Describe Group 2 physical and chemical trends
Use the Mg–Ba sequence to describe shared Group 2 properties and make cautious predictions from trends.
Section 10 · Group 2
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Group trends are qualitative unless data are supplied. Show physical states and balance equations; distinguish reaction from dissolution and precipitation. This sequence does not replace supervised practical experience. Written practice is self-checked
Before you begin
- Use Group 2 formulas, ionic charges, state symbols, balanced equations and acid reaction patterns.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What ion do Group 2 metals commonly form?
What you will learn
- Recognise the +2 ion pattern
- Describe radius and reactivity changes down Group 2
- Use supplied physical-property data without inventing a smooth trend
The same outer-shell pattern gives related chemistry
Magnesium, calcium, strontium and barium are Group 2 metals. Their atoms have two outer-shell electrons, and their common compounds contain M²⁺ ions. Formulae such as MO, M(OH)₂ and MCl₂ follow from the +2 charge paired with O²⁻, OH⁻ or Cl⁻.
Moving down Mg → Ca → Sr → Ba, atomic and M²⁺ ionic radii increase. The outer electrons are farther from the nucleus and more shielded, so they are removed more readily. The first ionisation energy generally falls and reactions become more vigorous down the group.
Describe measured physical patterns from the data
All four elements are metallic solids under ordinary conditions. For physical quantities such as density and melting point, read the values supplied in the question and describe the observed pattern; do not claim every value changes smoothly or in the same direction.
A prediction should name the trend and link it to the element's position. A down-group trend is qualitative: it does not supply an exact reaction rate, temperature or property value for a new element.
Pause and explain
How does the general reactivity of Group 2 metals change from Mg to Ba?
Worked example
Barium reacts more vigorously with water than magnesium. State the group trend that supports the comparison and one structural reason.
Show the worked solution
- Both metals are in Group 2 and form +2 ions.
- Reactivity increases down the group from Mg towards Ba.
- The outer electrons are farther from the nucleus and more shielded down the group, so they are lost more readily.
Answer Reactivity increases down Group 2; increased radius and shielding make the outer electrons easier to remove.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the formula of the oxide, chloride and hydroxide of a Group 2 element M.
Give me a hint
Pair M²⁺ with O²⁻, Cl⁻ and OH⁻, then make each compound neutral.
Compare my reasoning
- One M²⁺ balances one O²⁻, giving MO.
- Two Cl⁻ ions are needed for M²⁺, giving MCl₂.
- Two OH⁻ ions are needed for M²⁺, giving M(OH)₂.
MO, MCl₂ and M(OH)₂.
Look for these in your work
- M²⁺ used
- Each formula is charge-balanced
- Parentheses retained for two hydroxide groups
Arrange Mg, Ca, Sr and Ba in order of increasing ionic radius, then state the trend in reactivity.
Give me a hint
Follow the order down Group 2 and separate the size trend from chemical behavior.
Compare my reasoning
- The group order is Mg, Ca, Sr, Ba.
- Ionic radius increases down the group.
- Metal reactivity also generally increases down the group.
Ionic radius: Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺; reactivity increases down the group.
Look for these in your work
- Correct element order
- Ionic-radius direction correct
- Reactivity trend stated separately
A data table shows that melting point does not rise steadily from Mg to Ba. How should you report the pattern?
Give me a hint
Describe the actual values; use the general group trend only where the supplied evidence supports it.
Compare my reasoning
- Identify any rise, fall or local irregularity in the data.
- Do not replace the measured pattern with a smooth invented trend.
- Keep qualitative chemical reactivity distinct from melting point.
Describe the irregular melting-point pattern shown by the data; do not claim that every physical property increases or decreases smoothly down Group 2.
Look for these in your work
- Uses supplied data
- Acknowledges irregularity
- Separates physical and chemical trends
Common mistakes
- Writing M⁺ or M³⁺ for the usual Group 2 ion
- Claiming every physical property changes smoothly
- Using atomic radius alone as an exact reaction-rate calculation
- Confusing a qualitative trend with a guaranteed outcome in uncontrolled conditions
What can you explain now?
What ion do Group 2 metals commonly form?
Compare with the explanation
M²⁺
Each atom has two outer electrons and commonly loses both.
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Make it stick
What stays similar across Group 2, what changes down the group, and when must you rely on supplied physical data?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.