Trends in Group 2: The Alkaline Earth Metals (AQA A Level Chemistry): Revision Note

Group 2: Trends

Chemical trends

• All elements in Group 2, also known as the alkaline earth metals, have two electrons in their outer shell

• Group 2 metals form ionic compounds by losing these two outer electrons to form M²⁺ ions

  • In doing so, they act as reducing agents because they are oxidised (they lose electrons)

• Reactivity increases down the group

• This trend can be explained by considering ionisation energies:

  • Down the group, the outer electrons are further from the nucleus and experience greater shielding from inner shells

  • As a result, there is a weaker attraction between the nucleus and the outer electrons, so less energy is required to remove them

• The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

• The second ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

• The data show that both the first and second ionisation energies decrease down Group 2, meaning it becomes easier to remove the two outer electrons

• Although nuclear charge increases down the group (due to an increase in proton number), the outer electrons are further from the nucleus and experience greater shielding from additional inner shells

  • These factors reduce the nuclear attraction more than the increased nuclear charge increases it

• As a result, Group 2 elements become more reactive down the group because it becomes easier for the atoms to lose two electrons and form 2⁺ ions

• This trend can be observed in their reactions:

  • With dilute hydrochloric acid, the rate of hydrogen gas production increases down the group, indicating more vigorous reactions

  • With oxygen, reactivity also increases down the group

  • For example, barium is sufficiently reactive that it must be stored under oil to prevent reaction with oxygen in the air

Physical trends

• Down Group 2, the atomic radius increases

• This is because each successive element has an additional electron shell

  • The outer two electrons are therefore further from the nucleus and experience increased shielding, resulting in a larger atomic size

• The melting point of Group 2 elements generally decreases down the group

• As the atomic radius increases, the positive metal ions become larger, and the outer electrons are further from the nucleus

  • This weakens the electrostatic attraction between the positive ions and the delocalised electrons in the metallic lattice, resulting in weaker metallic bonding and therefore lower melting points

• As you go down Group 2, the density of the alkaline earth metals does not follow a perfectly regular trend

  • It decreases from beryllium to calcium, and then increases from calcium to barium

• Density depends on both atomic mass and atomic radius

  • Although the atomic radius increases down the group, the mass of the atoms also increases significantly

    In addition, differences in metallic packing structures influence density, so the trend is not perfectly smooth