In AP Physics 2, an ion is an atom with a nonzero net electric charge, created when a neutral atom loses or gains one or more electrons. Losing electrons makes a positive ion; gaining electrons makes a negative ion. The number of protons (and the element's identity) never changes.
An ion is an atom with a nonzero net electric charge. A neutral atom has equal numbers of protons (+) and electrons (−), so the charges cancel. Strip away an electron and the atom has more positive charge than negative, making it a positive ion. Add an extra electron and you get a negative ion. The key move is that only electrons leave or arrive. The protons stay put in the nucleus, so the element's identity never changes when an atom ionizes.
This fits directly into the picture of atomic structure in Topic 15.2. Atoms have a small, positively charged nucleus made of protons and neutrons, surrounded by electrons. The number of protons defines the element, the number of neutrons defines the isotope, and the number of electrons (relative to protons) defines whether the atom is neutral or an ion. In the Bohr model, electrons sit in quantized energy levels, and ionization happens when an electron absorbs enough energy to escape the atom entirely. For hydrogen, that ionization energy from the ground state is 13.6 eV, which is exactly why the Bohr energy levels are written as En = -13.6 eV/n².
Ions live in Unit 15 (Modern Physics) under Topic 15.2, The Bohr Model of Atomic Structure, supporting learning objective 15.2.A, which asks you to describe the properties of an atom. The essential knowledge states it directly: an ion is an atom with a nonzero net electric charge. But the real payoff is conceptual bookkeeping. The exam wants you to keep three counts straight at once. Protons tell you the element, neutrons tell you the isotope, and the electron-proton balance tells you the charge. Ions also bridge modern physics back to electrostatics, because once an atom is an ion, it feels electric forces and follows everything you learned about charges, fields, and potential earlier in the course.
Keep studying AP® Physics 2 Unit 15
Isotope (Unit 15)
Isotopes and ions are the two ways an atom can vary without becoming a different element. Change the neutron count and you get a different isotope; change the electron count and you get an ion. Protons never change in either case, and that's the fact MCQs love to test.
Bohr Model Energy Levels (Unit 15)
Ionization is what happens when an electron transition goes all the way. Using En = -13.6 eV/n², a hydrogen electron in the ground state needs 13.6 eV to escape to n = infinity. Give it that much energy and you've made an ion.
Electric Force, Field, and Potential (Unit 10)
An ion is just a charged particle, so every tool from electrostatics applies. It feels a Coulomb force, accelerates along field lines, and gains kinetic energy from potential differences. A neutral atom feels none of that, which is why the net charge matters.
Magnetism (Unit 12)
Moving ions curve in magnetic fields via F = qvB, while neutral atoms sail straight through. That single difference is how mass spectrometers separate particles, and it's a classic crossover question setup.
Ions show up mostly as a supporting concept rather than the star of the question. Multiple-choice stems on Topic 15.2 test whether you can keep atomic bookkeeping straight, like identifying what changes when an atom becomes an ion (electrons) versus an isotope (neutrons), or calculating the energy needed to ionize hydrogen from a given energy level using En = -13.6 eV/n². You'll also meet ions dressed up as 'charged particles' in electrostatics and magnetism problems, where you apply F = qE or F = qvB to predict their motion. No released FRQ centers on ions by name, but FRQs regularly hand you charged objects in fields and expect you to reason about forces and energy, which is exactly the skill the ion concept feeds.
Both are variations on a normal atom, but they change different particles. An ion has gained or lost electrons, so it has a net electric charge but the same nucleus. An isotope has a different number of neutrons, so it has a different mass but stays electrically neutral (unless it's also ionized). Quick test: charge changed means ion, mass changed means isotope, proton count changed means it's a different element entirely.
An ion is an atom with a nonzero net electric charge, created by losing electrons (positive ion) or gaining electrons (negative ion).
Ionization never changes the number of protons, so an ion is still the same element it started as.
Ions change electron count, isotopes change neutron count, and only a change in proton count changes the element.
In the Bohr model, ionizing hydrogen from the ground state takes 13.6 eV, the energy needed to remove the electron completely.
Once an atom is an ion, it responds to electric and magnetic fields just like any charged particle from Units 10 and 12.
The mass of an atom is dominated by the nucleus, so adding or removing electrons barely changes an ion's mass.
An ion is an atom with a nonzero net electric charge, which happens when a neutral atom loses or gains electrons. It's defined in Topic 15.2 (The Bohr Model of Atomic Structure) under learning objective 15.2.A.
No. Ionization only adds or removes electrons, and the element is defined by its proton count, which doesn't change. A sodium ion is still sodium, just with a net charge.
An ion has a different number of electrons than protons, giving it a net charge. An isotope has a different number of neutrons, giving it a different mass but no charge. Both keep the same proton count, so both stay the same element.
From the ground state (n = 1), it takes 13.6 eV, since the Bohr model gives En = -13.6 eV/n² and the electron must reach zero energy to escape. From higher levels it takes less, like only 3.4 eV from n = 2.
No, and this is a common trap. A positive ion has lost electrons, not gained protons. The nucleus is untouched; the atom is positive because it now has fewer negative electrons than positive protons.
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