Figure 20 Along the horizontal axis of the graph are two sets of labels. The first set, labelled 'Group 2', are Be, Mg, Ca, Sr and Ba. The second set, labelled 'Group 13' and positioned under the first, are B, Al, Ga, In and Tl.
The vertical axis is labelled 'ionisation energy in kJ mol superscript -1', and the values 500, 600, 700, 800 and 900 have been marked along the axis.
Two sets of five points have been plotted on the graph. One set has been joined together with a blue line, labelled 'Group 2', the other set has been joined together with an orange line labelled 'Group 13'.
For the 'Group 2' set the point above Be corresponds to about 900 on the vertical axis, the point above Mg to about 750, the point above Ca to about 600, the point above Sr to about 570, and the point above Ba to about 500.
For the 'Group 13' set, the point above B corresponds to about 800 on the vertical axis, the point above Al to about 580, the point above Ga to about 580, the point above In to about 570 , and the point above Tl to about 600.