Chemguide: Support for CIE A level Chemistry


Learning outcome 10.1(d)

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 statements 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. At the time, all you were interested were the facts.

Look at this page again, and this time read the explanation for the increase in reactivity as you go down the group.

It isn't clear from the syllabus that you need this explanation, but look at it anyway.

All you really need to know is that the reactivity increases because the activation energy for the reactions falls. The reason for that is mainly due to the fall in ionisation energies as you go down the group. It needs less energy to form the 2+ ions as you go down the group.


Note:  If you aren't happy about activation energy, follow this link to an introductory page in the rates of reaction section of Chemguide.


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

  • Radium will be more reactive with water than barium is.

  • Radium will react with water to make radium hydroxide and hydrogen - and you should be able to write an equation for this (because it will be just the same as the rest of the group).

  • Radium hydroxide will probably be more soluble than barium hydroxide, and its solution will be alkaline with a pH between 13 - 14.


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.

  • The elements all burn in oxygen to give an oxide. At the top of the group, this is a simple oxide, but barium gives a mixture of barium oxide and barium peroxide when it reacts with oxygen under normal conditions. That means that you could deduce that radium might burn to give mainly radium peroxide.

  • The oxides formed are white. You could deduce that radium oxide or peroxide will also be white solids.


Properties of the compounds of Group 2 elements

Oxides

Apart from beryllium oxide, the simple oxides are basic. (Beryllium oxide is amphoteric - you wouldn't be expected to know that.)

They 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.


Note:  There has been no mention of this in any of the pages you have read for section 10 of the syllabus, and so you might wonder why I expect you to know that basic oxides react with acids to form salts.

If you have done any chemistry before you started doing A level, you must have known this. You mustn't forget the chemical knowledge you have already built up in the past.

The reactions of basic oxides with acids were mentioned on the page I pointed you at regarding the acid-base behaviour of the Period 3 oxides when you were looking at periodicity in Section 9 of the syllabus - although you didn't specifically need to know about it for that bit of the syllabus. See the acid-base behaviour of the period 3 oxides if you have forgotten.




Hydroxides

Apart from beryllium hydroxide, the hydroxides are basic. (Beryllium hydroxide is amphoteric - you wouldn't be expected to know that.)

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.


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 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:

  • Radium oxide is a white solid which reacts with water to give a solution of radium hydroxide with a pH in the range 13 - 14.

  • Radium hydroxide solution or solid radium oxide will react with dilute hydrochloric acid to give a solution of radium chloride.

  • Radium nitrate will decompose on very strong heating to give radium oxide, nitrogen dioxide and oxygen.

  • Radium carbonate also decomposes on very strong heating - this time giving radium oxide and carbon dioxide.


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© Jim Clark 2010 (last modified May 2014)