Ion Formation and Electron Arrangement Revision Notes | GCSE Edexcel Higher Chemistry
An ion is an atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. Free GCSE Chemistry notes and 13 FAQs.
Updated 22 Mar 2026 · Revise Right Team
Ion Formation and Electron Arrangement is part of The periodic table in GCSE Edexcel Higher Chemistry.
4 min
Reading time
57
Lessons in this topic
GCSE Edexcel Higher
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Reviewed by Revise Right
Content review
What this lesson covers
An ion is an atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. Free GCSE Chemistry notes and 13 FAQs.
Explain how electron arrangement determines whether an element forms positive or negative ions
Predict whether an element is a metal or non-metal based on its position in the periodic table
Calculate the number of electrons in common ions using atomic number and charge
Apply knowledge of electron configuration to explain chemical reactivity patterns
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Use this lesson to stay inside The periodic table while connecting the detail back to the wider Topic 1: Key concepts in chemistry topic.
Topic 1: Key concepts in chemistry
The periodic table
GCSE Edexcel Higher Chemistry
Revision notes
Read the core explanations from Ion Formation and Electron Arrangement before testing yourself from memory.
1. Why Atoms Become Ions
Picture an atom as a perfectly balanced system - positive protons in the nucleus balanced by negative electrons in shells around it. But atoms don't always stay neutral. When they react, they can gain or lose electrons to become charged particles called ions.
This happens because atoms 'want' to have full outer electron shells - it makes them more stable. Think of it like musical chairs: atoms will gain or lose electrons to get a complete set of eight electrons in their outer shell (or two for the first shell).
2. Metals Form Positive Ions
Elements that react to form positive ions are metals. This isn't a coincidence - it's all about electron arrangement. Metals have few electrons in their outer shell (usually 1, 2, or 3). It's much easier for them to lose these few outer electrons than to gain many more to fill the shell.
When metals lose electrons, they form cations (positive ions). The name comes from the fact that in electrolysis, these positive ions move towards the cathode (negative electrode). **Trap**: Many students think metals 'want' to be positive - actually, they just find it easier to lose electrons than gain them.
Worked example: Working Out Ion Charges for Metals
Magnesium (atomic number 12) forms an ion. What is its charge and electron arrangement?
Mg atom: 2,8,2 (12 electrons total) — Atomic number = number of electrons in neutral atom
Outer shell has 2 electrons — These are the electrons that can be lost
Loses 2 electrons to get full outer shell — Easier to lose 2 than gain 6 electrons
12 protons - 10 electrons = +2 charge — More protons than electrons = positive
Mg²⁺ ion: 2,8 (10 electrons total) — Now has full outer shell of 8 electrons
Answer: Mg²⁺ with electron arrangement 2,8
3. Non-Metals Form Negative Ions
Elements that do not form positive ions are non-metals. Instead, non-metals form negative ions (anions) by gaining electrons. Non-metals have many electrons in their outer shell (usually 5, 6, or 7). It's easier for them to gain a few electrons to complete their outer shell than to lose many.
When non-metals gain electrons, they form anions (negative ions). These move towards the anode (positive electrode) in electrolysis. The pattern is clear: metals lose electrons, non-metals gain them.
Worked example: Working Out Ion Charges for Non-Metals
Chlorine (atomic number 17) forms an ion. What is its charge and electron arrangement?
Cl atom: 2,8,7 (17 electrons total) — Atomic number = number of electrons in neutral atom
Outer shell has 7 electrons — Needs 1 more electron for full shell of 8
Gains 1 electron to get full outer shell — Easier to gain 1 than lose 7 electrons
17 protons - 18 electrons = -1 charge — More electrons than protons = negative
Cl⁻ ion: 2,8,8 (18 electrons total) — Now has full outer shell of 8 electrons
Answer: Cl⁻ with electron arrangement 2,8,8
4. Electron Arrangement Patterns
The position of an element in the periodic table tells you everything about how it will form ions. Group 1 metals (like sodium) have 1 outer electron and form +1 ions. Group 2 metals (like magnesium) have 2 outer electrons and form +2 ions.
For non-metals, it's the reverse pattern. Group 7 elements (like chlorine) have 7 outer electrons and form -1 ions. Group 6 elements (like oxygen) have 6 outer electrons and form -2 ions. The group number tells you how many electrons they have in their outer shell.
Here's the key insight: all ions end up with the same electron arrangement as the nearest noble gas. Sodium (2,8,1) loses 1 electron to get 2,8 - the same as neon. Chlorine (2,8,7) gains 1 electron to get 2,8,8 - the same as argon. Noble gases are stable because they have full outer shells.
5. Exam Technique and Common Mistakes
Examiners test ion formation in predictable ways. They'll give you an element and ask for its ion charge, or give you an ion and ask for its electron arrangement. The key is methodical working - don't try to memorise every ion.
**Trap**: Many students confuse the number of electrons lost/gained with the final number of electrons. If calcium loses 2 electrons, it doesn't have 2 electrons left - it has 18 electrons left (started with 20, lost 2). Always subtract losses from the original number.
The most common exam mistake is getting the sign wrong. If an atom loses electrons, it becomes positive (fewer negative charges). If it gains electrons, it becomes negative (more negative charges). If you have 11 positive charges from protons and remove one negative charge from the electrons, the overall charge becomes more positive.
Atomic number tells you how many protons an atom has, and that proton number helps explain reactivity because it determines the electron arrangement. Elements in the same group have the same number of outer electrons, so they tend to lose or gain electrons in similar ways and therefore react similarly.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-22.
An ion is an atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. The number of protons stays the same; only the electrons change.
What is the difference between a cation and an anion?
A cation is a positively charged ion formed when an atom loses electrons, and all metal ions are cations. An anion is a negatively charged ion formed when an atom gains electrons, and all non-metal ions are anions. In electrolysis, cations move to the cathode and anions to the anode.
How do atoms form ions?
Atoms form ions by gaining or losing electrons to get a full outer shell. An ion is an atom or group of atoms with an overall electric charge; the number of protons stays the same and only the number of electrons changes.
Why are noble gases stable?
Noble gases are stable because they have full outer shells. Ions end up with the same electron arrangement as the nearest noble gas: sodium's ion is 2,8 like neon, and the Cl⁻ ion is 2,8,8 like argon.
Why do non-metals gain electrons?
Non-metals gain electrons because they have many electrons in their outer shell, usually 5, 6 or 7, so it is easier to gain a few to complete the shell than to lose many. They form negative ions called anions.
How do you work out the charge on an ion?
Work out an ion's charge from the electron arrangement: Mg (2,8,2) loses 2 electrons, so 12 protons - 10 electrons gives Mg²⁺. Cl (2,8,7) gains 1 electron, so 17 protons - 18 electrons gives Cl⁻.
What charge do Group 1, 2, 6 and 7 ions have?
Group 1 metals such as sodium form +1 ions and Group 2 metals such as magnesium form +2 ions. Group 7 elements such as chlorine form -1 ions and Group 6 elements such as oxygen form -2 ions, because the group number gives the number of outer electrons.
Why do ions have the same electron arrangement as noble gases?
Ions end up with the same electron arrangement as the nearest noble gas because they gain or lose electrons to get a full outer shell. Sodium (2,8,1) loses 1 electron to become 2,8, like neon, and chlorine (2,8,7) gains 1 to become 2,8,8, like argon.
What is the electron arrangement of a magnesium ion?
A magnesium ion, Mg²⁺, has the electron arrangement 2,8 with 10 electrons. A magnesium atom (2,8,2) loses its 2 outer electrons, so it has 12 protons and 10 electrons, so the charge is +2.
What is the electron arrangement of a chloride ion?
The Cl⁻ ion has the electron arrangement 2,8,8 with 18 electrons. A chlorine atom (2,8,7) gains 1 electron to fill its outer shell, giving 17 protons and 18 electrons, so the charge is -1.
How many electrons does a calcium ion have?
A calcium ion has 18 electrons: a calcium atom has 20 electrons and loses 2 to form Ca²⁺. A common mistake is thinking it has only 2 electrons left.
Does the number of protons change when an ion forms?
The number of protons does not change when an ion forms; only the number of electrons changes. Losing electrons gives a positive ion, and gaining electrons gives a negative ion.
Why are positive ions called cations?
Positive ions are called cations because in electrolysis they move towards the cathode, the negative electrode. Negative ions, anions, move towards the anode, the positive electrode.