Patterns in the Periodic Table
8. Patterns in the Periodic Table
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The core idea
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Learning objectives
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- describe the Periodic Table as an arrangement of the elements in the order of increasing proton (atomic) number
- describe how the position of an element in the Periodic Table is related to proton number and electronic configuration
- explain the similarities between the elements in the same group of the Periodic Table in terms of their electronic configuration
- describe the change from metallic to non-metallic character from left to right across a period of the Periodic Table
- describe the relationship between number of outer (valence) electrons and metallic/non-metallic character
- predict the properties of elements in Group 1 and Group 17 using the Periodic Table.
- describe lithium, sodium and potassium in Group 1 (the alkali metals) as a collection of relatively soft, low density metals showing a trend in melting point and in their reaction with water
- describe chlorine, bromine and iodine in Group 17 (the halogens) as a collection of diatomic non- metals showing a trend in colour, state and their displacement reactions with solutions of other halide ions
- describe the lack of reactivity of the elements in Group 18 (the noble gases) in terms of their electronic configurations.
- place in order of reactivity calcium, copper, (hydrogen), iron, lead, magnesium, potassium, silver, sodium and zinc by reference to the reactions, if any, of the metals with water, steam and dilute hydrochloric acid
- deduce the order of reactivity from a given set of experimental results
- describe the ease of obtaining metals from their ores by relating the elements to their positions in the reactivity series
- describe the essential conditions for the corrosion (rusting) of iron as the presence of oxygen and water; prevention of rusting can be achieved by placing a barrier around the metal, e.g. painting; greasing; plastic coating.
This G3 Science (Chemistry) lesson connects periodic position, group trends and metal-reactivity evidence to exam-ready predictions.
1. Definition
The Periodic Table is an arrangement of elements in increasing proton number that places elements with similar chemical properties in vertical groups.
- Group: vertical column; main-group members share an outer-electron pattern.
- Period: horizontal row; the period number gives the number of occupied shells.
- Displacement: a more reactive element replaces a less reactive element from its compound.
Transition-element properties and uses belong to G3 Pure / O-Level Chemistry (K324 / 6092), not G3 Science. The required Combined scope ends with Groups 1, 17 and 18, metals/non-metals, reactivity, extraction and corrosion.
2. Key Ideas
| Pattern | Direction or rule | Evidence used in an answer |
|---|---|---|
| Group 1 | reactivity increases down the group | water-reaction observations and easier electron loss |
| Group 17 | reactivity decreases down the group | displacement and harder electron gain |
| Group 18 | very unreactive | full outer electron shell |
| metal displacement | more reactive metal forms ions | solution/solid observations |
| obtaining a metal from its ore | higher reactivity usually means greater difficulty | position in the reactivity series |
| rust prevention | exclude oxygen and/or water | painting, greasing or plastic coating |
Across each main-group period, metallic character generally decreases and non-metallic character increases.
Moving from left to right across a main-group period, proton number and the number of outer electrons increase. Elements with few outer electrons tend to lose them and show metallic character; those with more outer electrons tend to gain or share electrons and show non-metallic character.
3. Detailed Explanations
Periodic position and ions
For electron configuration 2,8,2, three occupied shells give Period 3 and two outer electrons give Group 2. The atom tends to lose two electrons:
Position supports a prediction; it does not prove every physical property of an unfamiliar element.
Group trends and displacement
Group 1 metals lose one electron more readily down the group because the outer electron is farther from the nucleus and more shielded. Group 17 atoms gain an electron less readily down the group, so reactivity decreases.
In a halogen displacement, compare the elements, not the halide ions:
| Group 1 element | Useful description | Reaction with water |
|---|---|---|
| lithium | soft, low-density metal; highest melting point of the three | floats and fizzes steadily |
| sodium | soft, low-density metal | floats, melts into a ball and fizzes rapidly |
| potassium | soft, low-density metal; lowest melting point of the three | reacts very rapidly and may ignite with a lilac flame |
Down Group 1, melting point decreases and reaction with water becomes more vigorous.
| Group 17 element | State and colour at room temperature |
|---|---|
| chlorine | pale green gas |
| bromine | red-brown liquid |
| iodine | grey-black solid |
Down Group 17, the colour darkens, melting and boiling points increase, and reactivity decreases. A more reactive halogen displaces a less reactive halogen from its halide solution. Chlorine displaces bromide and iodide; bromine displaces iodide but not chloride.
Group 18 elements are very unreactive because their atoms already have full outer electron shells.
Reactivity, ores and corrosion
Learn this order, with hydrogen included as a reference point:
Potassium, sodium and calcium react with cold water. Magnesium reacts very slowly with cold water but reacts with steam; zinc and iron do not react with cold water but do react with steam. Lead does not react with water or steam. Metals above hydrogen react with dilute hydrochloric acid to form a salt and hydrogen, although lead soon reacts very slowly because a coating forms. Copper and silver do not react with water, steam or dilute hydrochloric acid.
Use experimental results to deduce order rather than forcing every result into a memorised list. More reactive metals form more stable compounds and are generally harder to obtain from their ores; less reactive metals are easier to obtain.
Rusting requires both oxygen and water. Painting, greasing and plastic coating form barriers that keep one or both away from the iron. If the barrier is broken, the exposed iron can rust.
4. Common Mistakes
- Explaining a group trend only as “atoms get bigger” without connecting it to electron loss or gain.
- Reversing halogen and halide roles in a displacement.
- Claiming displacement evidence proves comparisons that were not tested.
- Saying the metal highest in the reactivity series is easiest to obtain from its ore.
- Saying oxygen or water alone is enough for rusting.
- Treating transition-element questions as part of the Combined assessment.
5. Exam Tips
Give the structural or experimental evidence, state how it affects electron transfer or reactivity, then make the chemical prediction.
For Group 1 or Group 17, quote a visible trend as well as the reactivity trend. For a corrosion question, name the barrier and say that it prevents oxygen and/or water from reaching the iron.
Exam question 1: Evaluate displacement evidenceCore
P displaces Q, and R displaces Q. Can P and R be placed in order?
Show Answer
No. Both are more reactive than Q, but the evidence does not compare P directly with R. A further displacement test is needed.
6. Worked Examples
Modelled example 1
Deduce Position and Ion
Problem
Study the worked solution
Locate the element
Method
Use three shells and six outer electrons.Reason
Shell count gives period; outer-electron count gives main group.Working
Period 3, Group 16.Complete the outer shell
Method
Gain two electrons.Reason
Six outer electrons need two more for a full shell.Working
Likely ion: X²⁻.
Guided practice 2
Order Group 1 metals from observations
Problem
Use increasing vigour down the group
Hints
Hint 1: trend
Hint 2: distinctive signs
View solution step by step
Use the order
Method
Arrange the observations from least to most vigorous.Reason
Group 1 reactivity increases down the group.Working
A < B < C in reaction vigour.Match the identities
Method
Assign lithium, sodium and potassium in that order.Reason
The molten ball and lilac flame provide supporting observations.Working
A = lithium; B = sodium; C = potassium.
Common misconception 3
Explain the Group 17 Trend
Learner claim
Connect shells and shielding to attraction
View solution step by step
Compare electron structures
Method
Give chlorine fewer shells and less shielding.Reason
The incoming electron is closer to chlorine’s nucleus.Working
Chlorine: smaller distance, less shielding.Connect to reactivity
Method
State chlorine gains an electron more readily.Reason
Its nucleus attracts the incoming electron more strongly, making chlorine more reactive than bromine.Working
Group 17 reactivity decreases down the group.
Examiner practice 4
Explain why paint prevents rusting
Examination question
Write conditions, barrier action and limitation
View solution step by step
State both conditions
2 marksMethod
Name oxygen and water.Reason
Iron requires both for rusting.Working
oxygen + waterExplain the barrier
1 markMethod
State that paint prevents oxygen and water reaching the iron surface.Reason
Removing access to either required condition stops rusting.Working
paint → oxygen and water cannot contact ironExplain the scratch
1 markMethod
State that a scratch exposes iron to air and water.Reason
The barrier no longer separates the metal from both conditions at that point.Working
scratch → exposed iron can rust
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Award oxygen and water as separate conditions, then the barrier and scratch explanations.
Challenge 5
Deduce a Reactivity Order from Evidence
Problem
Place each metal relative to hydrogen
Hints
Hint 1: water-evidence
Hint 2: hydrogen-reference
View solution step by step
Use the water evidence
Method
Place P first.Reason
P reacts with cold water, while Q does not.Working
P > QUse hydrogen as the reference
Method
Place Q above hydrogen and R below it.Reason
Q reacts with dilute hydrochloric acid, but R does not.Working
Q > (H) > RCombine only supported comparisons
Method
Join the two evidence chains.Reason
Every comparison is supported by a stated reaction result.Working
Decreasing reactivity: P > Q > (H) > R.
7. Mind Stretchers
Mind stretcher 1: Constrain a predictionExtension
An unfamiliar element lies below chlorine in Group 17. Predict two properties and identify one claim that cannot be made from position alone.
Show Answer
It should be less reactive than chlorine and should form a 1− ion in simple compounds. Its exact melting point cannot be stated from position alone without data.
Mind stretcher 2: Choose a practical barrierExtension
Suggest a suitable barrier for an iron bicycle chain and one for an iron garden railing. Explain why the choices differ.
Show Answer
Grease suits the moving chain because it coats the surface while allowing movement. Paint or plastic coating suits the railing because it can form a lasting outer layer. Each barrier prevents oxygen and water from reaching the iron.
8. Quiz
Use Check your understanding, then practise periodic patterns. Before moving on, reconstruct the two group tables, the full metal order and the two conditions needed for rusting.