---
title: "Isotope — AP Physics 2 Definition & Exam Guide"
description: "An isotope is a version of an element with the same protons but different neutrons. Learn how nuclear notation and isotopes show up in AP Physics 2 Unit 15."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/isotope"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 15"
---

# Isotope — AP Physics 2 Definition & Exam Guide

## Definition

In AP Physics 2, an isotope is a variant of an element with the same number of protons but a different number of neutrons, identified by its mass number (total protons + neutrons) in nuclear notation, like hydrogen-1 versus hydrogen-2 (deuterium).

## What It Is

An isotope is a version of an element that has the same number of [protons](/ap-physics-2-revised/unit-15/2-the-bohr-model-of-atomic-structure/study-guide/gzEFr1ron8hpACPx "fv-autolink") but a different number of neutrons. The proton count (atomic number, Z) is what makes an element that element, so every isotope of hydrogen has exactly 1 proton. What changes is the neutron count, which changes the mass number (A), the total of protons plus neutrons. That's why hydrogen-1 and hydrogen-2 are both hydrogen, but they're different isotopes.

In nuclear notation, you write an isotope as ᴬ_Z X, with the mass number on top and the atomic number on the bottom. So ²³⁵₉₂U tells you this uranium isotope has 92 protons and 235 − 92 = 143 neutrons. Because protons and neutrons are far more massive than [electrons](/ap-physics-2-revised/key-terms/electrons "fv-autolink"), the mass of an atom is dominated by the [nucleus](/ap-physics-2-revised/key-terms/nucleus "fv-autolink"), which means different isotopes of the same element have noticeably different masses even though they behave the same chemically.

## Why It Matters

Isotopes live in **Topic 15.2 (The Bohr Model of Atomic Structure)** in **[Unit 15](/ap-physics-2-revised/unit-15 "fv-autolink"): Modern Physics**, supporting learning objective **15.2.A**: describe the properties of an atom. The CED's essential knowledge is direct about this: each element has a unique number of protons, nuclear notation represents the proton and neutron counts, and the total number of neutrons and protons identifies the isotope. This is also the bookkeeping skill that makes the rest of Unit 15 possible. When you balance a fusion reaction or track an [alpha decay](/ap-physics-2-revised/key-terms/alpha-decay "fv-autolink"), you're really just counting nucleons across isotopes. If you can't decode ²³⁹₉₄Pu in two seconds, nuclear physics problems become guesswork.

## Connections

### [Ion (Unit 15)](/ap-physics-2-revised/key-terms/ion)

Isotopes and ions are the two ways an atom can vary, and they're independent. Change the neutrons and you get a different isotope. Change the electrons and you get an ion with a nonzero [net charge](/ap-physics-2-revised/unit-10/2-the-process-of-charging/study-guide/fQJqVklZDhbQdyBa "fv-autolink"). An atom can be both at once.

### Nuclear notation and the Bohr model (Unit 15)

Nuclear notation is the language isotopes are written in. The bottom number (Z) names the element and tells you how many protons it has, and the top number (A) names the specific isotope. Subtract to get neutrons. Every Topic 15.2 atom-structure question starts here.

### Nuclear reactions: fusion and decay (Unit 15)

Fusion and decay problems are isotope arithmetic. When protium and deuterium fuse into helium-3 plus a neutron, the total [nucleon](/ap-physics-2-revised/key-terms/nucleon "fv-autolink") count stays the same (1 + 2 = 3 + 1... wait, 4 = 4). Conserving mass number and atomic number across a reaction is how you find the mystery daughter nucleus.

## On the AP Exam

Isotope questions are mostly multiple-choice and they reward fast nuclear-notation reading. Expect stems like: given ²³⁹₉₄Pu, describe the internal structure (94 protons, 239 − 94 = 145 neutrons). Or, given that ²³⁵₉₂U undergoes alpha decay, write the nuclear notation of the daughter nucleus by subtracting 4 from the mass number and 2 from the atomic number. Fusion questions ask you to track total nucleons, like protium plus deuterium producing helium-3 and a neutron, where the total number of nucleons doesn't change. The skill being tested is always the same: translate between notation and proton/neutron counts, then conserve A and Z across any reaction.

## isotope vs ion

Both are 'modified atoms,' but they're modified in different places. An isotope changes the nucleus (different neutron count, same element, neutral overall by default). An ion changes the electron cloud (different electron count, nonzero net charge, same nucleus). Quick check: neutrons make isotopes, electrons make ions. An oxygen atom (Z = 8) with 10 electrons is an ion with a 2− charge, not an isotope.

## Key Takeaways

- Isotopes of an element have the same number of protons but different numbers of neutrons, so they share an atomic number but have different mass numbers.
- In nuclear notation ᴬ_Z X, the bottom number Z is the proton count and the top number A is protons plus neutrons, so neutrons = A − Z.
- The number of protons defines the element, so changing neutrons gives you a new isotope of the same element, never a new element.
- An atom's mass is dominated by its nucleus, so different isotopes of the same element have different masses.
- In nuclear reactions like fusion and alpha decay, total mass number and total atomic number are conserved, which lets you solve for unknown isotopes.
- Isotopes are about neutrons in the nucleus; ions are about electrons and net charge. Don't mix them up.

## FAQs

### What is an isotope in AP Physics 2?

An isotope is a variant of an element with the same number of protons but a different number of neutrons. It's identified by the total number of protons and neutrons (the mass number), written in nuclear notation like ²³⁵₉₂U.

### What's the difference between an isotope and an ion?

An isotope differs in neutrons (nucleus changes, [charge](/ap-physics-2-revised/unit-10/1-electric-charge-and-electric-force/study-guide/E6OYkOGeroCXwgw1 "fv-autolink") stays neutral), while an ion differs in electrons (charge becomes nonzero, nucleus stays the same). An oxygen atom with Z = 8 and 10 electrons is an ion with a 2− charge, not a different isotope.

### Do isotopes of the same element have the same atomic number?

Yes. All isotopes of an element have the same atomic number (proton count) because protons define the element. Hydrogen-1 and hydrogen-2 (deuterium) both have Z = 1; they differ only in neutrons.

### How do I find the number of neutrons in an isotope?

Subtract the atomic number from the mass number: neutrons = A − Z. For ²³⁹₉₄Pu, that's 239 − 94 = 145 neutrons alongside 94 protons.

### Does the number of nucleons change in a nuclear reaction between isotopes?

No. The total number of nucleons (protons plus neutrons) is conserved. When protium and deuterium fuse into helium-3 plus a neutron, you start with 3 nucleons and end with 3 plus 1 in helium-3 and the free neutron, with mass number and atomic number both balanced.

## Related Study Guides

- [15.2 The Bohr Model of Atomic Structure](/ap-physics-2-revised/unit-15/2-the-bohr-model-of-atomic-structure/study-guide/gzEFr1ron8hpACPx)

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