Group 2; The Alkaline Earth Metals

Question 1

How does atomic radius differ down the group?

Answer 1

Atomic radii increases going down the group:

• an electron shell is added per period down; each element has an extra filled main level of electrons compared with the one above it.

Question 2

What is the trend in first ionisation energy?

Answer 2

Ionisation energy decreases down the group:

• greater shielding of the outermost electrons from the nuclear force
• electrons are further away from the nuclear force; extra electron shell compared to the one above

These factor make it easier to remove outer electrons, hence a lower ionisation energy.

The metals are more reactive as we go down the group.

(Nuclear charge does increase down the group with more protons per period however the effect is overridden by the greater no. of shellls)

Question 3

What is the trend in melting points?

Answer 3

Melting point decreases down the group:

• delocalised electrons are further away from cations (+)
• more spread out; less energy required to break metallic bonds; smaller charge/volume ratio.
• free electrons are further from the nuclei as size increases.
• weaker attraction between cations and delocalised electrons
• weaker metallic bonding

(excepting Mg due to crystal lattice structure)

Question 4

What occurs when the elements Mg-Ba react with water?

What is the trend?

What is the use of Mg(OH)₂ and Ca(OH)₂?

Answer 4

Metals get more reactive down the group; reactivity with water increases down the group.
Where M is any Group 2 metal:

M(s) + 2H₂O(l) → M(OH)₂ (aq) + H₂ (g)
Metal hydroxide

e.g. (oxidised from 0 to +2 forming Ca²⁺ ions)

0 +2
Ca + 2H₂O(l) → Ca(OH)₂ (aq) + H₂ (g)

Ca(OH)₂ is called ‘slaked lime’. It is used in agriculture to treat acidic soil through a neutralisation reaction.

Mg reacts differently; this happens with steam; rapid (because Mg(OH)₂ decomposes to MgO):

Mg(s) + H₂O(g) → MgO (aq) + H₂ (g)

Mg(OH)₂ reactioncan occur but is is slow; sparingly soluble; but magnesium hydroxide (milk of magnesia) is used in indigestion remedies to neutralise excess stomach acid; as an antacid.

Question 5

What are the solubilities of the hydroxides of Mg-Ba?

Answer 5

• Hydroxides become more soluble down the group.
• They are all white solids
• Mg(OH)₂ - milk of magnesia, used to treat indigestion caused by excess acid (neutralistion of H⁺ + OH^- → H₂O) – sold as a suspension in water
• Compounds like Mg(OH)₂ which have very low solubilities are said to be sparingly soluble.
• Ba(OH)₂ dissolves to produce a strongly alkaline solution:

Ba(OH)₂ + aq → Ba2⁺(aq) + 2OH^-(aq)

Question 6

What are the solubilities of the sulfates of the elements Mg - Ba?

Answer 6

• Sulfates become less soluble down the group.
• Magnesium is the most soluble; Barium the least. Hence its use as a barium meal.

Question 7

What is the test for sulfate ions? Why is it used?

Answer 7

• Acidified (nitric/hydrcochloric acid added) BaCl₂ solution is used.
• Acid rids of any sulfites/carbonates (carbonate ions rid of as CO₂) that may give an undesired precipitate; barium carbonate is also a white insoluble solid.
• When BaCl₂ is added to something with sulfate ions, a white precipitate is formed.

Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

Question 8

What is barium sulfate used for?

Answer 8

BaSO₄ is used as a barium meal to outline the gut in medical x-rays – the compound is toxic but it is virtually insoluble; safe – passes out of the body.

X-rays don’t pass through barium sulfate; it is opaque to them. Helps diagnose conditions, cancers etc.