Subatomic Particles
A = mass number, Z = atomic number, N = A − Z
Z = protons, N = A − Z, electrons = Z − charge. Methodology & sources →
Atomic structure chapter? The General Chemistry Workbook covers isotopes, ions and electron configuration with worked examples.As an Amazon Associate we earn from qualifying purchases.
View on Amazon →Reading Isotope Notation
An atom is written with the mass number on top and the atomic number on the bottom, before the symbol. From those:
- Protons = atomic number Z.
- Neutrons = mass number A − atomic number Z.
- Electrons = protons − charge (a neutral atom has electrons = protons).
To average all the isotopes of an element, use the Average Atomic Mass Calculator.
Worked Example — Reading Isotope Notation
Question: An atom is written as ³⁵₁₇Cl. Find its numbers of protons, neutrons, and electrons. Then write the notation for a chlorine isotope with 20 neutrons.
Step 1 — protons: the atomic number Z = 17 (the subscript), so there are 17 protons. This is fixed for every chlorine atom.
Step 2 — neutrons: N = A − Z = 35 − 17 = 18 neutrons (the mass number A is the superscript).
Step 3 — electrons: for a neutral atom, electrons = protons = 17.
Step 4 — the new isotope: keep Z = 17 (still chlorine), set N = 20, so A = Z + N = 37. The isotope is ³⁷₁₇Cl.
Answer: 17 protons, 18 neutrons, 17 electrons; the 20-neutron isotope is chlorine-37.
Common Mistakes
- Mixing up A and Z. The superscript (A) is the mass number = protons + neutrons. The subscript (Z) is the atomic number = protons only. Neutrons = A − Z, never A + Z.
- Assuming electrons always equal protons. That's only true for a neutral atom. For an ion, electrons = protons − charge (so a 2+ ion has 2 fewer electrons than protons).
- Isotopes vs. ions confused. Isotopes differ in neutron count (same element, different mass number). Ions differ in electron count (same element, different charge). An atom can be both an isotope and an ion at once.
Frequently Asked Questions
Neutrons = mass number − atomic number (N = A − Z).
Subtract the charge from the number of protons. A 2+ ion has two fewer electrons than protons; a 2− ion has two more.
Atomic number (Z) is the number of protons — it defines which element an atom is, and never changes for that element. Mass number (A) is protons + neutrons, and varies between isotopes of the same element. Z is written as a subscript, A as a superscript, in the notation ᴬZX.
Isotopes of an element have the same number of protons (same Z) but different numbers of neutrons, giving different mass numbers. Ions have the same number of protons but a different number of electrons, giving a net charge. A single atom can be both — for example, a chlorine-37 atom that has gained an electron is both an isotope (37Cl rather than the more common 35Cl) and an ion (Cl⁻).