3.4 Ionisation energy and the periodic table

You may see periodic tables labeled with s-block, p-block, d-block and f-block. This is simply another way to group the elements. When we group elements like this we are simply noting which orbitals are being filled in each block. This method of grouping is not very useful to the work covered at this level.

Trends in ionisation energy

Refer to the data table below which gives the ionisation energy (in kJ $·$ mol ${}^{-1}$ ) and atomic number (Z) for a number of elements in the periodic table:

1. Draw a line graph to show the relationship between atomic number (on the x-axis) and ionisation energy (y-axis).
2. Describe any trends that you observe.
3. Explain why...
   1. the ionisation energy for Z=2 is higher than for Z=1
   2. the ionisation energy for Z=3 is lower than for Z=2
   3. the ionisation energy increases between Z=5 and Z=7

Elements in the periodic table

Refer to the elements listed below:

Lithium (Li); Chlorine (Cl); Magnesium (Mg); Neon (Ne); Oxygen (O); Calcium (Ca); Carbon (C)

Which of the elements listed above:

1. belongs to Group 1
2. is a halogen
3. is a noble gas
4. is an alkali metal
5. has an atomic number of 12
6. has 4 neutrons in the nucleus of its atoms
7. contains electrons in the 4th energy level
8. has only one valence electron
9. has all its energy orbitals full
10. will have chemical properties that are most similar
11. will form positive ions

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). 1 u = 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 ${}_Z^A$ 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.
• An ion is a charged atom. A cation is a positively charged ion and an anion is a negatively charged ion.
• When forming an ion, an atom will lose or gain the number of electrons that will make its valence energy level full.
• An element's ionisation energy is the energy that is needed to remove one electron from an atom.
• Ionisation energy increases across a period in the periodic table.
• Ionisation energy decreases down a group in the periodic table.

Summary

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. ${}_{10}^{20}$ Ne and ${}_{10}^{22}$ 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 ${}_{92}^{234}$ U. Which of the following statements is true ?
      1. The element is an isotope of ${}_{94}^{234}$ Pu
      2. The element contains 234 neutrons
      3. The element has the same electron configuration as ${}_{92}^{238}$ 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. 1s ${}^2$ 2s ${}^8$ 3s ${}^7$
      2. 1s ${}^2$ 2s ${}^2$ 2p ${}^6$ 3s ${}^2$ 3p ${}^5$
      3. 1s ${}^2$ 2s ${}^2$ 2p ${}^6$ 3s ${}^2$ 3p ${}^6$
      4. 1s ${}^2$ 2s ${}^2$ 2p ${}^5$
4. The following table shows the first ionisation energies for the elements of period 1 and 2.

   Period	Element	First ionisation energy ( $kJ.mo{l}^{-1}$ )
   1	H	1312
   	He	2372
   	Li	520
   	Be	899
   	B	801
   	C	1086
   2	N	1402
   	O	1314
   	F	1681
   	Ne	2081

   1. What is the meaning of the term first ionisation energy ?
   2. Identify the pattern of first ionisation energies in a period.
   3. Which TWO elements exert the strongest attractive forces on their electrons? Use the data in the table to give a reason for your answer.
   4. Draw Aufbau diagrams for the TWO elements you listed in the previous question and explain why these elements are so stable.
   5. It is safer to use helium gas than hydrogen gas in balloons. Which property of helium makes it a safer option?
   6. 'Group 1 elements readily form positive ions'. Is this statement correct? Explain your answer by referring to the table.