# Ions (cations and anions)

> What the small charge on Fe²⁺ means


## Objectives

- State that an ion forms when an atom loses or gains electrons, while its protons stay the same.
- Tell a positive ion (cation) from a negative ion (anion).
- Find the number of electrons in an ion from Z and its charge, e.g. $\ce{Fe^{2+}}$.

## 1. Ions (cations and anions): from zero




## 2. What does the 2+ mean?

Predict: an iron atom has 26 protons and 26 electrons. How many electrons does $\ce{Fe^{2+}}$ have?

- 28
- 26
- 24 ✓

$\ce{Fe^{2+}}$ has 24 electrons: the atom loses 2 electrons while its 26 protons stay the same, so 26 positive charges against 24 negative ones leave a charge of 2+.

[ar-EG] الـ $\ce{Fe^{2+}}$ فيه 24 electrons: الذرة بتفقد 2 electrons والـ 26 protons بتوعها زي ما هم، فـ 26 positive charges قصاد 24 negative charges بيسيبوا charge قيمتها 2+.
## 3. Ion

Remember: a neutral atom has equal numbers of protons (+) and electrons (−). Ion An atom, or a group of atoms, that carries an electric charge because it lost or gained electrons. Sodium forms a positive ion When an ion forms, only electrons move. The protons stay the same, so it is still the same element.


## 4. Make iron ions

Take electrons away from an iron atom (Z = 26). Watch the charge. electrons lost protons electrons positive charge Iron (II) ion 26 protons and 24 electrons: this is $\ce{Fe^{2+}}$. Iron (III) ion 26 protons and 23 electrons: this is $\ce{Fe^{3+}}$. Cation A positive ion. Its charge equals the number of electrons the atom lost.


## 5. Anion

In ionic compounds, non-metal atoms such as Cl and O gain electrons. Anion A negative ion. Its charge equals the number of electrons the atom gained. Cl gains 1 electron $\ce{Cl-}$: 17 protons, 18 electrons O gains 2 electrons $\ce{O^{2-}}$: 8 protons, 10 electrons Non-metal atoms form negative ions In $\ce{FeCl3}$, the cation $\ce{Fe^{3+}}$ meets three anions $\ce{Cl-}$.


## 6. Count the electrons

A particle has 25 protons and 23 electrons. What is it? (Mn: Z = 25, V: Z = 23)

- $\ce{Mn^{2+}}$ ✓
- a neutral Mn atom
- $\ce{V^{2+}}$
- $\ce{Mn^{2-}}$

Protons (25) fix the element: Mn. Charge = 25 − 23 = +2.

[ar-EG] الـ protons (25) هي اللي بتحدد العنصر: Mn. والـ charge = 25 − 23 = +2.
## 7. Summary

An ion forms when an atom loses or gains electrons. Its protons do not change. Cation: positive, electrons lost (e.g. $\ce{Fe^{3+}}$). Anion: negative, electrons gained (e.g. $\ce{Cl-}$). Electrons in a cation = Z − charge: $\ce{Fe^{2+}}$ has 26 − 2 = 24. Next: which electrons does iron lose first, 4s or 3d? That is the next lesson. Can you explain why Fe and $\ce{Fe^{3+}}$ are the same element, though their electrons differ? Fe and $\ce{Fe^{3+}}$ both have 26 protons. When an ion forms, only electrons move; the protons stay the same, so it is still the same element.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-chem.ions