Chemguide: Support for CIE A level Chemistry Learning outcome 10.1 Group 2 Similarities and trends in the properties of the Group 2 metals (magnesium to barium) and their compounds Learning outcome 10.1.4 This statement is asking you to pick out the trends in the properties of the Group 2 elements and their compounds, and use these trends to make predictions. This could be a prediction about the radioactive element radium at the bottom of the group. Alternatively, you could be given some details about, say, calcium or one of its compounds, and asked to predict what the corresponding strontium chemistry might be. Before you go on, you should find and read the statement in your copy of the syllabus. Properties of the elements Trends in atomic properties This is a good time to revise what you know about atomic radius, ionisation energy and electronegativity. You will find all this discussed with regard to Group 2 elements on the page Atomic and physical properties of Periodic Table Group 2. Read the first half of the page down to the end of the bit about electronegativity. Don't forget that you are concentrating on trends as you go down the group. It is possible you might also get asked about ionic radii. This isn't strictly a property of the element, because it is only relevant in its compounds, but it fits in here better than anywhere else. The elements form simple 2+ ions with a noble gas structure. Obviously, as you go down the group, you are adding more layers of electrons, and so the ions get bigger. | |
Note: The Be2+ ion is so small with a high charge density (a lot of charge in a small volume) that it tends to pull electrons towards itself when bonds are made. That means that even in compounds with very electronegative elements like oxygen or fluorine, which you might expect to be ionic, there is a large degree of covalent character. | |
Trends in physical properties These are all metallic elements, and therefore conduct electricity and heat. Their melting and boiling points are higher than the Group 1 metals, but lower than typical transition metals. If you look again at the page Atomic and physical properties of Periodic Table Group 2, you will find bar charts for melting points, boiling points, and atomisation energies. The melting points show a general decrease as you go down the group, but it is broken completely at magnesium. There is no obvious pattern at all for the boiling points or atomisation energies which are better measures of the strengths of the metallic bonds than melting points are. If you are interested, you could read the discussion on the rest of that page, but it isn't essential. Trends in chemical properties These are reactive elements, with their reactivity tending to increase as you go down the group. They react with both water and oxygen, for example, to form compounds in which the oxidation number (oxidation state) of the element is +2. These compounds contain the 2+ ions. | |
Note: Beryllium is an exception to this, which you probably needn't worry about. It has an oxidation state of +2 in its compounds, but these tend to be either entirely, or largely, covalent. | |
The reaction with water This best illustrates the increase in reactivity. If you have been working through this section of the syllabus in order, you will already have read the page Reactions of the Group 2 elements with water. There is no suggestion in the syllabus that you should be able to explain the increase in reactivity. You could be asked to predict the reaction of, say, the radioactive element radium (at the very bottom of Group 2) with water. You would be able to predict that
The reaction with oxygen Again, if you have been working through this section of the syllabus in order, you will already have read the first bit of the page Reactions of the Group 2 elements with oxygen. You may remember that it is difficult to pick out any trends in reactivity in the elements, but there are trends that you can pick out.
Properties of the compounds of Group 2 elements Oxides You don't need to know about beryllium oxide. The other oxides react with water to produce hydroxides, although the reaction between magnesium oxide and water is fairly slight. They all react with acids to make salts - see statement 10.1.2. Hydroxides Again, remember that you only need to know about elements from magnesium to barium. The hydroxides get more soluble as you go down the group. Solutions of the hydroxides have alkaline pHs. How high the pH is depends on how soluble the hydroxide is. Because solubility increases down the group, the pH of a saturated solution increases down the group. The hydroxides all react with acids to make salts - again see statement 10.1.2. Nitrates The nitrates all decompose on heating to give the oxide, nitrogen dioxide and oxygen. As you go down the group, decomposition is more difficult, and so you have to heat more strongly. | |
Note: Apart from barium nitrate, the other nitrates in this Group have water of crystallisation. That means that if you actually do any of these reactions, you will also see steam produced. If you are asked about the nitrates in an exam just give the three main products (oxide, nitrogen dioxide and oxygen) unless you are specifically asked something like "Describe the effect of heating hydrated calcium nitrate crystals.". If the word "hydrated" (or the equivalent) isn't used, don't mention the water. | |
Carbonates The carbonates are insoluble in water. They react with dilute acids to give a salt, carbon dioxide and water, but remember that the reaction with dilute sulfuric acid is only straightforward in the case of the reaction with magnesium carbonate - again see statement 10.1.2. They decompose on heating to give the oxide and carbon dioxide. As you go down the group, decomposition is more difficult, and so you have to heat more strongly. Making predictions from these patterns Because beryllium's properties aren't all typical of the group as a whole, it isn't safe to make predictions about beryllium compounds at the top of the group. You can, though, safely make predictions about the compounds of radium at the bottom. Some things you might predict would include:
© Jim Clark 2020 |