
Relative atomic mass is a weighted average of the isotopes
Definition
Relative atomic mass
The weighted mean mass of the atoms of an element compared with one twelfth of the mass of a carbon-12 atom.
- Because an element can be a mixture of isotopes, we use an average mass called the relative atomic mass, ArA_rAr.
- It is a weighted mean, so isotopes that are more abundant count for more.
- Use the equation:
- Ar=∑(isotope mass×abundance)100A_r = \frac{\sum(\text{isotope mass} \times \text{abundance})}{100}Ar=100∑(isotope mass×abundance)
- Worked example for chlorine, which is 75% chlorine-35 and 25% chlorine-37:
- Ar=(35×75)+(37×25)100=2625+925100=35.5A_r = \frac{(35\times75) + (37\times25)}{100} = \frac{2625 + 925}{100} = 35.5Ar=100(35×75)+(37×25)=1002625+925=35.5
- This is why ArA_rAr values are often not whole numbers: they are averages of isotopes.
Self review
- What are isotopes?
- Why do isotopes have the same chemistry?
- Write the equation for relative atomic mass.
- A sample is 75% chlorine-35 and 25% chlorine-37. Find ArA_rAr.
