1 Recap Atomic Structure Nucleus contains p + and n. The number of p + defines the element. Mass of atom = Σ (p + + n) Electrons occupy orbits of defined energy. Electron configuration related to position in Periodic Table Ne 10 20

2 Ions Neutral atoms always have the same number of electrons as the number of protons in the nucleus. Many atoms may gain or lose electrons to form ions, which have a charge.

3 Gain of electrons gives an anion. e.g. F + e -  F - Ions Loss of electrons gives a cation. e.g. Li  Li + + e - 9 e - 10 e - 3 e - 2 e -

4 Ions More than one electron may be gained/lost but >3 electrons not common. O  O 2  N  N 3  Mg  Mg 2+ Al  Al 3+ Note: charge is always shown at the top right the sign comes after the number. Eg.

5 Why do ions form? Note on Noble Gases: Last column of periodic table. 8 electrons in outermost shell (2 for He). Stable electronic structure. Do not gain/lose electrons. Occur as isolated atoms.

6 Noble Gases He: 2 electrons therefore n = 1 totally full. Ne: 10 electrons therefore n = 1 and 2 totally full.

7 Predicting which ions form? Atoms gain or lose electrons to become isoelectronic as the nearest Noble gas. Non-metals tend to form anions Metals tend to form cations

8 Ions , 2+ or 3+

9 Ions Examples: Li + + Ne He Li  Li + + e - F F + - F + e -  F -

Ions Examples: O, Mg O + O 2-2- Ne O + 2 e -  O Mg + 2+ Mg  Mg e -

11 Na + Cl  Na + + Cl - Cl Na + + Cl - +

12 Ionic Bonding The electrostatic attraction of cations and anions results in ionic bonds being formed. Cl - Na + Crystal structure of sodium chloride, NaCl

13 Ionic Bonding There must always be metal atom (to lose electrons) and a non-metal atom (to gain them). Production of ions always results from complete transfer of electrons between bonding atoms.

14 Mg + O  Mg 2+ + O 2- O 2-2- Mg 2+ + O Mg + One Mg 2+ for every one O 2- so compound is MgO

15 2Na + O  2Na + + O 2- O 2-2- Na O + + Two Na + for every one O 2- so compound is Na 2 O

16 2Al + 3O  2Al O 2- O OO Al O 2- Al 3+ O 2- O Al 3+ Two Al 3+ for every three O 2- so compound is Al 2 O 3

17 Ionic Bonding Produces a solid - crystal lattice. Cations and anions are packed so as to maximise the attractive forces and minimise repulsion. Ratio of cations to anions ensures ZERO overall charge on the compound. Cation-anion attraction does not depend on direction: non-directional bonding.

18 Ionic Bonding Different ionic compounds have different crystal structure Sodium Chloride NaCl Zinc Sulfide ZnS

Learning Outcomes: By the end of this lecture, you should: –be able to work out the number of electrons an ion has from its symbol –recognize that most ions have a noble gas configuration –predict whether an element will form a cation or an anion –predict the charge on the cation or anion an element will form –be able to balance the charges –explain the characteristics of ionic bonding –be able to complete the worksheet (if you haven’t already done so….) 19

Questions to complete for the next lecture: 20 Predict the formula of the ionic compound formed between the following pairs: a)Li and Br b)Li and S c)Li and N d)Mg and Br e)Mg and S f)Mg and N g)Al and F h)Al and O i)Al and N