The periodic table is divided up into blocks. You should be familiar with the s-block, p-block and d-block, as shown in Figure 1.

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1 Electron configuration and the periodic table Specification reference Elements of life (e) (f(i)) (f(iii)) Introduction Working out electronic configurations of atoms can be quite a challenge, requiring you to be able to remember the order in which electronic sub-shells are filled and the number of electrons that can occupy each type of Luckily, there is a close connection between electronic configuration and the periodic table. In this activity, you will see how a copy of the periodic table can make it easy for you to predict the electronic configuration of atoms and ions. Learning outcomes After completing the worksheet you should be able to: state the periodic trend in electron configurations across Period 2 and Period 3 deduce the electron configuration of atoms and ions (up to atomic number = 36), using a periodic table. Background The periodic table is divided up into blocks. You should be familiar with the s-block, p-block and d-block, as shown in Figure 1. Figure 1 Blocks in the periodic table Some of the elements in Periods 6 and 7 are also placed in an f-block, but you will not be asked about the electron configuration of these elements. This resource sheet may have been changed from the original 1

2 What is the connection between an element s position in the periodic table and its electron configuration? There is a periodic trend in the electron configuration of atoms. This means that information about electronic configuration can be deduced from the position of the element in the periodic table. This is easy if the element is in the s-block or the p-block. Take chlorine as an example: Chlorine (atomic number 17) is found in the p-block. From this you can tell that the highest energy sub-shell is a p Chlorine is in the 3rd period. This means that the highest energy sub-shell will be a 3p- Chlorine is in the 5th column of the p-block. This means that the highest energy sub-shell will contain five electrons. Therefore, the electronic configuration of the highest energy sub-shell of chlorine is 3p 5. What is different about elements in the d-block? You will have learnt about electron configurations, including that 4s orbitals fill before 3d orbitals. This means that you may need to think a bit more carefully about the electron configuration of elements in the d-block. Take iron as an example: Iron (atomic number 26) is found in the d-block. From this you can tell that the highest energy sub-shell is a d Iron is in the 4th period because it contains some electrons in the 4th shell (the two electrons in the 4s sub-shell). However the 3d sub-shell has a higher energy than the 4s sub-shell so the highest energy sub-shell is actually the 3d Iron is in the 6th column of the d-block. This means that the highest energy sub-shell will contain six electrons. Therefore, the electronic configuration of the highest energy sub-shell of iron is 3d 6. Can the periodic table help you to remember the order in which sub-shells fill? The answer is yes. All you need to do is to remember the way in which the periodic table is broken up into blocks. You can then imagine gradually filling up the orbitals in an atom one electron at a time, starting with the 1s electrons. As you do this, you can mentally imagine moving through the periodic table, using it to remind you when a sub-shell is full. Figure 2 may help you to visualise this. Figure 2 Sub-shells in the periodic table This resource sheet may have been changed from the original 2

3 Take phosphorus as an example: The first two electrons go into the 1s sub-shell (shown on the top line of the periodic table). Notice that the second element in the periodic table (helium) shouldn t really be put into the p-block, but you can probably remember that it will have electron configuration 1s 2. At this point the 1s sub-shell is now full, and so you now imagine jumping to the next row in the periodic table and starting to fill up the next This is in the s-block, so it is a 2s There are only two columns in the s-block, so you can only fit two electrons into the 2s Now you need to jump across to the p-block and start filling the 2p There are six columns in the p-block so you can fit six electrons into the 2p You will now jump to the next row in the periodic table and start filling the 3s When this is full, you will need to start putting electrons into the 3p By the time you reach phosphorus, you will have put three electrons into this Therefore, the complete electron configuration is 1s 2 2s 2 2p 6 3s 2 3p 3. The first two shells have an identical electron configuration to that of neon, so the configuration is sometimes written [Ne]3s 2 3p 3. If the question requires it, you could also then show how the electrons are arranged in the orbitals of these sub-shells, as is demonstrated in Figure 3 for phosphorus. Figure 3 3s and 3p electron arrangement of phosphorus How do I predict the configuration of ions? The trick is to start by deducing the configuration of the atom from which the ion is derived. Then think about how many electrons have been lost or gained. For s-block and p-block elements, these electrons are lost or gained from the highest energy Note about transition metals At this stage in your A Level course, all the questions about writing electron configurations that you are likely to encounter can be answered by using the ideas in this support sheet. However, later on you will discover that predicting the configurations of some d-block atoms and ions will require some extra information. This is covered in detail in the second year of your A Level course. This resource sheet may have been changed from the original 3

4 Questions 1 Find the electron configuration of the highest energy sub-shell in atoms of the following elements. a F b Li c Si d Ar 2 Construct the full electron configuration of atoms of the following elements. a N b Ca c Ar d Ni e Se (4 marks) 3 The electron configuration of atoms can be written in a shorthand way by including the symbol for a noble gas. So sulfur, for example, can be represented by the shorthand configuration [Ne]3s 2 3p 4. Construct shorthand configurations of the following atoms or ions. a Na b Na + c O d O 2- e Ca 2+ f Br - 4 Describe the trend in electron configuration across Period 3 of the periodic table. Exam-style question 1 Group 3 of the periodic table contains elements including aluminium (Al), boron (B) and thallium (Tl). Most Group 3 elements react with chlorine to form ionic compounds with the general formula MCl 3 in which the Group 3 element has a 3+ charge. Thallium is unusual for a Group 3 element because it can form a 1+ ion, Tl 1+ in compounds such as TlCl. a i State the electron configuration of the highest energy sub-shell of a boron atom. ii Explain why both aluminium and boron are placed in the p-block of the periodic table. (1 mark) b i State the charge on a chloride ion in the compound AlCl 3. (1 mark) This resource sheet may have been changed from the original 4

5 ii Give the symbol for a noble gas that has an identical electron configuration to a chloride ion. (1 mark) c i Construct the electron configuration of an aluminium atom. (3 marks) ii Draw a diagram to show the relative energies of the orbitals of the outer shell electrons in an aluminium atom. d The highest energy outer shell configuration of thallium is similar to that of both boron and aluminium. Use ideas about the electron configuration of thallium to discuss how thallium can form a 1+ ion rather than a 3+ ion in some of its compounds. (3 marks) This resource sheet may have been changed from the original 5