In this section, we look at the patterns across period 3, Na to Ar. There are several trends that you need to know and explain.
- Atomic radius – Na is the largest despite having least mass, this is because as you add a proton, the nuclear charge and attraction increases so the force on the outer electron increases so it is pulled closer in.
- 1st Ionisation energy – Following on from the above reasons, the outer electron is closer to the nucleus so more energy will be required to liberate it. Hence, more energy needed to ionise Chlorine than Sodium.
- Electronegativity – Once again, the closer the outer electrons and so the bonding electrons in a covalent bond, the more pull is exerted on them from the nucleus. This higher pull makes them more electronegative so Chlorine is the most electronegative and Sodium the least.
This section appears to be about learning facts and trends again, however, the real skill is in applying these facts and explaining another situation or observation. Learn the trends but you need to be able to spot when an observation is due to variances in group 2 metals.
- Atomic radius – This increases as you descend the group. This is because, each successive element has an extra energy level.
- 1st Ionisation energy – This decreases as you descend the group as the outer electron is further from the nucleus and there is an increase in shielding due to the extra electron shells.
- Melting points – The melting point decreases as you descend the group. The ions all have a 2+ charge but the ions are much larger at the bottom so they have a lower charge density so less energy is needed to overcome the attraction.
- Solubility of Sulfates – Solubility decreases as you descend the group. BaSO4 is insoluble, MgSO4 is soluble. CaSO4 is sparingly soluble.
- Solubility of Hydroxides – Solubility increases as you descend the group. Ba(OH)2 is insoluble, Mg(OH)2 is soluble. Ca(OH)2 is sparingly soluble. Milk of magnesia is used as an antacid, it will not dissolve and raise the pH over 7, it simply neutralises acid present. BaSO4 is the ingredient in barium meals, these have to be insoluble to prevent the poisoning of patients. BaCl2(aq) is the test for sulfates in water, it will produce a white precipitate as a positive result as it forms BaSO4 (S).
Carrying on in this section, we once again need to learn some trends AND be able to understand the reasons enough to APPLY this knowledge to situations that you are likely not to have met in lessons.
- Physical appearance at RTP – F2 is a pale yellow gas, Cl2 is a pale green gas, Br2 is a brown liquid and I2 is a grey solid that sublimes into a purple vapour.
- Most have uses as antiseptics, particularly Chlorine and Iodine.
- As you go down the group, the atomic radius increases as there is an extra energy level for each period. This is why the top of the group is most reactive and the strongest oxidising agent.
- The first ionisation energy increases as you go down the group as the outer electron is further from the nucleus and there is much more shielding as you go further down.
- The trend in reducing power of the halide ions increases as you go down the group.
- Cl– does not reduce the sulfur in H2SO4 at all, there is simply an acid-base reaction in which HCl is made.
- Br– ions reduce the sulfur in H2SO4 from 6+ to 4+ in the form of SO2.
- I– ions reduce the sulfur in H2SO4 from 6+ to 4+ in the form of SO2, then to 0 in the form of elemental S and to 2- in the form of H2S.
Practical 4 – Testing for anions: Cl–, Br–, I–, OH–, CO32- & SO42- and cations from group 2 metals and NH4+. You may test for other ions just to expand your experience but the ones listed are crucial. As well as conducting these tests, you must learn the methods, the reagents involved and the positive results.