Explain positive and negative ion formation
Track electron loss or gain without changing the nucleus or element identity.
Section P5.1.1 · The atom
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Balance signed charges.
- Know neutral atomic charge balance.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
An electron has charge −1. What happens to the remaining atom when one is removed?
What you will learn
- Track electron loss or gain without changing the nucleus or element identity.
- Explain ion sign by electron transfer and preserve the unchanged nucleus.
Count charges before and after transfer
An initially neutral atom becomes a positive ion when it loses electrons: its positive nuclear charge now exceeds its negative electron charge. It becomes a negative ion when it gains electrons. The nucleus is unchanged in ordinary ion formation, so the proton count and element identity stay the same.
For a supplied atom with 11 protons, 11 electrons is neutral. After losing one electron it has 10 electrons and relative charge +1. An atom with 17 protons and 18 electrons has relative charge −1. The ion’s sign refers to the total charge difference, not the sign of every constituent.
Keep the transfer account complete
Losing a negative electron makes the remaining atom more positive; it does not remove a positive proton. A detached electron still carries negative charge. If the original atom and transferred electrons form a closed system, their total charge is conserved even though each part’s charge changes.
An ion is not the same as a new isotope. Electron transfer does not change nucleon number. Do not infer an actual atom’s likely ion from a supplied arithmetic example unless its chemistry is also given. This lesson checks charge accounting, not all chemical bonding rules.
Sources for this lesson
Pause and explain
A supplied atom has 17 protons and 18 electrons. What is its net relative charge?
Worked example
An initially neutral atom with 12 protons loses two electrons. Find its remaining electron count and ion charge.
Show the worked solution
- It begins with 12 electrons.
- Losing two leaves 10.
- Relative charge is 12 − 10 = +2.
Answer 10 electrons; charge +2.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the charge of 12 protons and 10 electrons.
Give me a hint
Add signed relative charge totals.
Compare my reasoning
- Protons contribute +12.
- Electrons contribute −10.
- The total is +2.
+2.
Look for these in your work
- Retain both signs.
- Do not count neutrons as charge.
A neutral atom with nine protons gains one electron. Find electron count and charge.
Give me a hint
Keep its nucleus unchanged.
Compare my reasoning
- It starts with nine electrons.
- It gains one, giving ten.
- Nine positive and ten negative charges give −1.
10 electrons; −1.
Look for these in your work
- Use electron gain.
- Keep nine protons.
A neutral atom loses two electrons into a closed collecting system. A report says total charge was created. Correct it.
Give me a hint
Account for the atom and both removed electrons.
Compare my reasoning
- The ion has relative charge +2.
- The two electrons have total −2.
- The full system still has zero total charge.
Charge separates; the closed-system total remains zero.
Look for these in your work
- Include transferred electrons.
- Use the complete system boundary.
Common mistakes
- Electron loss produces positive charge.
- Proton number stays fixed.
- Charge is conserved across the complete transfer.
What can you explain now?
Which transfer makes a neutral atom negative?
Compare with the explanation
Gaining electrons
Added negative charge exceeds the unchanged positive nuclear charge.
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Make it stick
Explain ion sign by electron transfer and preserve the unchanged nucleus.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.