2.2 group 2

what are group 2 elements called?

the alkaline earth metals

what ion charge do group 2 elements form?

what oxidation state do group 2 elements have in compounds?

2+

+2

as 2 electrons are lost when they react, in order to achieve a full outer shell

explain the trend in atomic radius down group 2

• atomic radius increases down the group.

• increase in number of electron shells.

• more inner electrons, electrons repel each other.

describe the bonding in group 2 elements

• metallic bonding

• electrostatic attraction between giant lattice of 2+ ions

• and delocalised electrons

explain the trend in melting point down group 2

• melting point decreases.

• increase in ion size down the group.

• weaker electrostatic attraction between nucleus and delocalised electrons, so weaker metallic bonding.

explain why magnesium has a higher melting point than sodium

• Mg²⁺ has a higher ion charge than Na⁺

• Mg atoms are bigger

• Mg has more delocalised electrons

• so stronger metallic bonding

define first ionisation energy

the amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1⁺ ions

explain the trend in first ionisation energy down group 2

• first ionisation energy decreases

• increase in no. of electron shells down the group

• so increased shielding effect and increased atomic radius

• so weaker electrostatic attraction between nucleus and outer electron

• so outer electron is easier to remove

write an equation for the first ionisation energy of magnesium

Mg_(g) → Mg⁺_(g) + e^-

define second ionisation energy

the amount of energy required to remove one mole of electrons from one mole of gaseous 1⁺ ions to form one mole of gaseous 2⁺ ions

write an equation for the second ionisation energy of magnesium

Mg⁺_(g) → Mg²⁺_(g) + e^-

explain why the second ionisation energy is greater than the first ionisation energy

• after first ionisation energy, the atom has the same nuclear charge but one less electron

• so stronger electrostatic attraction between the nucleus and outer electrons (as the no. of protons outweighs the no. of electrons)

• so next outer electron becomes harder to remove

explain why the second ionisation energy of calcium is lower than the second ionisation energy of potassium

• Ca⁺ loses electron from 4s orbital whereas K⁺ loses electron from 3p orbital

  • (4s has a higher energy than 3p so outer electron is further from the nucleus in Ca⁺)

• more shielding in Ca⁺

explain the trend in reactivity of group 2 elements

• reactivity increases

• group 2 loses 2 electrons when they react

• increase in no. of electron shells down the group

• so increased shielding effect and increased atomic radius

• so weaker electrostatic attraction between nucleus and outer electrons

• so outer electrons are more easily lost

describe the trend in reactivity with liquid water down group 2

reaction gets more rapid/vigorous/violent down the group:

• Mg: very slowly

• Ca: quite rapid/fairly vigorously

• Sr: rapid

• Ba: violent

describe the reaction between group 2 elements and liquid water

• X _(s) + 2H₂O _(l) → X(OH)_{2 (aq)} + H_{2 (g)}

• redox reaction

• exothermic

• white precipitate produced

• metal hydroxide forms as an alkaline solution

• effervescence, as bubbles of hydrogen gas are produced