Chemistry · Unit 1

Atoms

Atomic structure and elements

Everything in chemistry is a consequence of atomic structure, and almost all of it is a consequence of one part: where the electrons are.

The unit covers protons, neutrons and electrons, the nucleus, and atomic number - the count of protons that decides which element you are dealing with and cannot be changed by any chemical process.

This unit breaks down into 16 short steps and 95 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 16
Questions: 95
Difficulty: 1-3

What this unit covers

Proton
Neutron
Atomic Number
Electron
Nucleus

Where this fits

The first unit in Chemistry. The Periodic Table and Chemical Bonding both assume it completely.

Where people slip

Atomic number and mass number are different counts. Protons identify the element; adding neutrons gives you an isotope of the same element, not a new one.

How the unit is structured

Atoms runs as 16 short steps that unlock in order. 12 are practice rounds and 4 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 16 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 95 in the unit altogether.

Atomic Number

Fill the blankLevel 1
1. In the modern periodic table, elements are arranged in order of increasing ____ number.
- atomiccorrect
- mass
- neutron
- shell
Elements are ordered by atomic number, which increases by one from each element to the next.
Match the pairsLevel 1
2. Match each element to its chemical symbol.
Answer: Sodium = Na; Potassium = K; Oxygen = O; Hydrogen = H
Sodium is Na, potassium is K, oxygen is O, and hydrogen is H.
Multiple choiceLevel 1
3. The atomic number of an element tells you the number of what?
- Protonscorrect
- Neutrons
- Electron shells
- Molecules
An element's atomic number equals the number of protons in each atom's nucleus.
Sequence recallLevel 1
4. Memorize the first four elements in order of atomic number, then repeat that order.
Answer: Hydrogen -> Helium -> Lithium -> Beryllium
By atomic number the first four elements are hydrogen (1), helium (2), lithium (3), and beryllium (4).
Build the sentenceLevel 2
5. Build the sentence that defines atomic number.
Answer: The atomic number equals the number of protons
Atomic number is defined as the number of protons in an atom's nucleus.
Guess the numberLevel 2
6. How many protons does an atom of carbon have?
Answer: 6 protons
Carbon's atomic number is 6, so every carbon atom has 6 protons.

Electron

Build the sentenceLevel 1
7. Build a short sentence describing how electrons are organised in an atom.
Answer: electrons are arranged in shells
Electrons are arranged in shells, also called energy levels, around the nucleus.
Fill the blankLevel 1
8. An atom that has become electrically charged by gaining or losing electrons is called an ____.
- ioncorrect
- isotope
- element
- molecule
A charged atom is called an ion.
Guess the numberLevel 1
9. What is the maximum number of electrons that the second electron shell can hold?
Answer: 8 electrons
The second shell can hold up to 8 electrons.
Fact or fibLevel 2
10. Electrons fill the shell closest to the nucleus before filling shells further out.
Answer: True
Electrons occupy the lowest energy shells, which are nearest the nucleus, first.
Match the pairsLevel 2
11. Match each subatomic particle to its charge or location.
Answer: Proton = Positive charge; Electron = Negative charge; Neutron = No charge; Nucleus = Center of the atom
Protons are positive, electrons are negative, neutrons are neutral, and the nucleus holds them at the atom's center.
Choose all that applyLevel 3
12. Which subshells are present in the third shell (n = 3)? Select all that apply.
- 3scorrect
- 3pcorrect
- 3dcorrect
- 3f
The n=3 shell contains 3s, 3p and 3d subshells; there is no 3f.

Neutron

Fill the blankLevel 1
13. The mass number of an atom is the total number of protons plus ____.
- neutronscorrect
- electrons
- ions
- shells
Mass number counts the particles in the nucleus: protons plus neutrons.
Multiple choiceLevel 1
14. Which subatomic particle has no electric charge?
- Neutroncorrect
- Proton
- Electron
- Ion
Neutrons are electrically neutral, meaning they carry no charge.
True or falseLevel 1
15. Neutrons are located inside the nucleus of an atom.
Answer: True
Both protons and neutrons are packed together in the nucleus.
Guess the numberLevel 2
16. A fluorine atom has a mass number of 19 and an atomic number of 9. How many neutrons does it have?
Answer: 10 neutrons
Neutrons = mass number - atomic number = 19 - 9 = 10.
Fill the blankLevel 2
17. In a mass spectrometer, ions are separated according to their mass-to-____ ratio.
- chargecorrect
- volume
- speed
- colour
Ions are sorted by their mass-to-charge ratio, written as m/z.
Guess the numberLevel 2
18. An atom of carbon-14 has an atomic number of 6. How many neutrons does it contain?
Answer: 8 neutrons
Neutrons = mass number - atomic number = 14 - 6 = 8.

Nucleus

Fill the blankLevel 1
19. The small, dense region at the center of an atom is called the ____.
- Nucleuscorrect
- Shell
- Orbit
- Membrane
The nucleus sits at the atom's center and holds protons and neutrons.
Multiple choiceLevel 1
20. Almost all of an atom's mass is concentrated in which part?
- The nucleuscorrect
- The electron shells
- The empty space around the nucleus
- The outer electrons
Protons and neutrons in the tiny nucleus account for nearly all of an atom's mass.
True or falseLevel 1
21. The nucleus of an atom has an overall positive charge.
Answer: True
The nucleus contains positively charged protons and neutral neutrons, so it is positively charged overall.
Choose all that applyLevel 2
22. Which particles are found inside the nucleus? (Select all that apply.)
- Protonscorrect
- Neutronscorrect
- Electrons
- Photons
The nucleus contains protons and neutrons; electrons move around outside it.
Picture questionLevel 2
23. ⚛️ What does this common science symbol represent?
- An atomcorrect
- A single cell
- A galaxy
- A molecule of water
The symbol shows a nucleus circled by electron paths, representing an atom.
Put in orderLevel 2
24. Put these atomic models in the order they were developed, earliest first.
Answer: Dalton's solid sphere -> Thomson's plum pudding -> Rutherford's nuclear atom -> Bohr's shell model
The models were developed in the order Dalton, Thomson, Rutherford, then Bohr.

Proton

Guess the numberLevel 1
25. On the scale used to compare subatomic particles, what is the relative mass of a single proton?
Answer: 1 relative mass units
A proton is given a relative mass of 1 on the scale used to compare subatomic particles.
True or falseLevel 1
26. A proton carries a positive electric charge.
Answer: True
Protons are positively charged particles found in the nucleus.
Choose all that applyLevel 2
27. Which subatomic particles have a relative mass of about 1? (Select all that apply.)
- Protoncorrect
- Neutroncorrect
- Electron
- Photon
Protons and neutrons each have a relative mass of about 1, while an electron is roughly 1/1836 as heavy.
Odd one outLevel 2
28. Which of these is NOT a subatomic particle?
- Moleculecorrect
- Proton
- Neutron
- Electron
Protons, neutrons, and electrons are subatomic particles; a molecule is a group of bonded atoms.
Multiple choiceLevel 3
29. Why does first ionisation energy generally increase across a period?
- Nuclear charge increases while shielding stays similarcorrect
- Atomic radius increases across the period
- Shielding increases sharply across the period
- Each element adds electrons to a brand-new shell
Nuclear charge (proton number) rises while shielding stays similar, so outer electrons are pulled in more strongly.
Put in orderLevel 3
30. Put the stages of a time-of-flight mass spectrometer in the correct order.
Answer: Ionisation -> Acceleration -> Ion drift -> Detection
The sample is ionised, the ions are accelerated, they drift by mass, then reach the detector.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Atoms

95 questions across 16 steps. Start with step one and crawl at your own pace.

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Read about Atoms

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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