AQA A Level Chemistry复习笔记2.2.1 Trends in Group 2: The Alkaline Earth Metals

Chemical trends

• All elements in Group 2 (also called alkali earth metals) have two electrons in their outermost principal quantum shell
• All Group 2 metals can form ionic compounds in which they donate these two outermost electrons (so they act as reducing agents) to become an ion with +2 charge (so they themselves become oxidised)
• Going down the group, the metals become more reactive
• This can be explained by looking at the Group 2 ionisation energies:

The graph shows that both the first and second ionization energies decrease going down the group

• The first ionisation energy is the energy needed to remove the first outer electron of an atom
• The second ionisation energy is the energy needed to remove the second outer electron of an atom
• The graph above shows that going down the group, it becomes easier to remove the outer two electrons of the metals
• Though the nuclear charge increases going down the group (because there are more protons), factors such as an increased shielding effect and a larger distance between the outermost electrons and nucleus outweigh the attraction of the higher nuclear charge
• As a result of this, the elements become more reactive going down the group as it gets easier for the atoms to lose two electrons and become 2+ ions
• This trend is shown by looking at reactions of the Group 2 metals:
  • With dilute hydrochloric acid: bubbles of hydrogen gas are given off much faster indicating that the reactions become more vigorous
  • With oxygen: the metals get more reactive with oxygen down the group (Ba is so reactive, that it must be stored in oil to prevent it from reacting with oxygen in air)

   

Physical trends

• Going down the group, the elements become larger as the outer two electrons occupy a new principal quantum shell which is further away from the nucleus

The atomic radius of the Group 2 elements increases going down the group due to the addition of an extra principal quantum shell

The graph shows a increase in atomic radius going down the group

• The melting point of the elements decreases going down the group as the outer electrons get further away from the nucleus
• This means that the attraction between the nucleus and the bonding electrons decreases causing a decrease in melting point

The graph shows a general decrease in melting point going down the group with a slight anomaly in magnesium

• As you go down the group, the density of the alkali earth metals drops and then increases
• Density is also affected by the packing structure of the metals, not just the atomic radius - no trend is perfect!

The graph show the broad trend in density going down the group