3.3 Electronic structure  (Page 5/5)

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

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

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). $1u=1,67\times 10{}^{-24}g$ .
• 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 ${}_Z^A\mathrm{X}$ , 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.

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
2. For each of the following, say whether the statement is True or False. If it is False, re-write the statement correctly.
   1. ${}_{10}^{20}\mathrm{Ne}$ and ${}_{10}^{22}\mathrm{Ne}$ 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.
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
   2. The charge of an atom is...
      1. positive
      2. neutral
      3. negative
   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
   4. Consider the isotope ${}_{92}^{234}\mathrm{U}$ . Which of the following statements is true ?
      1. The element is an isotope of ${}_{94}^{234}\mathrm{Pu}$
      2. The element contains 234 neutrons
      3. The element has the same electron configuration as ${}_{92}^{238}\mathrm{U}$
      4. The element has an atomic mass number of 92
   5. The electron configuration of an atom of chlorine can be represented using the following notation:
      1. ${1\mathrm{s}}^2{2\mathrm{s}}^8{3\mathrm{s}}^7$
      2. ${1\mathrm{s}}^2{2\mathrm{s}}^2{2\mathrm{p}}^6{3\mathrm{s}}^2{3\mathrm{p}}^5$
      3. ${1\mathrm{s}}^2{2\mathrm{s}}^2{2\mathrm{p}}^6{3\mathrm{s}}^2{3\mathrm{p}}^6$
      4. ${1\mathrm{s}}^2{2\mathrm{s}}^2{2\mathrm{p}}^5$
4. Give the standard notation for the following elements:
   1. beryllium
   2. carbon-12
   3. titanium-48
   4. fluorine
5. Give the electron configurations and aufbau diagrams for the following elements:
   1. aluminium
   2. phosphorus
   3. carbon
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. ${}_{78}^{195}\mathrm{Pt}$
   2. ${}_{18}^{40}\mathrm{Ar}$
   3. ${}_{27}^{59}\mathrm{Co}$
   4. ${}_3^7\mathrm{Li}$
   5. ${}_5^{11}\mathrm{B}$
   Click here for the solution
8. For each of the following elements give the element or number represented by 'x':
   1. ${}_{45}^{103}\mathrm{X}$
   2. ${}_{\mathrm{x}}^{35}\mathrm{Cl}$
   3. ${}_4^{\mathrm{x}}\mathrm{Be}$
9. Which of the following are isotopes of ${}_{12}^{24}\mathrm{Mg}$ :
   1. ${}_{25}^{12}\mathrm{Mg}$
   2. ${}_{12}^{26}\mathrm{Mg}$
   3. ${}_{13}^{24}\mathrm{Al}$
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.
    
11. Complete the following table:

    Element	Electron configuration	Core electrons	Valence electrons
    Boron (B)
    Calcium (Ca)
    Silicon (Si)
    Lithium (Li)
    Neon (Ne)

12. Draw aufbau diagrams for the following elements:
    1. beryllium
    2. sulphur
    3. argon