3 The Periodic Table is arranged such that the properties of the elements show a number of trends. (a) A plot of the first ionisation energies for the first 18 elements is shown. 2500- M 2000- ionisation 1500- energy /kJ mol-1 1000- 500- 0+ 2 4 6 10 12 14 16 18 8 atomic number (i) Explain why the values show a general increase from atomic number 11 to 18. [2]. ​