Isotopes
Isotopes
- Isotopes are atoms of the same element with the same protons but different neutrons.
- Same proton number → same element; different neutron number → different mass number.
- They have the same chemical properties, because chemistry depends on electrons (which are the same).
Practice
Isotopes of an element have:
Isotopes share the proton number (same element) but differ in neutrons, so they have different mass numbers.
Practice
Isotopes of an element have the same chemical properties.
Chemistry depends on electrons; isotopes have the same electron arrangement, so they react the same.
Relative atomic mass
- The relative atomic mass ($A_r$) is the average mass of the atoms, weighted by how common each isotope is (its abundance).
$$A_r = \frac{\sum(\text{isotope mass} \times \text{abundance})}{100}$$
- Example — chlorine is 75% $^{35}\text{Cl}$ and 25% $^{37}\text{Cl}$:
$$A_r = \frac{(35 \times 75) + (37 \times 25)}{100} = 35.5$$
Practice
An element is 50% of mass 10 and 50% of mass 11. What is its relative atomic mass?
Ar = (10×50 + 11×50)/100 = 1050/100 = 10.5.
You've got it
Key idea
- isotopes = same protons, different neutrons → same element, different mass
- they share the same chemical properties (same electrons)
- $A_r$ = average isotope mass, weighted by abundance (chlorine → 35.5)