# Nakafa Learning Content > For AI agents: use [llms.txt](https://nakafa.com/llms.txt) for the site index. Markdown versions are available by appending `.md` to content URLs or sending `Accept: text/markdown`. URL: https://nakafa.com/en/subjects/chemistry/structure-matter/ion Source: https://raw.githubusercontent.com/nakafaai/nakafa.com/refs/heads/main/packages/contents/material/lesson/chemistry/structure-matter/ion/en.mdx Learn how atoms become ions, how to distinguish cations and anions, and how to calculate electrons from ion charge. --- ## When an Atom Is No Longer Neutral A neutral atom has the same number of protons and electrons. An **ion** forms when an atom or group of atoms has an overall charge. Here, we focus first on ions made from one atom. The Atomic Structure and Symbolism section from OpenStax explains that ions form when the number of electrons is not equal to the number of protons. The source can be opened through [OpenStax's Atomic Structure and Symbolism](https://openstax.org/books/chemistry-atoms-first-2e/pages/2-3-atomic-structure-and-symbolism). Protons are in the nucleus and usually do not move during simple chemical processes. **Electrons** are the particles that change. So element identity stays locked by the number of protons, while ion charge is determined by the difference between protons and electrons. A simple way to read charge is by its sign. Protons add positive count, while electrons add negative count. If the counts match, the result is $$0$$ or neutral. If electrons decrease, positive charge remains. If electrons increase, negative charge remains. Visible text: A simple way to read charge is by its sign. Protons add positive count, while electrons add negative count. If the counts match, the result is or neutral. If electrons decrease, positive charge remains. If electrons increase, negative charge remains. ## Cations and Anions A positively charged ion is called a **cation**. A cation forms when an atom loses electrons. A negatively charged ion is called an **anion**. An anion forms when an atom gains electrons. The IUPAC Gold Book defines an ion as an atomic or molecular particle that has a net electric charge. The term reference can be opened through [IUPAC Gold Book's ion entry](https://goldbook.iupac.org/terms/view/I03158). Component: IonLab Props: - title: Ion Formation Simulator - description: Follow the left-to-right path: a neutral atom loses or gains electrons, then becomes a cation or anion. Protons and neutrons stay the same. - labels: { beforeChange: "Before", electronChange: "Electrons change", afterChange: "After", chooseIon: "Choose an ion example", neutralAtom: "Neutral atom", protons: "Protons", neutrons: "Neutrons", electrons: "Electrons", charge: "Charge", samples: { "sodium-cation": { name: "Sodium ion", type: "Cation", action: <>Loses $$1$$ electron, }, "fluoride-anion": { name: "Fluoride ion", type: "Anion", action: <>Gains $$1$$ electron, }, "lithium-cation": { name: "Lithium ion", type: "Cation", action: <>Loses $$1$$ electron, }, "oxide-anion": { name: "Oxide ion", type: "Anion", action: <>Gains $$2$$ electrons, }, }, } Visible text: { beforeChange: "Before", electronChange: "Electrons change", afterChange: "After", chooseIon: "Choose an ion example", neutralAtom: "Neutral atom", protons: "Protons", neutrons: "Neutrons", electrons: "Electrons", charge: "Charge", samples: { "sodium-cation": { name: "Sodium ion", type: "Cation", action: <>Loses electron, }, "fluoride-anion": { name: "Fluoride ion", type: "Anion", action: <>Gains electron, }, "lithium-cation": { name: "Lithium ion", type: "Cation", action: <>Loses electron, }, "oxide-anion": { name: "Oxide ion", type: "Anion", action: <>Gains electrons, }, }, } ## Charge Comes from the Difference The safest formula for reading ion charge is: ```math q = p^+ - e^- ``` The symbol $$q$$ is the ion charge, $$p^+$$ is the number of protons, and $$e^-$$ is the number of electrons. Visible text: The symbol is the ion charge, is the number of protons, and is the number of electrons. To find the electron count of an ion, rearrange the same formula: ```math e^- = p^+ - q ``` Sodium example: ```math \begin{aligned} {}^{23}_{11}\mathrm{Na}^{+} &: p^+ = 11,\ q = +1 \\ e^- &= 11 - (+1) \\ &= 10 \end{aligned} ``` Fluoride example: ```math \begin{aligned} {}^{19}_{9}\mathrm{F}^{-} &: p^+ = 9,\ q = -1 \\ e^- &= 9 - (-1) \\ &= 10 \end{aligned} ``` The negative sign in $$\mathrm{F}^{-}$$ means there are more electrons than protons. That is why $$9 - (-1)$$ becomes $$10$$. Visible text: The negative sign in means there are more electrons than protons. That is why becomes . ## What Changes and What Stays When an atom becomes an ion, do not change every number at once. Read each part separately. | Part to read | When an ion forms | Effect | | :----------- | :---------------- | :----- | | Protons | Stay the same | The element does not change. | | Neutrons | Stay the same for the same ion | The mass number does not change. | | Electrons | Change | The ion charge changes. | So $$\mathrm{Na}$$ and $$\mathrm{Na}^{+}$$ are both sodium because the proton count stays $$11$$. The difference is that $$\mathrm{Na}^{+}$$ has $$10$$ electrons, not $$11$$ electrons. Visible text: So and are both sodium because the proton count stays . The difference is that has electrons, not electrons. ## Worked Table for Ion Counts Use the table below as a completed example. Read each row from left to right: atomic number $$Z$$ gives the proton count, $$A - Z$$ gives the neutron count, and $$q = p^+ - e^-$$ connects charge with the electron count. Visible text: Use the table below as a completed example. Read each row from left to right: atomic number gives the proton count, gives the neutron count, and connects charge with the electron count. | Symbol | Atomic number | Mass number | Charge | Protons | Electrons | Neutrons | | :----- | :------------ | :---------- | :----- | :------ | :-------- | :------- | | $${}^{7}_{3}\mathrm{Li}$$ | $$3$$ | $$7$$ | $$0$$ | $$3$$ | $$3$$ | $$4$$ | | $${}^{7}_{3}\mathrm{Li}^{+}$$ | $$3$$ | $$7$$ | $$+1$$ | $$3$$ | $$2$$ | $$4$$ | | $${}^{16}_{8}\mathrm{O}$$ | $$8$$ | $$16$$ | $$0$$ | $$8$$ | $$8$$ | $$8$$ | | $${}^{16}_{8}\mathrm{O}^{2-}$$ | $$8$$ | $$16$$ | $$-2$$ | $$8$$ | $$10$$ | $$8$$ | | $${}^{27}_{13}\mathrm{Al}$$ | $$13$$ | $$27$$ | $$0$$ | $$13$$ | $$13$$ | $$14$$ | | $${}^{27}_{13}\mathrm{Al}^{3+}$$ | $$13$$ | $$27$$ | $$+3$$ | $$13$$ | $$10$$ | $$14$$ | Visible text: | Symbol | Atomic number | Mass number | Charge | Protons | Electrons | Neutrons | | :----- | :------------ | :---------- | :----- | :------ | :-------- | :------- | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | ### Checking the Lithium Row Check one row, such as $${}^{7}_{3}\mathrm{Li}^{+}$$. Its atomic number is $$3$$, so it has $$3$$ protons. Its charge is $$+1$$, so the electron count is: Visible text: Check one row, such as . Its atomic number is , so it has protons. Its charge is , so the electron count is: ```math e^- = 3 - (+1) = 2 ``` Its neutron count comes from the mass number: ```math n^0 = 7 - 3 = 4 ``` If you feel unsure, start from the charge calculation. Cations always have fewer electrons than protons, while anions have more electrons than protons.