Why do Group 2 carbonates become more stable when heated as we move from magnesium to barium? All four metals form M²⁺ ions, but the ions become larger down Group 2. Small Mg²⁺ has a greater charge density and polarises the carbonate ion more strongly, distorting its electron cloud and making decomposition easier. Large Ba²⁺ polarises CO₃²⁻ less strongly, so its carbonate is more thermally stable. The familiar decomposition is MCO₃(s) → MO(s) + CO₂(g). The change is not caused by the metal ions having different charges: all remain +2.
For Hougang families searching for JC H2 Chemistry tuition, Group 2 carbonate decomposition, cation polarisation, Periodic Table trends, SEAB 9476 revision or a Chemistry tutor, the quickest way to check the student’s understanding is to ask three questions. Which property remains constant? The +2 charge. Which property changes? Ionic radius. What does that change cause? A different ability to polarise the carbonate anion, leading to different thermal stabilities. This guide includes a balanced-equation calculation, three explanation examples, an examination-error checklist, the official 2027 syllabus and connected eduKate Science resources.
Group 2 Carbonates: Make Ionic Polarisation Explain the Stability Trend
One property is unchanged while another is changing
Magnesium, calcium, strontium and barium commonly form Group 2 cations with a +2 charge. Moving down the group, the ions become larger. Because the positive charge stays the same while radius increases, charge density decreases. A small, highly charge-dense cation has a stronger polarising effect on a nearby carbonate anion than a larger +2 ion. If the learner says ‘barium carbonate is stable because barium has a larger charge’, they have chosen the wrong variable. The charge does not increase down the group; ionic radius and consequent charge density change.
Why anion polarisation belongs in the explanation
The carbonate ion has a distribution of electron density that can be distorted by a nearby cation. The smaller, more charge-dense Mg²⁺ exerts greater polarising influence than the larger Ba²⁺. In the familiar H2 model, greater polarisation destabilises the carbonate against its thermal decomposition into the corresponding oxide and carbon dioxide. Thus magnesium carbonate generally decomposes more readily than barium carbonate under comparable heating conditions. The trend is a structure-and-electrostatics explanation, not an unsupported assertion that carbon dioxide is a weaker gas near magnesium.
Keep thermodynamic stability separate from reaction speed
Thermal stability describes the relative resistance to decomposition at higher temperatures; observed decomposition in a real sample also depends on kinetics, heating, particle size and equipment conditions. A small quantity of barium carbonate that appears unchanged during a short demonstration does not by itself prove a specific decomposition temperature. A proper school question should use its stated information and standard trend, while live heating of metal carbonate powders must remain in suitably equipped supervised laboratories. Some barium compounds are hazardous.
Worked equation: the ion count survives decomposition
The idealised thermal decomposition equation is MCO₃(s) → MO(s) + CO₂(g) for the relevant Group 2 metal. Taking magnesium carbonate: MgCO₃(s) → MgO(s) + CO₂(g). There is one magnesium atom and one carbon atom on either side, while three oxygen atoms appear as one in MgO plus two in CO₂. This simple conservation check helps a learner connect a named stability trend with actual substances. Greater stability affects the conditions required, not the one-to-one theoretical stoichiometry after complete decomposition.
Worked amount: the same mole ratio despite different stabilities
With school relative atomic masses Mg = 24, C = 12 and O = 16, the formula mass of MgCO₃ is 84. If a question assumes 4.20 g of pure magnesium carbonate decomposes completely, that is 0.0500 mol MgCO₃ and can produce 0.0500 mol CO₂. Under an explicitly stated school RTP convention of 24 dm³ mol⁻¹, the theoretical gas volume is 1.20 dm³. The result says nothing about how fast this complete conversion occurs. If only partial decomposition took place, a measured gas amount could be lower.
An unfamiliar carbonate comparison is the decisive test
Give two Group 2 cations of the same +2 charge but different radii and ask which carbonate is expected to be more thermally stable. The answer must connect ion size → charge density → carbonate polarisation → susceptibility to decomposition. Then ask whether the smaller or larger carbonate formula produces a different number of CO₂ molecules per formula unit after complete decomposition. This changed question checks whether the learner is using a particle model or merely reciting a trend arrow from the Periodic Table.
How the Secondary-to-JC bridge should be taught
At Secondary Chemistry level, a learner may first meet metal carbonates as salts that can thermally decompose under appropriate conditions. In H2 Chemistry, the explanation adds ionic polarisation and a more detailed periodic trend. A useful three-student tutorial starts by checking ion formulae, balanced decomposition equations and electron arrangement, then asks each student to explain a different comparison without hints. Advanced polarisation reasoning should not replace missing foundational charge-balance or mole-ratio skills.
Distinct reference owner and onward JC learning
This page owns Group 2 carbonate thermal stability through cation polarisation. The Secondary 4 thermal decomposition guide explains the earlier observable reaction. For more advanced Periodic Table trends, read Period 3 oxides and chlorides. The JC H2 Chemistry hero and Hougang Chemistry hub own the broader tuition and syllabus path.
Group 2 Carbonates: Five Examination Checks Beyond the Trend Arrow
Check 1 — an ion’s charge and charge density are not synonyms
A student writes: ‘The Group 2 metals become more stable because the charge on their cations decreases.’ That is incorrect: the cations in the familiar set are Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺. Their formal charge remains +2. The relevant comparison is ionic radius: it increases down the group, lowering the charge density. Ask which variable the examiner actually wants compared before selecting the polarisation explanation. If a learner memorises ‘less polarisation’ without naming the reason, the response is incomplete.
Check 2 — the more stable salt still gives CO₂ when fully decomposed
Suppose a question asks whether one mole of magnesium carbonate yields more carbon dioxide after complete thermal decomposition than one mole of barium carbonate. Under the usual equation MCO₃(s) → MO(s) + CO₂(g), each mole of carbonate gives one mole of CO₂ if decomposition is complete. Thermal stability determines how readily decomposition occurs under specified heating conditions; it does not change this stoichiometric 1:1 relationship. A correct response can therefore state both ‘BaCO₃ is more thermally stable’ and ‘the theoretical mole ratio is still 1:1’ without contradiction.
Check 3 — changing the mass is not the same as changing the number of moles
For 8.40 g of MgCO₃ with Mr 84, the amount is 0.100 mol. For a hypothetical 8.40 g sample of BaCO₃ with a much greater molar mass, the number of moles is smaller; equal gram masses need not produce equal theoretical CO₂ quantities after complete decomposition. The learner should compute each formula mass before comparing. The trend in thermal stability is a separate question from formula mass. Ask which quantity is supplied and which is sought rather than repeatedly using one memory rule.
Check 4 — the Group 2 comparison is not a solubility question
A solid carbonate’s decomposition on heating and its solubility in water concern different physical and chemical relationships. A student cannot infer carbonate solubility trends simply by reversing the thermal stability order. Nor should they use the trend in hydroxide solubility as though it were the same thing. Begin by identifying the stated substance and property: MCO₃ thermal decomposition, M(OH)₂ dissolution, or a different salt altogether. The correct model depends on the specified phenomenon, not merely the periodic group name.
Check 5 — compare with Period 3 oxide classifications without confusing them
Group 2 carbonate thermal stability is explained at H2 level using cation size and polarising power. Period 3 oxide questions often focus on bonding, acid–base character and reaction with water or acid/alkali. Both topics involve ions and periodic trends, but they own different questions. A tutor can ask a student to classify MgO as a basic oxide, then separately explain why MgCO₃ decomposes more readily than CaCO₃. The first answer does not automatically prove the second. Linking the two ideas is useful only if their chemical jobs remain distinct.
A six-line model answer that is concise but complete
A suitable response might read: ‘Group 2 carbonate stability generally increases down the group. All the metal cations carry +2 charge, but ionic radius increases. Consequently the cation charge density decreases. The larger cation polarises CO₃²⁻ less strongly. The carbonate ion is less destabilised toward decomposition, so a higher temperature is generally required to decompose the carbonate under comparable conditions.’ A learner who can generate that chain from an unfamiliar pair without prompts has learned more than a copied table.
Safety and evidence remain part of the lesson
Heating carbonate salts and working with metal compounds must not be turned into an improvised household experiment. Some relevant substances have toxicity or dust hazards, and suitable laboratory supervision, equipment and procedures are essential. For tuition, use provided school observations and exam data, rather than claiming the group runs thermal-decomposition experiments onsite. The teacher can diagnose reasoning with diagrams, balanced equations and changed comparisons while following the student’s actual H2 examination syllabus.
A Secondary Chemistry fact becomes a JC explanation
Students learn early that calcium carbonate can produce carbon dioxide when heated sufficiently. At JC, Chemistry asks why closely related carbonates behave differently under similar heating conditions.
The earlier reaction is not discarded. Instead, the periodic table supplies a more precise model of the metal ions and how they affect their surrounding anions.
The best way to learn this chapter is to start with what is common to the compounds, then identify what differs. That is a powerful technique beyond Group 2: controlled comparisons let students isolate the chemical variable responsible for an observation.
When a child can reconstruct the trend from ionic structure rather than memorising a list, the explanation is more likely to survive an unfamiliar examination question.
The four familiar Group 2 carbonates
The relevant compounds are MgCO₃, CaCO₃, SrCO₃ and BaCO₃.
Each consists of M²⁺ metal ions and CO₃²⁻ carbonate ions. A 1:1 ratio makes a neutral ionic formula unit.
If a learner writes Mg₂CO₃ or Ba(CO₃)₂, the underlying issue is ionic charge balancing, not yet thermal stability. Repair that foundation before moving on.
The useful comparison is that the carbonate anion is the same species throughout, while the cation changes from Mg²⁺ to Ca²⁺ to Sr²⁺ to Ba²⁺.
The charge does not change. The ionic radius does.
Ionic radius and charge density
Down Group 2, the number of occupied electron shells increases. The familiar ionic radii increase from Mg²⁺ to Ba²⁺.
The +2 charge of Mg²⁺ is concentrated into a smaller region than the same +2 charge of Ba²⁺. This is the meaning of higher charge density in the qualitative school model.
A small, highly charged ion can exert a strong polarising influence on a nearby anion. It attracts and distorts the anion’s electron cloud.
In Group 2, Mg²⁺ has the greatest polarising power of these common examples; Ba²⁺ has the least.
The student should not claim that Ba²⁺ is less positively charged. Its charge is still +2. It has lower charge density because its ionic radius is larger.
What polarisation does to carbonate
The carbonate anion contains an electron cloud distributed over its carbon–oxygen framework. A neighbouring positive cation attracts that cloud.
The stronger the polarising power of the cation, the more the carbonate’s electron distribution can be distorted.
In the syllabus’s qualitative explanation, strong polarisation of CO₃²⁻ by a small cation makes the carbonate more readily decomposed on heating.
Conversely, the large Ba²⁺ ion distorts CO₃²⁻ less strongly, and BaCO₃ is more resistant to thermal decomposition.
The complete reasoning chain is ion size → charge density → carbonate polarisation → thermal stability. Omitting the last two links makes the explanation incomplete.
Worked example 1: write the equation
For magnesium carbonate:
MgCO₃(s) → MgO(s) + CO₂(g).
For calcium carbonate:
CaCO₃(s) → CaO(s) + CO₂(g).
For barium carbonate:
BaCO₃(s) → BaO(s) + CO₂(g).
Count the atoms. There is one metal, one carbon and three oxygen atoms on each side.
Each equation has the same 1:1 ratio of carbonate to carbon dioxide when the decomposition occurs. But the ease of decomposition is different.
Students should not confuse identical stoichiometry with identical stability, reaction temperature or reaction rate.
Worked example 2: rank three carbonates
Rank MgCO₃, SrCO₃ and BaCO₃ in increasing thermal stability.
The order is:
MgCO₃ < SrCO₃ < BaCO₃.
Explain it in full. The metal-ion radii increase in that sequence while the ionic charges remain +2. The cation charge density and polarising power decrease. The carbonate ion is progressively less distorted.
Therefore the carbonate becomes more thermally stable.
A response saying only “barium is heavier” misses the particle explanation required by SEAB. Atomic mass is not the relevant causal explanation of this trend.
Worked example 3: compare MgCO₃ and CaCO₃
Why is magnesium carbonate less thermally stable than calcium carbonate?
A good answer is:
Mg²⁺ is smaller than Ca²⁺, although both carry a +2 charge. Mg²⁺ therefore has greater charge density and polarises CO₃²⁻ more strongly, making MgCO₃ more susceptible to thermal decomposition.
This answer works because each sentence adds the next link in the causal chain.
A weaker response says, “Magnesium is above calcium so its carbonate decomposes first.” That describes a remembered ordering, not the mechanism explaining it.
A well-designed lesson should ask the learner to apply the same reasoning to a new pair such as SrCO₃ and BaCO₃ without hints.
Worked example 4: calculate the carbon dioxide volume
Suppose an examination question says 10.0 g of pure CaCO₃ decomposes completely.
Using the supplied relative atomic masses Ca = 40.1, C = 12.0 and O = 16.0, the molar mass is 100.1 g mol⁻¹.
Amount CaCO₃ = 10.0 ÷ 100.1 ≈ 0.0999 mol.
The balanced equation gives one mole CO₂ per mole CaCO₃, so the theoretical CO₂ amount is 0.0999 mol.
If the question provides a molar gas volume of 24.0 dm³ mol⁻¹ at RTP, the CO₂ volume is approximately 2.40 dm³.
That result describes a theoretical complete decomposition and gas-volume assumption. It does not give the temperature needed to decompose the carbonate or the speed of its decomposition.
Stability is not the same thing as rate
“Less thermally stable” does not mean that every possible magnesium carbonate sample will lose mass faster than any possible barium carbonate sample under arbitrary conditions.
Particle size, heating profile, heat transfer and apparatus affect how quickly an actual sample responds. Thermal stability concerns its relative resistance to decomposition, while reaction rate concerns the speed observed under defined conditions.
A student who gives a kinetic explanation in place of an ionic polarisation explanation is answering a different question.
For the distinction, see H2 Initial Rates and Reaction Kinetics.
Why the simple lattice-energy slogan is not enough
Students sometimes remember that smaller ions produce stronger electrostatic attractions and predict that the magnesium carbonate lattice should be the most stable.
But thermal decomposition transforms the carbonate into an oxide and carbon dioxide; the energetic and structural comparison involves both the reactants and products.
The 2027 syllabus specifically frames this trend using cation charge density and the polarisability of the carbonate anion. Repeating “stronger ionic bond” without addressing carbonate distortion is not the requested argument.
More advanced thermodynamic comparisons can use enthalpy cycles, but these do not eliminate the need to explain the specific periodicity trend.
The Hess’s Law and Born–Haber Cycles Guide gives that separate framework.
A connected idea: Group 2 metals as reducing agents
Another syllabus objective compares Group 2 metals’ relative tendency to act as reducing agents.
A metal is oxidised when it loses two electrons:
M(s) → M²⁺(aq) + 2e⁻.
The thermodynamic reducing tendencies can be compared using the relevant standard electrode potentials, with the qualifications about aqueous ion formation and standard conditions.
That is not the same process as decomposing MCO₃. One question concerns electron donation by the metal; the other concerns the thermal stability of an ionic compound.
The Hougang Standard Electrode Potentials Guide helps students connect without confusing these two aspects of periodicity.
Why carbonate Chemistry matters outside the examination
Limestone and other carbonate minerals appear in industrial materials, construction and chemical processing. When calcium carbonate is thermally converted to calcium oxide, carbon dioxide is released.
That brings energy and emissions into the story. The mass and energy requirements of a manufacturing process can be large even when its balanced equation looks simple.
The surrounding Hougang landscape contains buildings and infrastructure, but this general Chemistry article does not pretend to know the composition or supply chain of any particular building.
Read How Materials Age in Hougang to explore how materials behave in the real environment and How Chemistry Works in Hougang for a wider town-level framework.
Six mistakes to find before an exam
- Saying the charge on Group 2 ions changes from +2 to +1 down the group.
- Reversing ionic sizes and writing Ba²⁺ is smaller than Mg²⁺.
- Forgetting to mention carbonate polarisation in a thermal-stability explanation.
- Using atomic mass alone as the reason for the trend.
- Mistaking the ease of decomposition for the stoichiometric amount of CO₂ formed.
- Writing an unbalanced MCO₃ decomposition equation.
The first faulty step matters more than the number of marks lost. A tutor can target the error rather than demand another full set of memorised notes.
SEAB 2027 H2 Chemistry scope
The official 2027 H2 Chemistry 9476 syllabus includes Group 2 from magnesium to barium and specifically requires the explanation of carbonate thermal stability in terms of cation charge density and polarisability of the large anion.
The SEAB 2027 A-Level subject directory distinguishes H2 Chemistry 9476 from H1 Chemistry 8873. Use the correct course code and examination year when selecting advanced practice material.
Students need a reliable mechanism first, before moving to exceptional or industrially complex chemistry.
A seven-day revision plan
Day 1: balance the decomposition equations for the four familiar Group 2 carbonates.
Day 2: compare Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ radii without changing their +2 charges.
Day 3: explain charge density using the smaller-ion, stronger-polarisation connection.
Day 4: apply that explanation to a pair of carbonates and practise the complete causal chain.
Day 5: use a new comparison to test whether the concept transfers without notes.
Day 6: solve a mole and gas-volume calculation and keep it distinct from the stability discussion.
Day 7: complete a mixed periodicity question, then identify the first incorrect idea behind any error.
A short, structured sequence often creates more stable understanding than repeating the same explanation all afternoon.
What good Hougang JC Chemistry tutoring should do
One child may remember the ranking but reverse the radii. Another may know charge density but not carbonate polarisation. Another may explain the chemistry well but make a conversion mistake in the numerical question.
A thoughtfully managed three-student small-group tutorial allows the tutor to inspect each learner’s reasoning, repair the first broken link and check the correction using an unfamiliar problem.
For programme and syllabus planning, use Hougang JC Chemistry Tuition Guide, H1/H2 Syllabus Library and Hougang Science Learning Hub. Confirm actual lesson venue, subject fit and available timetable directly rather than assuming an area-based learning guide lists a physical Hougang classroom.
Frequently asked questions
Why is BaCO₃ more stable than MgCO₃?
Ba²⁺ is larger, with lower charge density, so it distorts the carbonate ion less strongly.
Do Group 2 cations have different charges?
The common metal ions in these carbonates are all +2. Their sizes and polarising powers differ.
What is the decomposition equation?
MCO₃(s) → MO(s) + CO₂(g), under appropriate heating conditions.
Does the most stable carbonate produce less CO₂ after complete decomposition?
Not on a per-mole stoichiometric basis. One mole of a typical Group 2 carbonate gives one mole of carbon dioxide if fully decomposed.
What should I study next?
Continue with Successive Ionisation Energies, Thermochemistry and eduKate Sengkang Science Hub. The goal is to rebuild the stability trend from particle structure, not recite a list blindly.