Using the Periodic Table to Compare Elements
In 1886, Henri Moissan finally isolated fluorine after 74 years of failed attempts, it is so reactive it ignites steel wool on contact at room temperature.
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Q1 · How do you already use the periodic table to find information about an element?
Q2 · Without doing an experiment, do you think you could predict which element would be more reactive, fluorine or iodine? What clues would you use?
● Know
- element comparison can use several table features
- position, symbol, atomic number and category all matter
- good comparisons use evidence
● Understand
- one-feature comparisons are weaker than combined comparisons
- the periodic table supports judgement and communication
- comparison prepares you for property-use reasoning
● Can do
- compare two selected elements
- use several pieces of table evidence in one answer
- write precise comparison sentences
Pick up a brown-purple crystal of iodine and compare it with a diagram of fluorine gas, both sit in Group 17, yet one is handled safely in a school lab while the other ignites steel on contact. Strong comparisons use more than one feature from the periodic table to explain why. A weak comparison might say only 'both are metals.' A strong comparison adds group, period, atomic number, category and property information. The more features you use, the more precise and convincing your comparison becomes.
The skill is selecting features that actually help answer the question. If you are comparing reactivity, group number matters most. If you are comparing size, period number matters most. If you are comparing conductivity, category (metal vs non-metal) matters most. Choose the right evidence for the claim.
Weak comparison: 'Sodium and potassium are both metals.' Strong comparison: 'Sodium and potassium are both Group 1 alkali metals with one valence electron. Potassium is below sodium in Group 1, so it has more electron shells and a larger atomic radius. This makes potassium more reactive than sodium, as shown by its more vigorous reaction with water.' The strong version uses position, structure and property.
Australian scientific reasoning: ACER research shows that students who learn to support claims with multiple pieces of evidence perform better in science assessments. The periodic table provides multiple features, group, period, category, atomic number, that can be combined into strong comparative arguments.
'One feature is enough for a strong comparison.' It is not. Saying two elements are both metals is true but shallow. A strong comparison connects multiple features and explains how they relate. The goal is not to mention as many facts as possible, but to select the features that best support your specific claim.
Use the Element Comparator interactive below. What is one thing you learned from using it?
Rank these halogens from most reactive to least reactive.
- Astatine (At)
- Fluorine (F)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)
Not all periodic table features are equally useful for every question. The skill of strong comparison is choosing features that actually help. If you are comparing reactivity, group position is the most relevant feature. If you are comparing atomic size, period number is more relevant. If you are comparing conductivity, metal vs non-metal category is most relevant.
Using irrelevant features weakens your argument. Mentioning that two elements have different numbers of neutrons does not help explain why they react similarly. Select evidence that directly supports your claim.
When comparing fluorine and chlorine (both Group 7 halogens), the relevant feature is group position: both have seven valence electrons and both are highly reactive non-metals. The fact that fluorine has nine protons and chlorine has seventeen is true but irrelevant to their chemical similarity. Group number explains their similar behaviour; atomic number does not.
Australian forensic science: Forensic chemists at the Australian Federal Police select specific elemental features when comparing trace evidence. They do not analyse every property, they choose the ones most likely to distinguish between samples. This is exactly the skill we are practising: selecting the most useful evidence.
'More features always make a stronger comparison.' Not if the features are irrelevant. A comparison that mentions atomic number, neutron count, electron configuration and melting point is not necessarily stronger than one that focuses on group number and reactivity. Quality of evidence matters more than quantity.
- Fluorine
- Chlorine
- Iodine
- Water disinfection
- Antiseptic and thyroid health
- Toothpaste (fluoride)
Comparison needs precise language. Words like 'similar,' 'different,' 'more' and 'less' are useful but must be backed by specific features from the periodic table. A strong comparison sentence might read: 'Sodium and potassium are similar because they are both Group 1 metals with one valence electron, but potassium is more reactive because it is further down the group.'
The structure 'both... but...' is powerful because it acknowledges similarity while explaining difference. This is the language of scientific comparison, balanced, specific and evidence-based.
Weak: 'Lithium and sodium are the same.' Strong: 'Lithium and sodium are similar because both are Group 1 alkali metals with one valence electron, but sodium is more reactive because it has more electron shells and its outer electron is further from the nucleus.' The strong version uses specific features and explains the relationship between position and property.
Australian science education: The NSW Education Standards Authority explicitly rewards comparative reasoning in science assessments. Students who use precise language like 'both... but...' and support claims with multiple periodic table features consistently earn higher marks.
'Saying two things are similar is enough.' It is not. Similarity needs to be explained using specific shared features. And if there are differences, those need explanation too. Scientific comparison is not about finding sameness or difference, it is about explaining why those similarities and differences exist.
Predict: which reacts more with water, magnesium (Mg) or calcium (Ca)?
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
Improve a weak comparison sentence by adding position and category evidence.
Both sodium and potassium are in , but they have different and occupy different .
By the end of this block, you should be able to use the table to support simple evidence-based judgements.
That prepares them for the final block on properties, uses and scientific understanding.
The checkpoint will now test navigation, categories and descriptive patterns together.
Find the mistake in this comparison.
- Sodium and potassium are both metals.
- They are both in Group 1.
- They have the same atomic number.
- They are both reactive.
Compare two named elements using at least three features from the periodic table.
At the start of this lesson, you were asked about fluorine being the most reactive non-metal on Earth while iodine in the same group reacts far more gently, and how you use the periodic table to compare two elements without running an experiment.
Now that you have worked through everything, write your answer below. How has your thinking changed, and what surprised you most?
Q1. Explain why a strong element comparison should use more than one table feature.
Q2. Compare two elements using category and one other table feature.
Q3. Why is evidence-based comparison better than opinion-based comparison in science?
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: D. That combination provides the strongest evidence set.
2: B. It helps include both similarity and difference.
3: A. That sentence uses clear comparison evidence.
4: C. Comparison prepares you for later property-use judgements.
5: D. That gives weak, non-scientific evidence.
Short Answer 1
A strong comparison should use more than one table feature because one fact alone is often too limited. Using several features gives a clearer and more supported comparison.
Short Answer 2
Example: Both aluminium and copper are metals, but they have different atomic numbers and different positions in the periodic table.
Short Answer 3
Evidence-based comparison is better because science requires justified reasoning. Opinion does not show why the comparison is valid, while table evidence does.
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: D. That combination provides the strongest evidence set.
2: B. It helps include both similarity and difference.
3: A. That sentence uses clear comparison evidence.
4: C. Comparison prepares you for later property-use judgements.
5: D. That gives weak, non-scientific evidence.
Short Answer 1
A strong comparison should use more than one table feature because one fact alone is often too limited. Using several features gives a clearer and more supported comparison.
Short Answer 2
Example: Both aluminium and copper are metals, but they have different atomic numbers and different positions in the periodic table.
Short Answer 3
Evidence-based comparison is better because science requires justified reasoning. Opinion does not show why the comparison is valid, while table evidence does.
● Comparison
Strong comparisons use several table features.
● Language
Both... but... is a helpful comparison frame.
● Evidence
Position, category and atomic number can all support comparison.
● Checkpoint
You are now ready for Checkpoint 3.