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CI 3.2 The size of ions Ions in solution • Attractions to other ions and to water molecules depend on: • Charge on the ion • Size of the ion Highly charged ions are strongly attracted to other ions and to water molecules 3- 3+ 3+ 3- Small ions are strongly attracted to other ions and to water molecules because they can get close Water molecule Charge density • An ion which has a high charge and a small size has a high charge density 3+ Al3+ I- High charge density Low charge density Ions with high charge density • Attract water molecules strongly Become very hydrated • Attract other ions strongly, forming lattices with strong ionic bonds Have high melting points Trends in size • Moving down a group in the Periodic Table: • Atomic radius increases Why? Look at table 4, page 42 • Moving across the Periodic Table from left to right: • Atomic radius decreases • Na > Mg > Al Why?? Atomic radius • This depends on: • Number of protons in nucleus • Number of electron shells Moving from left to right • An extra proton is added each time • An extra electron is added, but into the same shell • Each electron is pulled more strongly towards the nucleus • Result = a smaller atom Trend in ionic radius • Moving from left to right • Ionic radius decreases initially, increases and then decreases across the period Why? Ionic radius - Cations • Moving across the period an extra proton is added each time • But one more electron is lost each time to form the ion • So, moving from left to right, the nuclear charge is increasing, but the negative charge stays the same so the electrons are pulled more tightly towards the nucleus • Result = size of cation decreases from left to right Na+ > Mg2+ > Al3+ Example Element Protons Electrons Electron configuration Na+ 11 10 1s22s22p6 Mg2+ 12 10 1s22s22p6 Al3+ 13 10 1s22s22p6 So why is there an increase in ionic radius in the middle? • Metal atoms lose their outer shell electrons to form ions, but non-metal atoms gain extra electrons into the outer shell • So as you move from a metal to a nonmetal, there is an extra shell of electrons • Result = a bigger ion Compare the following ions in period 3 • Al3+ 1s22s22p6 • P3- 1s2 2s2 2p63s23p6 Atomic no = 13 Atomic no = 15 P3- has one more shell of electrons than Al3+ Ionic radius - Anions • As you move further across to the right, the ionic radius decreases again ? • The ion of the next element has two more electrons than its atom • But it has one more proton & the same number of electrons as the previous ion • This results in the ion being larger than its atom, but smaller than the previous ion Compare the following anions in period 2 Element Protons Electrons Electron configuration N3- 7 10 1s2 2s2 2p6 O2- 8 10 1s2 2s2 2p6 F- 9 10 1s2 2s2 2p6 So, to sum up… • From left to right across the Periodic Table: • Metal ions decrease in size • Changing from metal ions to non-metal ions, there is a big increase in size • Non-metal ions decrease in size Hydration & Size • Hydration adds one or more layers of water molecules to an ion • The smaller the ion and the higher the charge, the more water molecules will be attracted • So small, highly charged ions become large when they are hydrated. Question time • Have a go at the problems ?