How to Answer Atomic Radius and Ionic Radius Exam Questions (GCSE & BTEC Applied Science Guide)

Have you ever seen a question like this in your chemistry paper?

“Discuss the atomic radius and ionic radius data in the table”

It’s a classic exam favourite — and one that can easily earn you full marks if you know what the examiner is looking for. Let’s break it down step by step.

🧪 Step 1: Understand What the Question Is Really Asking

This type of question wants you to:

1. Describe the data in the table (what happens to the numbers).

2. Explain the trends using your understanding of atomic structure and the periodic table.

3. Compare both the atomic and ionic radii for the different elements.

That’s how you turn simple data into a scientific discussion.

📊 Step 2: Look Carefully at the Data

From this, you can spot two clear patterns:

1. The ionic radius is larger than the atomic radius for every element.

2. Both atomic and ionic radii increase as you go down the group (from oxygen to sulfur).

Now let’s explain why.

⚛️ Step 3: Explain Why the Ionic Radius Is Larger

When a non-metal atom forms a negative ion, it gains electrons. For example:

• Oxygen becomes O²⁻ (gains two electrons)

• Fluorine becomes F⁻ (gains one electron)

Adding extra electrons increases repulsion between the negatively charged electrons in the outer shell. The number of protons in the nucleus stays the same, so the attractive pull on each electron becomes weaker.

👉 This causes the outer electrons to spread out more — so the ion is bigger than the atom.

That’s why O²⁻, F⁻, and S²⁻ all have larger radii than their neutral atoms.

🧭 Step 4: Explain the Trend Down the Group

As you go down the group in the periodic table:

• Atoms gain more electron shells, increasing the distance between the nucleus and outer electrons.

• The shielding effect also increases, reducing the attraction between the nucleus and the outermost electrons.

So, sulfur has more shells than oxygen or fluorine — and both its atomic and ionic radii are therefore larger.

🧩 Step 5: Compare Oxygen and Fluorine

Oxygen and fluorine are both in the same period (Period 2). Fluorine’s atomic radius is slightly smaller than oxygen’s because fluorine has more protons (9 compared to 8). This creates a stronger nuclear attraction that pulls electrons in a little closer to the nucleus.

That explains why F has a smaller atomic radius than O, even though they’re side by side.

🧠 Step 6: Write It All Together for a 6-Mark Answer

Here’s what a well-structured answer looks like:

The ionic radius is larger than the atomic radius for oxygen, fluorine and sulfur. This happens because when atoms gain electrons to form negative ions, electron–electron repulsion increases and the same number of protons must attract more electrons, reducing the pull on each electron.

The atomic and ionic radii increase down the group from oxygen to sulfur because more electron shells are added, increasing shielding and distance from the nucleus.

Within the same period, fluorine has a smaller atomic radius than oxygen because it has more protons, creating a stronger attraction between the nucleus and outer electrons.

That’s your 6/6 response — clear, comparative, and backed by data.

🧭 Top Tips for Exam Success

✅ Always use data from the question — quote numbers where possible.

✅ Include comparisons (larger/smaller, increases/decreases).

✅ Link each trend to atomic structure: number of shells, protons, and electrons.

✅ Keep your language scientific but concise.

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