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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. 3+. 3-. 3-. 3+. Highly charged ions are strongly attracted to other ions and to water molecules.
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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
3+ 3- 3- 3+ Highly charged ions are strongly attracted to other ions and to water molecules
Small ions are strongly attracted to other ions and to water molecules because they can get close Water molecule
3+ Al3+ I- Charge density • An ion which has a high charge and a small size has a high charge density High charge density Low charge density
Become very hydrated Have high melting points Ions with high charge density • Attract water molecules strongly • Attract other ions strongly, forming lattices with strong ionic bonds
Look at table 4, page 42 Why? Trends in size • Moving down a group in the Periodic Table: • Atomic radius increases
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+
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 non-metal, there is an extra shell of electrons • Result = a bigger ion
Compare the following ions in period 3 • Al3+ 1s22s22p6 Atomic no = 13 • P3- 1s2 2s2 2p63s23p6 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 ?
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