An ion is an atom (or group of atoms) that has lost or gained electrons, so it carries an overall charge. To find its particles, keep the protons and neutrons from the atom and change only the electrons: take electrons away for a positive charge, add them for a negative charge.
This builds directly on reading proton and nucleon numbers and leads into ionic ideas in bonding and structure.
What changes and what stays fixed?
In chemical changes the nucleus is left alone. The protons and neutrons stay exactly as they were, so the proton number and nucleon number do not change. Only electrons move.
- Positive ion (cation): electrons lost. Electrons = protons − charge.
- Negative ion (anion): electrons gained. Electrons = protons + charge.
The size of the charge is the number of electrons lost or gained. A charge of 3+ means three electrons lost. A charge of 2− means two electrons gained.
Worked example
Find the number of protons, neutrons and electrons in the ion 27/13 Al³⁺.
Step 1, protons: the proton number is 13, so 13 protons.
Step 2, neutrons: 27 − 13 = 14 neutrons.
Step 3, electrons: the neutral atom has 13 electrons. A 3+ charge means three electrons have been lost, so 13 − 3 = 10 electrons.
Step 4, check the charge: +13 from protons and −10 from electrons gives +3. That matches the ion’s charge.
Answer: 13 protons, 14 neutrons, 10 electrons.
The charge check in step 4 works for every ion and takes seconds.
The mistake to watch for
A common slip is to alter the protons because “the ion has a different charge”.
Mistaken answer for Al³⁺: 10 protons, 14 neutrons, 10 electrons.
The student changed the proton count to match the electron count, which turns the ion into a different element.
If the protons change, the element changes, and ion formation does not do that. The correction: protons are fixed by the element. Only the electron count moves, and the charge is the difference, protons minus electrons.
The same habit applies to a negative ion. For 16/8 O²⁻, protons stay at 8, neutrons stay at 16 − 8 = 8, and electrons are 8 + 2 = 10.
Check yourself
1. How many protons, neutrons and electrons are in 23/11 Na⁺?
Show answer
Protons = 11. Neutrons = 23 − 11 = 12. Electrons = 11 − 1 = 10. Check: +11 − 10 = +1.
11 protons, 12 neutrons, 10 electrons.
2. How many of each particle are in 32/16 S²⁻?
Show answer
Protons = 16. Neutrons = 32 − 16 = 16. Electrons = 16 + 2 = 18. Check: +16 − 18 = −2.
16 protons, 16 neutrons, 18 electrons.
3. A particle has 26 protons, 30 neutrons and 23 electrons. Write its symbol with nucleon number, proton number and charge.
Show answer
Nucleon number = 26 + 30 = 56. Proton number = 26, so the element is iron. Charge = 26 − 23 = +3.
56/26 Fe³⁺
Where this leads next
Next, see how atoms of the same element can differ in neutrons in comparing isotopes. The equation balance reasoning trainer also shows why charges must balance when ions combine, and the mixed practice set mixes atoms and ions together.
Some students know the rule but still slip when the charge is negative. A teacher watching that slip live can pinpoint it quickly, and that is part of online one-to-one Chemistry tuition.