What Are Isotopes?

• French B
• Geography
• German A
• German A Lit
• German ab initio
• German B
• Global Politics
• History
• History (First Exam 2028)
• Mathematics Analysis and Approaches (AA)
• Mathematics Applications & Interpretation (AI)
• Philosophy
• Physics
• Psychology
• Psychology (First Exam 2027)
• Spanish A
• Spanish A Lit
• Spanish ab initio
• Spanish B
• Sports, exercise and health science (SEHS)
• Theory of Knowledge (TOK)
• World Religions

Nuclear notation is written as $^{A}_{Z}\mathrm{X}$, where $\mathrm{X}$ is the chemical symbol.

Both nuclei are carbon because they have the same proton number, $Z=6$. They are different isotopes because their neutron numbers and nucleon numbers differ.

Isotopes have very similar chemical properties because neutral atoms of the isotopes have the same number and arrangement of electrons. However, they have different masses because their nuclei contain different numbers of neutrons. Their nuclear stability can also differ: an unstable isotope is a radioisotope and undergoes random, spontaneous radioactive decay.

For example, carbon-12 is stable, whereas carbon-14 undergoes beta-minus decay. During radioactive decay, the nucleus changes; ordinary chemical reactions do not change one isotope into another because they involve electron rearrangement rather than nuclear change.

A common misconception is that isotopes have different numbers of protons. This is incorrect: changing proton number changes the element, whereas changing only neutron number produces another isotope. Isotopes are also not ions; ions differ in electron number, while isotopes differ in neutron number.

IB Exam Technique

This is SL and HL content in E.3 Radioactive decay. For a State question, write “same proton number, different neutron number.” For Identify or Deduce questions, use $N=A-Z$ and compare proton numbers before naming isotopes. In nuclear equations, check that total nucleon number and total proton number are conserved.