In the nucleus of an atom, the number of protons (the atomic number \$Z\$) defines the element. However, the number of neutrons can vary. When atoms of the same element have different numbers of neutrons, they are called isotopes.
An isotope is an atom that has the same number of protons but a different number of neutrons. The mass number \$A\$ (protons + neutrons) changes, but the chemical identity stays the same.
Think of isotopes like different models of the same car brand. They all belong to the same brand (element), but each model has a different engine size (neutron count). The cars look similar and perform similar functions, but their weight and performance differ.
Carbon: \$^{12}\text{C}\$ (6 protons, 6 neutrons) • \$^{13}\text{C}\$ (6 protons, 7 neutrons)
Hydrogen: \$^{1}\text{H}\$ (1 proton, 0 neutrons) • \$^{2}\text{H}\$ (1 proton, 1 neutron) – also known as deuterium
Uranium: \$^{235}\text{U}\$ (92 protons, 143 neutrons) • \$^{238}\text{U}\$ (92 protons, 146 neutrons)
| Element | Isotope | Mass Number (\$A\$) | Neutrons |
|---|---|---|---|
| Carbon | \$^{12}\text{C}\$ | 12 | 6 |
| Carbon | \$^{13}\text{C}\$ | 13 | 7 |
| Hydrogen | \$^{1}\text{H}\$ | 1 | 0 |
| Hydrogen | \$^{2}\text{H}\$ | 2 | 1 |
Remember: Isotopes have the same atomic number \$Z\$ but different mass numbers \$A\$.
When you see a question about “different forms of the same element”, think “isotopes”. Use the formula \$A = Z + N\$ to find the number of neutrons.
🔍 Tip: If a problem mentions a specific mass number, you can immediately identify the isotope.