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Electron configuration

The energy of electrons

You will remember from our earlier discussions that an atom is made up of a central nucleus, which contains protons and neutrons and that this nucleus is surrounded by electrons. Although these electrons all have the same charge and the same mass, each electron in an atom has a different amount of energy. Electrons that have the lowest energy are found closest to the nucleus where the attractive force of the positively charged nucleus is the greatest. Those electrons that have higher energy, and which are able to overcome the attractive force of the nucleus, are found further away.

Electron configuration

We will start with a very simple view of the arrangement or configuration of electrons around an atom. This view simply states that electrons are arranged in energy levels (or shells) around the nucleus of an atom. These energy levels are numbered 1, 2, 3, etc. Electrons that are in the first energy level (energy level 1) are closest to the nucleus and will have the lowest energy. Electrons further away from the nucleus will have a higher energy.

In the following examples, the energy levels are shown as concentric circles around the central nucleus. The important thing to know for these diagrams is that the first energy level can hold 2 electrons, the second energy level can hold 8 electrons and the third energy level can hold 8 electrons.

  1. Lithium Lithium (Li) has an atomic number of 3, meaning that in a neutral atom, the number of electrons will also be 3. The first two electrons are found in the first energy level, while the third electron is found in the second energy level (Figure 1).
    Figure 1: The arrangement of electrons in a lithium atom.
    Figure 1 (CG10C3_005.png)
  2. Fluorine Fluorine (FF) has an atomic number of 9, meaning that a neutral atom also has 9 electrons. The first 2 electrons are found in the first energy level, while the other 7 are found in the second energy level (Figure 2).
    Figure 2: The arrangement of electrons in a fluorine atom.
    Figure 2 (CG10C3_006.png)
  3. Argon Argon has an atomic number of 18, meaning that a neutral atom also has 18 electrons. The first 2 electrons are found in the first energy level, the next 8 are found in the second energy level, and the last 8 are found in the third energy level (Figure 3).
    Figure 3: The arrangement of electrons in an argon atom.
    Figure 3 (CG10C3_007.png)

But the situation is slightly more complicated than this. Within each energy level, the electrons move in orbitals. An orbital defines the spaces or regions where electrons move.

Definition 1: Atomic orbital

An atomic orbital is the region in which an electron may be found around a single atom.

The first energy level contains only one 's' orbital, the second energy level contains one 's' orbital and three 'p' orbitals and the third energy level contains one 's' orbital and three 'p' orbitals (as well as 5 'd' orbitals). Within each energy level, the 's' orbital is at a lower energy than the 'p' orbitals. This arrangement is shown in Figure 4.

Figure 4: The positions of the first ten orbitals of an atom on an energy diagram. Note that each block is able to hold two electrons.
Figure 4 (CG10C3_008.png)

This diagram also helps us when we are working out the electron configuration of an element. The electron configuration of an element is the arrangement of the electrons in the shells and subshells. There are a few guidelines for working out the electron configuration. These are:

  • Each orbital can only hold two electrons. Electrons that occur together in an orbital are called an electron pair.
  • An electron will always try to enter an orbital with the lowest possible energy.
  • An electron will occupy an orbital on its own, rather than share an orbital with another electron. An electron would also rather occupy a lower energy orbital with another electron, before occupying a higher energy orbital. In other words, within one energy level, electrons will fill an 's' orbital before starting to fill 'p' orbitals.
  • The s subshell can hold 2 electrons
  • The p subshell can hold 6 electrons

In the examples you will cover, you will mainly be filling the s and p subshells. Occasionally you may get an example that has the d subshell. The f subshell is more complex and is not covered at this level.

The way that electrons are arranged in an atom is called its electron configuration.

Definition 2: Electron configuration

Electron configuration is the arrangement of electrons in an atom, molecule or other physical structure.

An element's electron configuration can be represented using Aufbau diagrams or energy level diagrams. An Aufbau diagram uses arrows to represent electrons. You can use the following steps to help you to draw an Aufbau diagram:

  1. Determine the number of electrons that the atom has.
  2. Fill the 's' orbital in the first energy level (the 1s1s orbital) with the first two electrons.
  3. Fill the 's' orbital in the second energy level (the 2s2s orbital) with the second two electrons.
  4. Put one electron in each of the three 'p' orbitals in the second energy level (the 2p2p orbitals) and then if there are still electrons remaining, go back and place a second electron in each of the 2p2p orbitals to complete the electron pairs.
  5. Carry on in this way through each of the successive energy levels until all the electrons have been drawn.

Tip:

When there are two electrons in an orbital, the electrons are called an electron pair. If the orbital only has one electron, this electron is said to be an unpaired electron. Electron pairs are shown with arrows pointing in opposite directions.

Note: Interesting fact:

Aufbau is the German word for 'building up'. Scientists used this term since this is exactly what we are doing when we work out electron configuration, we are building up the atoms structure.

You can think of Aufbau diagrams as being similar to people getting on a bus or a train. People will first sit in empty seats with empty seats between them and the other people (unless they know the people and then they will sit next to them). When all the seats are filled like this, any more people that get on will be forced to sit next to someone or stand. As the bus or train fills even more the people have to stand to fit on.

An Aufbau diagram for the element Lithium is shown in Figure 5.

Figure 5: The electron configuration of Lithium, shown on an Aufbau diagram
Figure 5 (CG10C3_009.png)

A special type of notation is used to show an atom's electron configuration. The notation describes the energy levels, orbitals and the number of electrons in each. For example, the electron configuration of lithium is 1s22s11s22s1. The number and letter describe the energy level and orbital and the number above the orbital shows how many electrons are in that orbital.

Aufbau diagrams for the elements fluorine and argon are shown in Figure 6 and Figure 7 respectively. Using standard notation, the electron configuration of fluorine is 1s22s22p51s22s22p5 and the electron configuration of argon is 1s22s22p61s22s22p6.

Figure 6: An Aufbau diagram showing the electron configuration of fluorine
Figure 6 (CG10C3_010.png)
Figure 7: An Aufbau diagram showing the electron configuration of argon
Figure 7 (CG10C3_011.png)

Exercise 1: Aufbau diagrams

Give the electron configuration for sodium (NaNa) and draw an aufbau diagram.

Solution

  1. Step 1. Write down the number of electrons: Sodium has 11 electrons.
  2. Step 2. Work out which orbitals to fill: We start by placing two electrons in the 1s1s orbital: 1s21s2. Now we have 9 electrons left to place in orbitals, so we put two in the 2s2s orbital: 2s22s2. There are now 7 electrons to place in orbitals so we place 6 of them in the 2p2p orbital: 2p62p6. The last electron goes into the 3s3s orbital: 3s13s1.
  3. Step 3. Write down the electron configuration: The electron configuration is: 1s22s22p63s11s22s22p63s1
  4. Step 4. Draw the Aufbau diagram: Using the electron configuration we get the following diagram:
    Figure 8
    Figure 8 (wexaufbau.png)

There are different orbital shapes, but we will be mainly dealing with only two. These are the 's' and 'p' orbitals (there are also 'd' and 'f' orbitals). The 's' orbitals are spherical and the 'p' orbitals are dumbbell shaped.

Figure 9: The shapes of orbitals. a) shows an 's' orbital, b) shows a single 'p' orbital and c) shows the three 'p' orbitals.
Figure 9 (orbitals.png)

Hund's rule and Pauli's principle

Sometimes people refer to Hund's rule for electron configuration. This rule simply says that electrons would rather be in a subshell on their own than share a subshell. This is why when you are filling the subshells you put one electron in each subshell and then go back and fill the subshell, before moving onto the next energy level.

Pauli's exclusion principle simply states that electrons have a property known as spin and that two electrons in a subshell will not spin the same way. This is why we draw electrons as one arrow pointing up and one arrow pointing down.

Core and valence electrons

Electrons in the outermost energy level of an atom are called valence electrons. The electrons that are in the energy shells closer to the nucleus are called core electrons. Core electrons are all the electrons in an atom, excluding the valence electrons. An element that has its valence energy level full is more stable and less likely to react than other elements with a valence energy level that is not full.

Definition 3: Valence electrons

The electrons in the outer energy level of an atom

Definition 4: Core electrons

All the electrons in an atom, excluding the valence electrons

The importance of understanding electron configuration

By this stage, you may well be wondering why it is important for you to understand how electrons are arranged around the nucleus of an atom. Remember that during chemical reactions, when atoms come into contact with one another, it is the electrons of these atoms that will interact first. More specifically, it is the valence electrons of the atoms that will determine how they react with one another.

To take this a step further, an atom is at its most stable (and therefore unreactive) when all its orbitals are full. On the other hand, an atom is least stable (and therefore most reactive) when its valence electron orbitals are not full. This will make more sense when we go on to look at chemical bonding in a later chapter. To put it simply, the valence electrons are largely responsible for an element's chemical behaviour and elements that have the same number of valence electrons often have similar chemical properties.

One final point to note about electron configurations is stability. Which configurations are stable and which are not? Very simply, the most stable configurations are the ones that have full energy levels. These configurations occur in the noble gases. The noble gases are very stable elements that do not react easily (if at all) with any other elements. This is due to the full energy levels. All elements would like to reach the most stable electron configurations, i.e. all elements want to be noble gases. This principle of stability is sometimes referred to as the octet rule. An octet is a set of 8, and the number of electrons in a full energy level is 8.

Experiment: Flame tests

Aim:


To determine what colour a metal cation will cause a flame to be.

Apparatus:


Watch glass, bunsen burner, methanol, bamboo sticks, metal salts (e.g. NaClNaCl, CuCl2CuCl2, CaCl2CaCl2, KClKCl, etc. ) and metal powders (e.g. copper, magnesium, zinc, iron, etc.)

Warning:
Be careful when working with bunsen burners as you can easily burn yourself. Make sure all scarves/loose clothing is securely tucked in and long hair is tied back. Ensure that you work in a well-ventilated space and that there is nothing flammable near the open flame.
Method:


For each salt or powder do the following:

  1. Dip a clean bamboo stick into the methanol
  2. Dip the stick into the salt or powder
  3. Wave the stick through the flame from the bunsen burner. DO NOT hold the stick in the flame, but rather wave it back and forth through the flame.
  4. Observe what happens

Results:


Record your results in a table, listing the metal salt and the colour of the flame.

Conclusion:


You should have observed different colours for each of the metal salts and powders that you tested.

The above experiment on flame tests relates to the line emission spectra of the metals. These line emission spectra are a direct result of the arrangement of the electrons in metals.

Energy diagrams and electrons

  1. Draw Aufbau diagrams to show the electron configuration of each of the following elements:
    1. magnesium
    2. potassium
    3. sulphur
    4. neon
    5. nitrogen
  2. Use the Aufbau diagrams you drew to help you complete the following table:
    Table 1
    ElementNo. of energy levelsNo. of core electronsNo. of valence electronsElectron configuration (standard notation)
    MgMg    
    KK    
    SS    
    NeNe    
    NN    
  3. Rank the elements used above in order of increasing reactivity. Give reasons for the order you give. Click here for the answer

Group work : Building a model of an atom

Earlier in this chapter, we talked about different 'models' of the atom. In science, one of the uses of models is that they can help us to understand the structure of something that we can't see. In the case of the atom, models help us to build a picture in our heads of what the atom looks like.

Models are often simplified. The small toy cars that you may have played with as a child are models. They give you a good idea of what a real car looks like, but they are much smaller and much simpler. A model cannot always be absolutely accurate and it is important that we realise this so that we don't build up a false idea about something.

In groups of 4-5, you are going to build a model of an atom. Before you start, think about these questions:

  • What information do I know about the structure of the atom? (e.g. what parts make it up? how big is it?)
  • What materials can I use to represent these parts of the atom as accurately as I can?
  • How will I put all these different parts together in my model?

As a group, share your ideas and then plan how you will build your model. Once you have built your model, discuss the following questions:

  • Does our model give a good idea of what the atom actually looks like?
  • In what ways is our model inaccurate? For example, we know that electrons move around the atom's nucleus, but in your model, it might not have been possible for you to show this.
  • Are there any ways in which our model could be improved?

Now look at what other groups have done. Discuss the same questions for each of the models you see and record your answers.

The following simulation allows you to build an atom
run demo

Figure 10: Build an atom simulation
Figure 10 (atom1.png)

This is another simulation that allows you to build an atom. This simulation also provides a summary of what you have learnt so far.
Run demo

Figure 11: Build an atom simulation 2
Figure 11 (atom2.png)

Summary

  • Much of what we know today about the atom, has been the result of the work of a number of scientists who have added to each other's work to give us a good understanding of atomic structure.
  • Some of the important scientific contributors include J.J.Thomson (discovery of the electron, which led to the Plum Pudding Model of the atom), Ernest Rutherford (discovery that positive charge is concentrated in the centre of the atom) and Niels Bohr (the arrangement of electrons around the nucleus in energy levels).
  • Because of the very small mass of atoms, their mass is measured in atomic mass units (u). 1u= 1,67×10-24g1u=1,67×10-24g.
  • An atom is made up of a central nucleus (containing protons and neutrons), surrounded by electrons.
  • The atomic number (Z) is the number of protons in an atom.
  • The atomic mass number (A) is the number of protons and neutrons in the nucleus of an atom.
  • The standard notation that is used to write an element, is ZAXZAX, where X is the element symbol, A is the atomic mass number and Z is the atomic number.
  • The isotope of a particular element is made up of atoms which have the same number of protons as the atoms in the original element, but a different number of neutrons. This means that not all atoms of an element will have the same atomic mass.
  • The relative atomic mass of an element is the average mass of one atom of all the naturally occurring isotopes of a particular chemical element, expressed in atomic mass units. The relative atomic mass is written under the elements' symbol on the Periodic Table.
  • The energy of electrons in an atom is quantised. Electrons occur in specific energy levels around an atom's nucleus.
  • Within each energy level, an electron may move within a particular shape of orbital. An orbital defines the space in which an electron is most likely to be found. There are different orbital shapes, including s, p, d and f orbitals.
  • Energy diagrams such as Aufbau diagrams are used to show the electron configuration of atoms.
  • The electrons in the outermost energy level are called valence electrons.
  • The electrons that are not valence electrons are called core electrons.
  • Atoms whose outermost energy level is full, are less chemically reactive and therefore more stable, than those atoms whose outer energy level is not full.

Figure 12

End of chapter exercises

  1. Write down only the word/term for each of the following descriptions.
    1. The sum of the number of protons and neutrons in an atom
    2. The defined space around an atom's nucleus, where an electron is most likely to be found
    Click here for the solution
  2. For each of the following, say whether the statement is True or False. If it is False, re-write the statement correctly.
    1. 1020Ne1020Ne and 1022Ne1022Ne each have 10 protons, 12 electrons and 12 neutrons.
    2. The atomic mass of any atom of a particular element is always the same.
    3. It is safer to use helium gas rather than hydrogen gas in balloons.
    4. Group 1 elements readily form negative ions.
    Click here for the solution
  3. Multiple choice questions: In each of the following, choose the one correct answer.
    1. The three basic components of an atom are:
      1. protons, neutrons, and ions
      2. protons, neutrons, and electrons
      3. protons, neutrinos, and ions
      4. protium, deuterium, and tritium
      Click here for the solution
    2. The charge of an atom is...
      1. positive
      2. neutral
      3. negative
      Click here for the solution
    3. If Rutherford had used neutrons instead of alpha particles in his scattering experiment, the neutrons would...
      1. not deflect because they have no charge
      2. have deflected more often
      3. have been attracted to the nucleus easily
      4. have given the same results
      Click here for the solution
    4. Consider the isotope 92234U92234U. Which of the following statements is true?
      1. The element is an isotope of 94234Pu94234Pu
      2. The element contains 234 neutrons
      3. The element has the same electron configuration as 92238U92238U
      4. The element has an atomic mass number of 92
      Click here for the solution
    5. The electron configuration of an atom of chlorine can be represented using the following notation:
      1. 1s22s83s71s22s83s7
      2. 1s22s22p63s23p51s22s22p63s23p5
      3. 1s22s22p63s23p61s22s22p63s23p6
      4. 1s22s22p51s22s22p5
      Click here for the solution
  4. Give the standard notation for the following elements:
    1. beryllium
    2. carbon-12
    3. titanium-48
    4. fluorine
    Click here for the solution
  5. Give the electron configurations and aufbau diagrams for the following elements:
    1. aluminium
    2. phosphorus
    3. carbon
    Click here for the solution
  6. Use standard notation to represent the following elements:
    1. argon
    2. calcium
    3. silver-107
    4. bromine-79
    Click here for the solution
  7. For each of the following elements give the number of protons, neutrons and electrons in the element:
    1. 78195Pt78195Pt
    2. 1840Ar1840Ar
    3. 2759Co2759Co
    4. 37Li37Li
    5. 511B511B
    Click here for the solution
  8. For each of the following elements give the element or number represented by 'x':
    1. 45103X45103X
    2. x35Clx35Cl
    3. 4xBe4xBe
    Click here for the solution
  9. Which of the following are isotopes of 1224Mg1224Mg:
    1. 2512Mg2512Mg
    2. 1226Mg1226Mg
    3. 1324Al1324Al
    Click here for the solution
  10. If a sample contains 69% of copper-63 and 31% of copper-65, calculate the relative atomic mass of an atom in that sample.
    Click here for the solution
  11. Complete the following table:
    Table 2
    Element Electron configuration Core electrons Valence electrons
    Boron (B)      
    Calcium (Ca)      
    Silicon (Si)      
    Lithium (Li)      
    Neon (Ne)      
    Click here for the solution
  12. Draw aufbau diagrams for the following elements:
    1. beryllium
    2. sulphur
    3. argon
    Click here for the solution

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