Group 2 Trends & Reactions (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about Group 2, the alkaline earth metals: the trends down the group, and the reactions of the metals and their compounds.

Every Group 2 element reacts by losing its two outer electrons, and going down the group those electrons become easier to lose, which is why the reactions get more vigorous down the group.

All the Group 2 elements have two electrons in their outer shell, and they form ionic compounds by losing those two electrons to form 2 plus ions. Going down the group, the outer electrons are further from the nucleus and experience greater shielding from the inner shells. So there is a weaker attraction between the nucleus and those electrons, and less energy is needed to remove them. That is why the elements get more reactive down the group, so the trends explain the reactions.

We start with the chemical trends, the ionisation energies and reactivity. Then the physical trends, atomic radius and melting point. Then the reactions of the metals themselves, with oxygen, water and acids. And last the reactions of their oxides, hydroxides and carbonates.

All the Group 2 elements, the alkaline earth metals, have two electrons in their outer shell. They form ionic compounds by losing those two electrons to form 2 plus ions, and in doing so they act as reducing agents, because they are themselves oxidised. Both the first and the second ionisation energy decrease down the group, so it becomes easier to remove those two outer electrons. Nuclear charge does increase down the group, because the proton number increases, but the outer electrons are further from the nucleus and experience greater shielding from the extra inner shells, and those factors reduce the nuclear attraction by more than the increased charge increases it. That is why Group 2 elements become more reactive down the group.

Down Group 2 the atomic radius increases, because each successive element has an additional electron shell, so the outer two electrons are further from the nucleus and experience increased shielding. The melting point generally decreases down the group. As the atomic radius increases, the positive metal ions get larger and the outer electrons sit further from the nucleus, and that weakens the electrostatic attraction between those positive ions and the delocalised electrons in the metallic lattice. Weaker metallic bonding means a lower melting point. So the same increase in size that makes the outer electrons easier to remove also makes the metallic bonding weaker.

The Group 2 elements react with oxygen, with water and with dilute acids, and the reactions get more vigorous down the group. With water the general equation is the metal plus two waters giving the metal hydroxide plus hydrogen. Beryllium is the exception, because it does not react with water at all. Magnesium is the other special case: it reacts extremely slowly with cold water, but heated in steam it reacts vigorously to give magnesium oxide and hydrogen. Calcium, strontium and barium all react with water to form a hydroxide, and they react faster the further down the group you go. With dilute hydrochloric acid all of them react vigorously to give the chloride and hydrogen. With dilute sulfuric acid the reaction can stop early, because the sulfate that forms coats the metal, and the sulfates get less soluble down the group.

The oxides react with water to form hydroxides, and those solutions are alkaline, getting more alkaline down the group. Magnesium oxide is only slightly soluble, so it gives a weakly alkaline solution at about pH 10, while calcium oxide reacts vigorously and releases a lot of energy, with a pH of about 11. Calcium hydroxide in solution is what you know as limewater.

The hydroxides react with dilute acids to give colourless solutions of the metal salt and water. The carbonates are all insoluble in water, except beryllium carbonate, and with dilute hydrochloric acid they give the chloride, water and carbon dioxide. With sulfuric acid the same blocking happens as with the metals: an insoluble sulfate layer forms on the solid carbonate and stops the reaction after the initial effervescence. So the compounds show trends down the group as well, the solutions getting more alkaline and the sulfates getting less soluble.

One reaction is flagged more than any other, and that is magnesium with steam. The observations examiners want are a bright white flame or light, and a white or grey ash or solid. Mark schemes explicitly do not accept effervescence or fizzing, and they do not accept precipitate.

The Group 2 elements have two outer electrons and lose them to form 2 plus ions. Down the group the atomic radius increases and the melting point decreases, because the metallic bonding gets weaker. Both the first and the second ionisation energies decrease down the group, so those two electrons get easier to remove. And that is why the reactions, with oxygen, with water and with acids, get more vigorous down the group: lower ionisation energy, more reactive metal.