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Science · Chemistry

Atomic Structure

Protons, neutrons, electrons, and how their arrangement determines an element's identity and behavior — the atomic number and mass number aren't just labels, they're the two numbers every other calculation in this subtopic builds from. Every question comes with a written explanation of exactly what each number represents.

Practise atomic structure nowSee sample questions
3
Difficulty tiers
25s
Target pace
58%
Average first-attempt score

What's covered

  • Subatomic particles — identify protons and neutrons in the nucleus, and electrons in orbitals around it, along with each particle's charge and relative mass.
  • Atomic number and mass number — read the atomic number as the number of protons (which defines the element), and the mass number as protons plus neutrons.
  • Isotopes — recognize that isotopes of an element share the same number of protons but differ in neutron count, giving them different mass numbers.
  • Ions — determine an ion's charge from the difference between its number of protons and electrons — more electrons than protons gives a negative charge, fewer gives a positive charge.
  • Electron configuration — fill electron shells in order of increasing energy, with each shell holding a specific maximum number of electrons.

Where students lose marks

Confusing atomic number with mass number

Atomic number is protons only; mass number is protons plus neutrons — using mass number where atomic number is needed, or vice versa, gives the wrong particle count.

Assuming isotopes are different elements

Isotopes of the same element have identical chemical behavior because they have the same number of protons and electrons — only the neutron count, and therefore the mass, differs.

Getting an ion's charge backwards

Gaining electrons makes an ion more negative, not more positive — losing electrons is what creates a positive charge, since protons don't change during ion formation.

Three sample questions

Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.

Sample 01Foundation

An atom of chlorine has an atomic number of 17 and a mass number of 35. How many neutrons does this atom have?

Sample 02Core

An ion has 20 protons, 20 neutrons, and 18 electrons. What is the charge of this ion?

Sample 03Advanced

Two isotopes of carbon, carbon-12 and carbon-14, both have an atomic number of 6. How many more neutrons does carbon-14 have compared to carbon-12?

How to practise this

Atomic structure questions are almost entirely arithmetic once the definitions are locked in — the fastest gains come from never confusing atomic number with mass number.

Write out protons, neutrons, and electrons separately for every particle question

Before answering, list all three counts explicitly, even when the question only asks for one. Untimed practice is where that habit prevents mixing up the numbers.

Then 25 seconds a question

Move to timed sessions once the atomic number/mass number distinction is automatic.

Fold into a mock

Pair atomic structure with the periodic table in a mock — atomic number is literally what the table is organized by.

Stop mixing up the numbers.
Start listing every particle count.

  • A particle-by-particle walkthrough on every item
  • Timed or untimed sessions, any length you like
  • Tracked separately — see your atomic structure accuracy and pace over time
Start practising free

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