0.3 The periodic table  (Page 5/5)

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.

Trends in ionisation energy

Refer to the data table below which gives the ionisation energy (in $\mathrm{kJ}·\mathrm{mol}{}^{-1}$ ) 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 $Z=7$

By now you should have an appreciation of what the periodic table can tell us. The periodic table does not just list the elements, but tells chemists what the properties of elements are, how the elements will combine and many other useful facts. The periodic table is truly an amazing resource. Into one simple table, chemists have packed so many facts and data that can easily be seen with a glance. The periodic table is a crucial part of chemistry and you should never go to science class without it.

The following presentation provides a summary of the periodic table

Summary

• Elements are arranged in periods and groups on the periodic table. The elements are arranged according to increasing atomic number.
• A group is a column on the periodic table containing elements with similar properties. A period is a row on the periodic table.
• The groups on the periodic table are labeled from 1 to 8. The first group is known as the alkali metals, the second group is known as the alkali earth metals, the seventh group is known as the halogens and the eighth group is known as the noble gases. Each group has the same properties.
• Several trends such as ionisation energy and atomic diameter can be seen across the periods of the periodic table
• 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.

End of chapter exercises

1. For the following questions state whether they are true or false. If they are false, correct the statement.
   1. The group 1 elements are sometimes known as the alkali earth metals.
   2. The group 2 elements tend to lose 2 electrons to form cations.
   3. The group 8 elements are known as the noble gases.
   4. Group 7 elements are very unreactive.
   5. The transition elements are found between groups 3 and 4.
2. Give one word or term for each of the following:
   1. A positive ion
   2. The energy that is needed to remove one electron from an atom
   3. A horizontal row on the periodic table
   4. A very reactive group of elements that is missing just one electron from their outer shells.
3. For each of the following elements give the ion that will be formed:
   1. sodium
   2. bromine
   3. magnesium
   4. oxygen
   Click here for the solution
4. The following table shows the first ionisation energies for the elements of period 1 and 2.

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