For any ion written as A/Z X with a charge, use three simple rules. Protons = atomic number Z. Neutrons = mass number A minus Z. Electrons = Z minus the charge (a + charge means fewer electrons, a - charge means extra electrons). Memory hook: "Protons never change, neutrons come from the top number, only electrons feel the charge."
Three rules for any ion: protons come from Z, neutrons from A minus Z, and electrons from Z minus the charge. Only the electron count changes when an ion forms.
Your doubts, answered
When an atom becomes an ion, do the protons or neutrons change?
No. Protons and neutrons are inside the nucleus, and the nucleus does not change when an ion forms. Only electrons are added or removed. So for an ion, protons = Z and neutrons = A - Z, exactly the same as for the neutral atom. The identity of the element never changes because protons stay the same.
Why does a positive charge mean fewer electrons, not more?
Charge tells you how many electrons were lost or gained. A positive (+) ion (a cation) formed by LOSING electrons, so it has fewer electrons than protons. Example: Na+ lost 1 electron, so electrons = 11 - 1 = 10. Rule: electrons = Z - charge. Since the charge is +1, you subtract, giving fewer electrons.
How do I find electrons in a negative ion like O2-?
A negative (-) ion (an anion) GAINED electrons. Use electrons = Z - charge, but the charge is negative, so subtracting a negative number ADDS. For O2-, Z = 8 and charge = -2, so electrons = 8 - (-2) = 8 + 2 = 10. Easy version: for a minus ion, ADD the charge number to Z.
How do I get neutrons when only the mass number is given?
Neutrons = mass number (A) - atomic number (Z). The mass number is the top-left number in the symbol A/Z X, and it counts protons plus neutrons together. Subtract the protons (Z) to leave only neutrons. Charge does NOT affect this at all, because electrons have almost no mass.
Which number is A and which is Z in the symbol?
In A/Z X, the top number is A (mass number = protons + neutrons) and the bottom number is Z (atomic number = number of protons). A is always the bigger number. Remember: A is Above and is the Atomic mass; Z is the smaller number at the bottom and equals protons.
What does isoelectronic mean and how is it linked to this?
Isoelectronic species have the SAME number of electrons. To check, find electrons for each using electrons = Z - charge. For example Na+ (11-1=10), Mg2+ (12-2=10), O2- (8+2=10) and F- (9+1=10) all have 10 electrons, so they are isoelectronic. NEET asks this every few years.
⚠️ The NEET trap ✗ Students think a positive ion has MORE electrons and a negative ion has FEWER, so they add for + and subtract for -. For Na+ they wrongly write 11 + 1 = 12 electrons. ✓ It is the opposite. A + ion LOST electrons (fewer), a - ion GAINED electrons (more). Use electrons = Z - charge with the sign: Na+ gives 11 - (+1) = 10; O2- gives 8 - (-2) = 10. 🧠 Plus means minus electrons: a + sign shows electrons LEFT, so you take them away.
Real NEET questions
2020
The number of protons, neutrons and electrons in \(^{175}_{71}\mathrm{Lu}\), respectively, are:
A · 71, 71 and 104
B · 175, 104 and 71
C · 71, 104 and 71 ✓
D · 104, 71 and 71
Solution: Lu here is a neutral atom (no charge shown). Protons = Z = 71. Neutrons = A - Z = 175 - 71 = 104. Electrons = protons = 71 (neutral atom). So the order protons, neutrons, electrons is 71, 104, 71, which is option C.
2023
Which one of the following represents all isoelectronic species (same number of electrons)?
A · Na+, Mg2+, O-, F-
B · Ca2+, Ar, K+, Cl- ✓
C · Na+, Cl-, O-, NO+
D · N2O, N2O4, NO+, NO
Solution: Count electrons using electrons = Z - charge. Ca2+ = 20 - 2 = 18; Ar = 18; K+ = 19 - 1 = 18; Cl- = 17 + 1 = 18. All have 18 electrons, so option B is fully isoelectronic. In option A, O- has 9 electrons (not 10), so it fails.
2021
From the following pairs of ions, which one is NOT an isoelectronic pair?
A · Mn2+, Fe3+
B · Fe2+, Mn2+ ✓
C · O2-, F-
D · Na+, Mg2+
Solution: Electrons = Z - charge. Fe2+ = 26 - 2 = 24; Mn2+ = 25 - 2 = 23. These are different (24 vs 23), so option B is NOT isoelectronic. Check others: Mn2+ (23) and Fe3+ (26-3=23) match; O2- (10) and F- (10) match; Na+ (10) and Mg2+ (10) match.
Solved Structure Of Atom NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the formula for protons, neutrons and electrons in an ion?
Protons = Z (atomic number). Neutrons = A - Z (mass number minus atomic number). Electrons = Z - charge. The charge only changes electrons, never protons or neutrons.
How many electrons does Al3+ have?
Aluminium has Z = 13. Al3+ lost 3 electrons, so electrons = 13 - 3 = 10. It is isoelectronic with neon.
Does the mass number change when an ion forms?
No. Mass number A = protons + neutrons, and neither changes when electrons are added or removed. The mass of electrons is tiny, so the mass number stays the same.
Why is finding electrons in ions important for NEET?
NEET regularly asks isoelectronic species and proton/neutron/electron counting (2020, 2021, 2023). These are quick, guaranteed marks if you know electrons = Z - charge.
How do I quickly spot isoelectronic species?
Compute electrons = Z - charge for each species and compare. If the electron counts are equal, they are isoelectronic. Common target counts are 10 (neon core) and 18 (argon core).
