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Ionisation energy and the periodic table

Module by: Free High School Science Texts Project. E-mail the author

Ionisation Energy and the Periodic Table

Ions

In the previous section, we focused our attention on the electron configuration of neutral atoms. In a neutral atom, the number of protons is the same as the number of electrons. But what happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element?

A change in the number of electrons of an atom does not change the type of atom that it is. However, the charge of the atom will change. If electrons are added, then the atom will become more negative. If electrons are taken away, then the atom will become more positive. The atom that is formed in either of these cases is called an ion. Put simply, an ion is a charged atom.

Definition 1: Ion

An ion is a charged atom. A positively charged ion is called a cation e.g. Na+, and a negatively charged ion is called an anion e.g. F--. The charge on an ion depends on the number of electrons that have been lost or gained.

But how do we know how many electrons an atom will gain or lose? Remember what we said about stability? We said that all atoms are trying to get a full outer shell. For the elements on the left hand side of the periodic table the easiest way to do this is to lose electrons and for the elements on the right of the periodic table the easiest way to do this is to gain electrons. So the elements on the left of the periodic table will form cations and the elements on the right hand side of the periodic table will form anions. By doing this the elements can be in the most stable electronic configuration and so be as stable as the noble gases.

Look at the following examples. Notice the number of valence electrons in the neutral atom, the number of electrons that are lost or gained and the final charge of the ion that is formed.

Lithium

A lithium atom loses one electron to form a positive ion (figure).

Figure 1: The arrangement of electrons in a lithium ion.
Figure 1 (CG10C3_012.png)

In this example, the lithium atom loses an electron to form the cation Li+.

Fluorine

A fluorine atom gains one electron to form a negative ion ((Reference)).

Figure 2: The arrangement of electrons in a fluorine ion.
Figure 2 (CG10C3_013.png)

You should have noticed in both these examples that each element lost or gained electrons to make a full outer shell.

Investigation : The formation of ions

  1. Use the diagram for lithium as a guide and draw similar diagrams to show how each of the following ions is formed:
    1. Mg2+2+
    2. Na++
    3. Cl--
    4. O2-2-
  2. Do you notice anything interesting about the charge on each of these ions? Hint: Look at the number of valence electrons in the neutral atom and the charge on the final ion.

Observations:

Once you have completed the activity, you should notice that:

  • In each case the number of electrons that is either gained or lost, is the same as the number of electrons that are needed for the atoms to achieve a full outer energy level.
  • If you look at an energy level diagram for sodium (Na), you will see that in a neutral atom, there is only one valence electron. In order to achieve a full outer energy level, and therefore a more stable state for the atom, this electron will be lost.
  • In the case of oxygen (O), there are six valence electrons. To achieve a full energy level, it makes more sense for this atom to gain two electrons. A negative ion is formed.

Ionisation Energy

Ionisation energy is the energy that is needed to remove one electron from an atom. The ionisation energy will be different for different atoms.

The second ionisation energy is the energy that is needed to remove a second electron from an atom, and so on. As an energy level becomes more full, it becomes more and more difficult to remove an electron and the ionisation energy increases. On the Periodic Table of the Elements, a group is a vertical column of the elements, and a period is a horizontal row. In the periodic table, ionisation energy increases across a period, but decreases as you move down a group. The lower the ionisation energy, the more reactive the element will be because there is a greater chance of electrons being involved in chemical reactions. We will look at this in more detail in the next section.

The formation of ions

Match the information in column A with the information in column B by writing only the letter (A to I) next to the question number (1 to 7)

Table 1
1. A positive ion that has 3 less electrons than its neutral atom A. Mg2+2+
2. An ion that has 1 more electron than its neutral atom B. Cl--
3. The anion that is formed when bromine gains an electron C. CO32-32-
4. The cation that is formed from a magnesium atom D. Al3+3+
5. An example of a compound ion E. Br2-2-
6. A positive ion with the electron configuration of argon F. K++
7. A negative ion with the electron configuration of neon G. Mg++
  H. O2-2-
  I. Br--

Click here for the solution

The arrangement of atoms in the periodic table

Figure 3

The periodic table of the elements is a method of showing the chemical elements in a table. Most of the work that was done to arrive at the periodic table that we know, can be attributed to a man called Dmitri Mendeleev in 1869. Mendeleev was a Russian chemist who designed the table in such a way that recurring ("periodic") trends in the properties of the elements could be shown. Using the trends he observed, he even left gaps for those elements that he thought were 'missing'. He even predicted the properties that he thought the missing elements would have when they were discovered. Many of these elements were indeed discovered and Mendeleev's predictions were proved to be correct.

To show the recurring properties that he had observed, Mendeleev began new rows in his table so that elements with similar properties were in the same vertical columns, called groups. Each row was referred to as a period. One important feature to note in the periodic table is that all the non-metals are to the right of the zig-zag line drawn under the element boron. The rest of the elements are metals, with the exception of hydrogen which occurs in the first block of the table despite being a non-metal.

Figure 4: A simplified diagram showing part of the Periodic Table
Figure 4 (CG10C3_014.png)

Groups in the periodic table

A group is a vertical column in the periodic table and is considered to be the most important way of classifying the elements. If you look at a periodic table, you will see the groups numbered at the top of each column. The groups are numbered from left to right as follows: 1, 2, then an open space which contains the transition elements, followed by groups 3 to 8. These numbers are normally represented using Roman numerals. In some periodic tables, all the groups are numbered from left to right from number 1 to number 18. In some groups, the elements display very similar chemical properties and the groups are even given separate names to identify them.

The characteristics of each group are mostly determined by the electron configuration of the atoms of the element.

  • Group 1: These elements are known as the alkali metals and they are very reactive.
    Figure 5: Electron diagrams for some of the Group 1 elements, with sodium and potasium incomplete; to be completed as an excersise.
    Figure 5 (CG10C3_015.png)
  • Group 2: These elements are known as the alkali earth metals. Each element only has two valence electrons and so in chemical reactions, the group 2 elements tend to lose these electrons so that the energy shells are complete. These elements are less reactive than those in group 1 because it is more difficult to lose two electrons than it is to lose one.
  • Group 3 elements have three valence electrons.

    Tip:

    The number of valence electrons of an element corresponds to its group number on the periodic table.
  • Group 7: These elements are known as the halogens. Each element is missing just one electron from its outer energy shell. These elements tend to gain electrons to fill this shell, rather than losing them. These elements are also very reactive.
  • Group 8: These elements are the noble gases. All of the energy shells of the halogens are full and so these elements are very unreactive.
    Figure 6: Electron diagrams for two of the noble gases, helium (He) and neon (Ne).
    Figure 6 (CG10C3_016.png)
  • Transition metals: The differences between groups in the transition metals are not usually dramatic.

Investigation : The properties of elements

Refer to Figure 5.

  1. Use a periodic table to help you to complete the last two diagrams for sodium (Na) and potassium (K).
  2. What do you notice about the number of electrons in the valence energy level in each case?
  3. Explain why elements from group 1 are more reactive than elements from group 2 on the periodic table (Hint: Think back to 'ionisation energy').

It is worth noting that in each of the groups described above, the atomic diameter of the elements increases as you move down the group. This is because, while the number of valence electrons is the same in each element, the number of core electrons increases as one moves down the group.

Periods in the periodic table

A period is a horizontal row in the periodic table of the elements. Some of the trends that can be observed within a period are highlighted below:

  • As you move from one group to the next within a period, the number of valence electrons increases by one each time.
  • Within a single period, all the valence electrons occur in the same energy shell. If the period increases, so does the energy shell in which the valence electrons occur.
  • In general, the diameter of atoms decreases as one moves from left to right across a period. Consider the attractive force between the positively charged nucleus and the negatively charged electrons in an atom. As you move across a period, the number of protons in each atom increases. The number of electrons also increases, but these electrons will still be in the same energy shell. As the number of protons increases, the force of attraction between the nucleus and the electrons will increase and the atomic diameter will decrease.
  • Ionisation energy increases as one moves from left to right across a period. As the valence electron shell moves closer to being full, it becomes more difficult to remove electrons. The opposite is true when you move down a group in the table because more energy shells are being added. The electrons that are closer to the nucleus 'shield' the outer electrons from the attractive force of the positive nucleus. Because these electrons are not being held to the nucleus as strongly, it is easier for them to be removed and the ionisation energy decreases.
  • In general, the reactivity of the elements decreases from left to right across a period.

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-1-1) and atomic number (Z) for a number of elements in the periodic table:

Table 2
Z Ionisation energy Z Ionisation energy
1 1310 10 2072
2 2360 11 494
3 517 12 734
4 895 13 575
5 797 14 783
6 1087 15 1051
7 1397 16 994
8 1307 17 1250
9 1673 18 1540
  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

Click here for the solution

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

Click here for the solution

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 ×× 10-24-24 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 ZAZAX, 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.

Figure 7

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
    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. 10201020Ne and 10221022Ne 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 9223492234U. Which of the following statements is true?
      1. The element is an isotope of 9423494234Pu
      2. The element contains 234 neutrons
      3. The element has the same electron configuration as 9223892238U
      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. 1s22 2s88 3s77
      2. 1s22 2s22 2p66 3s22 3p55
      3. 1s22 2s22 2p66 3s22 3p66
      4. 1s22 2s22 2p55
      Click here for the solution
  4. The following table shows the first ionisation energies for the elements of period 1 and 2.
    Table 3
    PeriodElementFirst ionisation energy (kJ.mol-1kJ.mol-1)
    1H1312
     He2372
     Li520
     Be899
     B801
     C1086
    2N1402
     O1314
     F1681
     Ne2081
    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.
    Click here for the solution

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