WACE Chemistry

Atomic Structure and Electron Configuration: A WACE Chemistry Guide

Published 2026-09-10 · Updated 2026-09-10

Many students lose marks in the ATAR course examination not because they don't know the facts, but because they mix up the terms or write electron configurations in the wrong order. WACE markers see the same errors every year: students who confuse mass number with atomic number, or who forget that the valence shell, not the whole atom, is what drives bonding. Get these foundations right and a surprising number of later topics fall into place.

What the syllabus asks

This concept sits inside Unit 1: Chemical Fundamentals, and it covers three tightly connected ideas:

• The nuclear model of the atom, including the role of subatomic particles and the forces that hold an atom together.
• How electrons are arranged in energy levels (shells), and how to write an electron configuration.
• Why the valence shell matters for chemical stability and bonding.

Everything below maps directly to those ideas.

The idea, explained

The nuclear model

An atom is modelled as a small, dense nucleus surrounded by electrons arranged in distinct energy levels, also called shells. The nucleus contains protons (charge +1, relative mass 1) and neutrons (charge 0, relative mass 1). Electrons (charge −1, negligible mass) occupy the energy levels around the nucleus.

Two numbers define any atom:

• Atomic number (Z): the number of protons. This is what makes an element what it is. Carbon always has 6 protons; sodium always has 11.
• Mass number (A): protons plus neutrons combined.

A neutral atom has exactly as many electrons as protons, so its overall charge is zero. Almost all the mass of an atom is concentrated in the nucleus, while almost all the volume is the space occupied by the electrons around it.

The whole structure is held together by the electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons. Without this attraction, the electrons would simply fly away.

Electron configuration

Electrons do not sit randomly around the nucleus. They fill energy levels starting from the one closest to the nucleus, because that level is lowest in energy. The shells fill in order, and each shell has a maximum capacity:

• 1st shell: up to 2 electrons
• 2nd shell: up to 8 electrons
• 3rd shell: up to 8 electrons (for the first 20 elements)
• 4th shell: fills next, after the 3rd is full

An electron configuration is written by listing the number of electrons in each shell from the innermost outward, separated by commas. Here are three examples you should know well:

• Sodium (Z = 11): 2, 8, 1
• Chlorine (Z = 17): 2, 8, 7
• Calcium (Z = 20): 2, 8, 8, 2

To write any configuration yourself, simply work out the atomic number (total electrons in a neutral atom), then fill the shells in order until you run out of electrons.

The valence shell and why it drives bonding

The outermost occupied energy level is called the valence shell, and the electrons in it are the valence electrons. This is the part of the atom that interacts with other atoms.

An atom is most stable when its valence shell is full, matching the electron arrangement of a noble gas. For most elements that means eight electrons in the valence shell; for elements whose valence shell is the first shell, two electrons is the stable number.

Atoms that do not already have a full valence shell will gain, lose, or share electrons to reach that stable arrangement, and this is precisely what chemical bonding is. Two examples make this concrete:

• Sodium (2, 8, 1) has one electron in its valence shell. It loses that electron to reach the stable arrangement 2, 8, matching the noble gas neon.
• Chlorine (2, 8, 7) has seven electrons in its valence shell, one short of a full shell. It gains one electron to reach 2, 8, 8, matching the noble gas argon.

The number of valence electrons therefore tells you a great deal about how an element will behave chemically.

What the exam asks

In the ATAR course examination, questions on this concept typically ask you to:

• State the charge, relative mass, and location of each subatomic particle.
• Calculate the number of protons, neutrons, or electrons from a given atomic number and mass number.
• Write the electron configuration for any of the first 20 elements.
• Identify the valence shell and the number of valence electrons for a given element.
• Explain, using electron configuration, why an atom tends to gain or lose a specific number of electrons when bonding.

Practise writing configurations quickly and accurately, and always connect the valence shell back to stability when a question asks you to explain bonding behaviour.

Common mistakes

• Swapping atomic number and mass number. Atomic number (Z) is protons only. Mass number (A) is protons plus neutrons. Confusing them leads to wrong electron configurations and wrong particle counts.
• Writing the configuration in the wrong order. Always list shells from the innermost (closest to the nucleus) outward. Writing 1, 8, 2 instead of 2, 8, 1 for sodium will cost marks.
• Forgetting that a neutral atom has equal protons and electrons. When you are asked for the number of electrons in a neutral atom, the answer is always the same as the atomic number.
• Overfilling the 3rd shell. For the first 20 elements, the 3rd shell holds a maximum of 8 electrons. Calcium (Z = 20) is 2, 8, 8, 2, not 2, 8, 10.
• Describing bonding without mentioning the valence shell. WACE markers expect you to use the term "valence shell" (or "valence electrons") and to link the electron arrangement explicitly to stability when explaining why atoms bond.

If you want to work through practice questions and get instant feedback on your electron configurations, Avocado's AI-native tutoring service has a dedicated lesson ready for you: Atomic Structure and Electron Configuration.