The Elements of Groups 1 and 2

Full Notes

Ionisation Energy down Group 2

(First) Ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms.

First ionisation energy decreases down the group:

• This is because more electron shells increase shielding, reducing the attraction between the positively charged nucleus and outer electrons.
• The outermost electron is further from the nucleus, making it easier to remove.

This trend helps explain increasing reactivity down the group.

Reactivity Trend in Group 2

Reactivity increases down Group 2.

Group 2 metals lose two electrons to form M²⁺ ions in reactions.

• Lower ionisation energies down the group means electrons are more easily lost
• Reactions with water become more vigorous down the group, Mg to Ba

With Oxygen

Group 2 metals burn in oxygen to form white ionic oxides:

General equation:
2M + O₂ → 2MO

These oxides are basic and react with water to form hydroxides (see below).

With Chlorine

Group 2 metals react with chlorine to form metal chlorides that are white solids:

General equation:
M + Cl₂ → MCl₂

With Water

Group 2 metals react with water to produce a metal hydroxide and hydrogen gas:

General equation:
M + 2H₂O → M(OH)₂ + H₂

• Magnesium reacts slowly with cold water, faster with steam.
• Barium reacts rapidly even with cold water.
• Reactivity increases down the group.

Reactions of Group 2 Oxides and Hydroxides

You also need to know the reactions of group 2 oxides and hydroxides with water and dilute acids.

With Water

Group 2 metal oxides react with water to form metal hydroxides.

General Reaction:
MO + H₂O → M(OH)₂

• Group 2 Hydroxide solubility increases down the group (making the solution more alkaline, higher pH).

With Dilute Acids

Group II Oxides react with acids to form salt + water.

Group II hydroxides also react with acids to form salt + water.

Solubility of Hydroxides and Sulfates

The solubility of group 2 hydroxides and sulphates show opposite trends.

Hydroxide Solubility

Hydroxide Solubility increases down the group.

• Higher solubility means more OH^- ions dissolve = higher pH

Sulfate Solubility

Sulfate solubility decreases down the group.

• BaSO₄ is insoluble and forms a white precipitate.

Thermal Stability of Carbonates and Nitrates

Group 1 and 2 carbonates and nitrates undergo thermal decomposition:

Group II Metal Carbonates:
Decompose to metal oxide + CO₂ when heated

Group II Metal Nitrates:
Decompose to oxide + NO₂ + O₂

Group I Metal Nitrates:
Decompose to metal nitrite + O₂

• Group 2 nitrates decompose more readily than Group 1 nitrates
• Lithium Nitrate (LiNO₃) behaves more like the group II metal nitrates and decomposes to form lithium oxide
• 4LiNO₃ → 2Li₂O + 4NO₂ + O₂

Trend in thermal stability

Thermal stability increases down the group from Mg to Ba.

Explanation:

• Smaller cations (like Mg²⁺) have a higher charge density, which polarises the large nitrate (NO₃^-) or carbonate (CO₃^2-) anion more.

• This distorts the nitrate’s or carbonate’s electron cloud, weakening its bonds and making decomposition easier.
• As you go down the group, the cations get larger and their polarising ability decreases, meaning the anions remain more stable and require more heat to decompose.

Flame Colours of Group 1 and 2 Compounds

Group 1 and 2 compounds produce characteristic flame colours due to electron transitions:

• Heating excites outer electrons to higher energy levels
• As excited electrons return to a lower energy (ground state), light is emitted
• Different metal ions have different energy gaps between energy levels, meaning different wavelengths of light are emitted, producing unique colours depending on the metal.

Flame colours you need to know:

Experimental Procedures

You need to be able to describe how to determine the trend in thermal decomposition of group 2 nitrates and carbonates and carry out flame test analysis on group 1 and group 2 compounds.

Thermal Decomposition Patterns

Procedure: Heat a small sample of the carbonate or nitrate, gradually increasing the heat supplied. Bubble any gas produced through limewater to test for CO₂ (for carbonates).

Observations: Look for gas evolution, colour changes, and time taken for limewater to turn milky or any changes to observed.

Conclusion: Faster observational changes means a less thermally stable carbonate or nitrate compound.

Flame Tests for Group 1 and 2

To carry out a flame test:

• Use a platinum or nichrome wire loop
• Dip into concentrated hydrochloric acid and place in non-luminous Bunsen flame and make sure no colour is observed. Repeat if necessary.
• Dip clean wire loop into sample and then place back in non-luminous Bunsen flame.
• Observe flame colour

Summary

• First and Second Ionisation energy decreases down Group 2 due to increased shielding and distance from nucleus.
• Reactivity increases down Group 2 as electrons are lost more easily.
• Group 2 metals react with oxygen, chlorine and water to form oxides, chlorides and hydroxides.
• Group 2 oxides and hydroxides are basic and neutralise acids.
• Hydroxide solubility increases down Group 2, but sulfate solubility decreases.
• Carbonates and nitrates become more thermally stable down Groups 1 and 2.
• Flame tests show characteristic colours due to electron transitions.
• Thermal decomposition and flame test experiments help identify stability trends and metal ions.