Group 2

What is the name given to the elements in Group 2?

The alkaline earth metals.

What is the electron configuration of the outer shell of Group 2 elements?

They have two electrons in their outermost s subshell, giving a general outer configuration of ns².

What ion do Group 2 metals generally form?

2+ ions, by losing their two outermost electrons.

Why do Group 2 elements readily form 2+ ions?

They lose their two outermost s electrons, producing a stable noble-gas electron configuration.

Give the electron configurations of Be and Be²⁺.

Be: 1s² 2s²
Be²⁺: 1s²

Give the electron configurations of Mg and Mg²⁺.

Mg: 1s² 2s² 2p⁶ 3s²
Mg²⁺: 1s² 2s² 2p⁶

Give the electron configurations of Ca and Ca²⁺.

C

What happens to atomic radius down Group 2?

Atomic radius increases down the group.

Why does atomic radius increase down Group 2?

More electron shells are added down the group, so the outer electrons are in higher energy levels and are further from the nucleus.

How does electron shielding change down Group 2?

Electron shielding increases because more inner electron shells are added.

How does increased shielding affect the attraction between the nucleus and outer electrons?

Inner electrons shield the outer electrons from the attraction of the nucleus, reducing the effective nuclear attraction.

Give the full explanation for why atomic radius increases down Group 2.

More electron shells are added, so the outer electrons are further from the nucleus and there is increased electron shielding. This decreases the electrostatic attraction between the nucleus and outer electrons, so the atomic radius increases.

What happens to first ionisation energy down Group 2?

It decreases down the group.

What is first ionisation energy?

The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

What three factors affect first ionisation energy down Group 2?

Nuclear charge
Atomic radius
Electron shielding

How does nuclear charge change down Group 2?

It increases because the number of protons increases.

How does atomic radius affect first ionisation energy?

A larger atomic radius means the outer electron is further from the nucleus, so there is weaker electrostatic attraction and the electron is easier to remove.

How does electron shielding affect first ionisation energy?

Increased shielding means inner electrons reduce the attraction between the nucleus and the outer electron, making the outer electron easier to remove.

Why does first ionisation energy decrease down Group 2 despite increasing nuclear charge?

Although nuclear charge increases, atomic radius and electron shielding also increase. The increased distance and shielding have a greater effect, so the attraction between the nucleus and outer electron decreases. Therefore, less energy is required to remove the outer electron.

Give some first ionisation energy values for Group 2 elements.

Be = 900 kJ mol⁻¹
Mg = 738 kJ mol⁻¹
Ca = 590 kJ mol⁻¹
Sr = 547 kJ mol⁻¹

Why is it easier to remove the outer electron from Ba than Be?

Ba has more electron shells, so its outer electron is further from the nucleus and experiences greater shielding. This reduces the attraction between the nucleus and outer electron, so less energy is required to remove it.

What is the general trend in melting points down Group 2?

Melting points generally decrease down the group.

What type of structure do Group 2 metals have?

A giant metallic lattice containing positive metal ions surrounded by a sea of delocalised electrons.

What holds the particles together in metallic bonding?

Strong electrostatic attraction between positive metal ions and delocalised electrons.

Why does metallic bonding generally become weaker down Group 2?

The metal ions become larger, so the delocalised electrons are further from the positive nuclei. This weakens the electrostatic attraction between the metal ions and delocalised electrons.

Why does the melting point generally decrease down Group 2?

The ionic radius of the metal ions increases, so the delocalised electrons are further from the positive nuclei. This weakens the electrostatic attraction in the metallic lattice, meaning less energy is required to overcome the metallic bonding.

What is the anomaly in the Group 2 melting-point trend?

Magnesium has an unexpectedly high melting point compared with the general trend.

Why does magnesium have an unusually high melting point?

Magnesium forms an unusually stable crystal structure, giving it a higher melting point than expected.

How does the reactivity of Group 2 metals with water change down the group?

Reactivity increases down the group.

What is the general equation for a Group 2 metal reacting with cold water?

M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)

What are the products when a Group 2 metal reacts with water?

A metal hydroxide and hydrogen gas.

Write the equation for calcium reacting with cold water.

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

What happens to the oxidation state of a Group 2 metal when it reacts with water?

It is oxidised from 0 to +2.

What happens to the reactivity of Group 2 metals with water down the group?

It becomes increasingly vigorous because it becomes easier to remove the two outer electrons.

How does the reaction with water change from Be to Ba?

Be: no reaction
Mg: very slow reaction
C

Why does reactivity increase down Group 2?

Atomic radius and electron shielding increase down the group, reducing the attraction between the nucleus and outer electrons. Therefore, the two outer electrons are easier to remove, so the metal reacts more readily.

What is formed when a Group 2 metal burns in oxygen?

A solid metal oxide.

What is the general equation for a Group 2 metal reacting with oxygen?

2M(s) + O₂(g) → 2MO(s)

Write the equation for calcium reacting with oxygen.

2Ca(s) + O₂(g) → 2CaO(s)

What happens to calcium's oxidation state when it reacts with oxygen?

Calcium is oxidised from 0 to +2.

What happens to oxygen's oxidation state when it reacts with calcium?

Oxygen is reduced from 0 to −2.

What happens to the solubility of Group 2 hydroxides down the group?

Solubility increases down the group.

What are the approximate solubilities of the Group 2 hydroxides?

Mg(OH)₂: sparingly soluble
Ca(OH)₂: moderately/sparingly soluble
Sr(OH)₂: soluble
Ba(OH)₂: soluble

Why is Mg(OH)₂ important in the Group 2 solubility trend?

It is sparingly soluble in water.

What happens to the alkalinity of Group 2 hydroxide solutions down the group?

It generally increases, because the hydroxides become more soluble and therefore produce more OH⁻ ions in solution.

What happens when a Group 2 oxide reacts with water?

It forms the corresponding metal hydroxide.

Write the general equation for a Group 2 oxide reacting with water.

MO(s) + H₂O(l) → M(OH)₂(aq)

Why are Group 2 oxides basic?

They react with water to form metal hydroxides, which produce OH⁻ ions and therefore alkaline solutions.

Give an example of a Group 2 oxide reacting with water.

BaO(s) + H₂O(l) → Ba²⁺(aq) + 2OH⁻(aq)

What is the exception to the general behaviour of Group 2 oxides with water?

MgO reacts slowly with water and has low solubility.

What happens to the solubility of Group 2 sulfates down the group?

Solubility decreases down the group.

What is the general sulfate solubility trend from Mg to Ba?

MgSO₄: soluble
CaSO₄: sparingly soluble
SrSO₄: insoluble
BaSO₄: insoluble

Which Group 2 sulfate is particularly important because it is insoluble?

Barium sulfate, BaSO₄.

What are the two opposite solubility trends for Group 2 hydroxides and sulfates?

Hydroxide solubility increases down the group.
Sulfate solubility decreases down the group.

Why are the different solubilities of Group 2 hydroxides and sulfates important?

They determine their practical uses, including medicines and changing soil pH.

What is magnesium hydroxide used for?

It is used as an antacid to neutralise excess stomach acid.

Why is Mg(OH)₂ suitable as an antacid?

It is a base that neutralises excess hydrochloric acid in the stomach.

Write the ionic equation for the neutralisation of stomach acid by Mg(OH)₂.

H⁺(aq) + OH⁻(aq) → H₂O(l)

What is calcium hydroxide also known as?

Slaked lime.

What is calcium hydroxide used for?

It is used in agriculture to neutralise acidic soils and increase soil pH.

Why is Ca(OH)₂ suitable for neutralising acidic soil?

It is an alkali that provides OH⁻ ions, which react with H⁺ ions in acidic soil to form water.

Write the ionic equation for the neutralisation of acidic soil by Ca(OH)₂.

H⁺(aq) + OH⁻(aq) → H₂O(l)

What is magnesium used for in the extraction of titanium?

Magnesium is used as a reducing agent to extract titanium from titanium(IV) chloride, TiCl₄.

What happens to TiCl₄ before it is reduced by magnesium?

TiO₂ is converted to TiCl₄, which is then purified by fractional distillation.

Write the equation for the reduction of TiCl₄ by magnesium.

TiCl₄ + 2Mg → Ti + 2MgCl₂

Why is magnesium suitable for extracting titanium?

Magnesium is sufficiently reactive to reduce Ti⁴⁺ to titanium metal, while magnesium itself is oxidised to Mg²⁺.

Why does sulfur dioxide need to be removed from flue gases?

SO₂ is an atmospheric pollutant that contributes to acid rain.

Which Group 2 compounds can be used to remove SO₂ from flue gases?

Calcium oxide (CaO) or calcium carbonate (CaCO₃).

What is produced when CaO reacts with SO₂?

Calcium sulfite, CaSO₃.

Write the equation for CaO reacting with SO₂.

CaO(s) + SO₂(g) → CaSO₃(s)

What is produced when CaCO₃ reacts with SO₂?

Calcium sulfite, CaSO₃, and carbon dioxide.

Write the equation for CaCO₃ reacting with SO₂.

CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)

What can happen to calcium sulfite after it is formed?

It can be oxidised to calcium sulfate, CaSO₄.

Write the equation for oxidation of CaSO₃ to CaSO₄.

CaSO₃(s) + ½O₂(g) → CaSO₄(s)

What reagent is used to test for sulfate ions?

Acidified barium chloride solution, BaCl₂.

What observation indicates sulfate ions are present?

A white precipitate of barium sulfate forms.

Write the ionic equation for the sulfate ion test.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why is barium chloride used to test for sulfate ions?

BaSO₄ is extremely insoluble, so sulfate ions react with Ba²⁺ ions to produce a visible white precipitate.

Why is the barium chloride solution acidified?

The acid removes/interferes with carbonate ions and other ions that could produce precipitates with Ba²⁺, reducing false positive results.

What colour is the precipitate formed when sulfate ions are present?

White.

What is barium sulfate used for?

It is used in medical imaging, particularly as a barium meal/barium swallow to help image the digestive tract using X-rays.

Why is BaSO₄ suitable for use in medical imaging?

BaSO₄ is very insoluble, so it remains in the digestive tract rather than dissolving and being absorbed into the bloodstream. It also absorbs X-rays, making the digestive tract visible.

Why is it important that BaSO₄ is insoluble when used medically?

Soluble barium compounds can be toxic, whereas insoluble BaSO₄ is not significantly absorbed into the bloodstream.

Why does reactivity increase down Group 2?

Atomic radius and electron shielding increase, reducing attraction between the nucleus and outer electrons. The outer electrons are therefore easier to remove, so reactivity increases.

How are the trends in atomic radius and first ionisation energy down Group 2 related?

Atomic radius increases, meaning the outer electron is further from the nucleus. Shielding also increases, so attraction to the outer electron decreases. Therefore, first ionisation energy decreases.

How are the trends in first ionisation energy and reactivity down Group 2 related?

First ionisation energy decreases, so the outer electrons require less energy to remove. Therefore, the metals become more reactive down the group.

How are the trends in hydroxide and sulfate solubility different?

Hydroxide solubility increases down Group 2, whereas sulfate solubility decreases.

What are the four Group 2 compounds you should particularly associate with their uses?

Mg(OH)₂ → antacid
Ca(OH)₂ → neutralises acidic soil
CaO/CaCO₃ → removes SO₂ from flue gases
BaSO₄ → medical imaging

What are the three major trends you need to know for Group 2 elements Mg–Ba?

Atomic radius increases
First ionisation energy decreases
Melting point generally decreases

What is the key explanation linking atomic radius, ionisation energy and reactivity down Group 2?

Down the group, more electron shells are added, increasing atomic radius and electron shielding. This reduces the attraction between the nucleus and outer electrons, so first ionisation energy decreases and the outer electrons are easier to remove. Therefore, reactivity increases