Physical & Chemical Trends in Group 2 (Cambridge (CIE) AS Chemistry)
Revision Note
Physical & Chemical Trends in Group 2
Chemical trends
All elements in Group 2 (also called alkali earth metals) have the 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:
Graph of the first and second ionisation energies of the Group 2 elements
The graph shows that both the first and second ionisation energies decrease going down the table
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 on the nucleus 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 the 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 the 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
The atomic radius of the Group 2 elements increases going down the group due to the addition of an extra principal quantum shell
Graph of the atomic radius descending Group 2
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
Graph of the melting points descending Group 2
The graph shows a general decrease in melting point going down the group
As you go down the group, the density of the alkali earth metals increases
Graph of the density descending Group 2
The graph shows an overall increase in density going down the group
Worked Example
Predicting properties of radium
Radium, Ra, is a radioactive element found below barium, at the bottom of Group 2.
Applying your knowledge of the Group 2 elements, predict:
The formula of the ion formed by radium.
The formula radium's oxide and hydroxide.
The first ionisation energy of radium.
The reactivity of radium compared to barium.
The relative pH of a saturated radium hydroxide solution compared with a saturated calcium hydroxide solution.
The solubility of radium sulfate compared with strontium sulfate.
The equation for the reaction of solid radium oxide with dilute hydrochloric acid.
What you would expect to see if you mixed radium hydroxide solution with dilute sulfuric acid.
Answer
Answer 1:
Since Ra is in Group 2, it will form an ion with a +2 charge to give Ra2+
Answer 2:
The Group 2 oxides and hydroxides have the general formula XO and X(OH)2 respectively, where X is the Group 2 element
Therefore, radium oxide is RaO and radium hydroxide is Ra(OH)2
Answer 3:
Radium is below barium so its atomic radius is larger than the atomic radius of barium.
This means that radium’s outermost electrons are even further away and are therefore even more easily removed than barium’s outermost electron pair.
The first ionisation energy is between 450 - 480 kJ mol-1
Answer 4:
Radium’s outermost electrons are even further away than in barium and are therefore more easily removed making radium more reactive than barium
Answer 5:
The Group 2 hydroxides become more soluble going down the group
Radium hydroxide will therefore be more soluble than calcium hydroxide
Answer 6:
The Group 2 sulfates become less soluble going down the group
Radium sulfate will therefore be less soluble than strontium sulfate
Answer 7:
The general equation for the reaction of Group 2 oxides with dilute hydrochloric acid is:
XO (s) + 2HCl (aq) → XCl2 (aq) + H2O (l)
where X is the Group 2 element
The reaction of radium oxide with dilute hydrochloric acid is therefore:
RaO (s) + 2HCl (aq) → RaCl2 (aq) + H2O (l)
Answer 8:
Radium sulfate will be formed in this reaction
However, the solubility of Group 2 sulfates decreases going down the group
Therefore, a white precipitate of radium sulfate will be formed in this reaction
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?