Chlorine's relative mass of 35.5 is the perfect puzzle to open this topic — it can't be a single whole-number mass, so what's going on? This video answers that question by introducing isotopes: atoms of the same element with different numbers of neutrons. You'll learn how to write isotope notation, calculate relative atomic mass as a weighted average, and see why radioactive isotopes like carbon-14 have real-world applications from carbon dating to medical imaging.

What You Will Learn

01:39What isotopes are and how they differ from each other in terms of neutron number.

01:53How to write isotope notation using element name plus mass number.

02:57What proton number and mass number represent and how to calculate neutron count from them.

03:43What relative atomic mass is and why it is not always a whole number.

04:20How to calculate relative atomic mass as a weighted average of isotope masses.

04:46What isotope abundance means and how to interpret abundance data from tables or graphs.

07:38What makes some isotopes stable and others radioactive.

09:04What beta decay is and how it changes the composition of a nucleus.

09:24How carbon dating uses carbon-14 decay to determine the age of organic materials.