VCE Chemistry

Elements, Isotopes and Ions: Atomic Number, Mass Number and Nuclide Notation

Published 2026-09-22 · Updated 2026-09-22

Few topics in VCE Chemistry Unit 1 look simpler on the surface yet cost students more marks in the VCAA Chemistry Examination than this one. The numbers on a nuclide symbol are small, but mixing up which is which, or forgetting to adjust the electron count when an ion forms, are among the most common errors VCAA examiners report. Getting this concept locked in early pays dividends across the entire course.

What the syllabus asks

This concept sits within Unit 1: How can the diversity of materials be explained? You need to understand what defines an element, what isotopes are, how ions form, and how to read and interpret nuclide notation, including identifying the number of protons, neutrons and electrons for any given atom or ion.

The idea, explained

The nuclear model of the atom

Atoms consist of a small, dense central nucleus surrounded by electrons arranged in distinct energy levels (also called shells). Almost all the mass of an atom sits in the nucleus; almost all the volume is the space occupied by electrons.

There are three subatomic particles you need to know:

Because electrons have negligible mass compared with protons and neutrons, the mass of an atom is essentially the mass of its nucleus.

Elements and atomic number

An element is a pure substance whose atoms all share the same atomic number, given the symbol Z. The atomic number equals the number of protons in the nucleus. Z is the identity tag of an element: change Z and you change the element entirely. The periodic table lists elements in order of increasing Z.

Isotopes

Isotopes are atoms of the same element (same Z, same number of protons) that have different numbers of neutrons, and therefore different mass numbers. The mass number, given the symbol A, is defined as:

A = number of protons + number of neutrons

So the number of neutrons = A − Z.

Because isotopes of an element have the same number of protons, they have the same electron configuration and therefore the same chemical properties. They differ in physical properties such as mass and nuclear stability. Carbon-12 (12 nucleons: 6 protons, 6 neutrons) and carbon-14 (14 nucleons: 6 protons, 8 neutrons) are a classic example. Both are carbon because both have Z = 6.

Ions

A neutral atom has equal numbers of protons and electrons, so its overall charge is zero. An ion forms when an atom gains or loses electrons. Crucially, the number of protons does not change when an ion forms.

• A cation is positively charged because electrons have been lost (for example, Na+, Mg2+, Al3+).
• An anion is negatively charged because electrons have been gained (for example, Cl−, O2−).

Nuclide notation

The standard nuclide symbol is written as A on the top left and Z on the bottom left of the element symbol X, with any ionic charge at the top right. In plain text this is written as (A, Z)X(charge), but you will see it in your textbook and on exam papers with A above Z to the left of the symbol.

From any nuclide symbol you can read off:

• Protons = Z
• Neutrons = A − Z
• Electrons = Z − (charge as a signed integer)

For a neutral atom the charge is zero, so electrons = Z. For a cation with charge +2, electrons = Z − 2. For an anion with charge −1, electrons = Z − (−1) = Z + 1.

Worked examples

• 12C (Z = 6): 6 protons, 12 − 6 = 6 neutrons, 6 electrons (neutral).
• 14C (Z = 6): 6 protons, 14 − 6 = 8 neutrons, 6 electrons. 12C and 14C are isotopes of carbon.
• 35Cl− (Z = 17): 17 protons, 35 − 17 = 18 neutrons, 17 − (−1) = 18 electrons.
• 24Mg2+ (Z = 12): 12 protons, 24 − 12 = 12 neutrons, 12 − 2 = 10 electrons.
• 56Fe3+ (Z = 26): 26 protons, 56 − 26 = 30 neutrons, 26 − 3 = 23 electrons.

What the exam asks

VCAA examiners regularly ask students to determine the number of protons, neutrons and/or electrons from a nuclide symbol, including for ions. You may also be asked to identify whether two species are isotopes, ions of the same element, or different elements entirely. The decision rules are straightforward:

• Same Z, different A: isotopes of the same element.
• Same Z, same A, different charge: an ion and its parent atom (or two different ions of the same isotope).
• Different Z: different elements, regardless of A or charge.

Expect short-answer questions that give you a nuclide symbol and ask for a specific particle count, or that ask you to explain the difference between two species.

Common mistakes

• Swapping A and Z. Students often read the bottom number as the mass number and the top number as the atomic number. Remember: Z (atomic number) is at the bottom left; A (mass number) is at the top left.
• Forgetting to adjust electrons for ions. A neutral atom has electrons equal to Z. When a cation forms, electrons decrease; when an anion forms, electrons increase. The proton count never changes.
• Confusing mass number with atomic mass. Mass number A is always a whole number (count of protons plus neutrons). It is not the same as the relative atomic mass listed on the periodic table, which is a weighted average across all naturally occurring isotopes.
• Claiming isotopes have different chemical properties. Isotopes share the same electron configuration and therefore the same chemical properties. Only physical properties (such as mass) differ.
• Miscalculating neutrons when a charge is present. Neutrons = A − Z always, regardless of ionic charge. The charge only affects the electron count, not the neutron count.

If you want to practise reading nuclide symbols, working through ion formation and testing yourself with exam-style questions, Avocado's AI-native tutoring service has you covered. Work through the Elements, Isotopes and Ions lessons at your own pace and get instant feedback on every answer.