Isotopes and Nuclide Notation Explained

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Nuclide notation

${}^A_Z X$

where $A$ is the nucleon number (mass number), $Z$ is the proton number (atomic number), and $X$ is the chemical symbol.

Example: Carbon-14 is written ${}^{14}_6\text{C}$.

• Protons: 6
• Neutrons: $14 - 6 = 8$
• Electrons (neutral atom): 6

Properties of isotopes

All isotopes of an element have:

• The same proton number (same element, same chemical properties).
• Different nucleon numbers (different numbers of neutrons).
• Different physical properties (e.g. mass, density, some are radioactive).

Examples

Isotope	Protons	Neutrons	Stable?
Carbon-12 (${}^{12}_6\text{C}$)	6	6	Yes
Carbon-14 (${}^{14}_6\text{C}$)	6	8	No (radioactive)
Uranium-235 (${}^{235}_{92}\text{U}$)	92	143	No (radioactive)
Uranium-238 (${}^{238}_{92}\text{U}$)	92	146	No (radioactive, very long half-life)

Worked example

How many protons, neutrons and electrons are in a neutral atom of ${}^{56}_{26}\text{Fe}$?

• Protons: 26
• Neutrons: $56 - 26 = 30$
• Electrons: 26 (same as protons in a neutral atom)

Common errors and how to correct them

Confusing the top and bottom numbers. $A$ (nucleon number, larger) is on top; $Z$ (proton number, smaller) is on the bottom.

Saying isotopes have different chemical properties. Isotopes have the same chemical properties because they have the same number of electrons and protons. Their physical properties (mass, radioactivity) differ.

How to apply this in an exam

Write nuclide notation with $A$ on top and $Z$ on the bottom. Calculate neutrons as $A - Z$. When defining isotopes, always say “same number of protons, different number of neutrons.”