Recap Atomic Structure

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 20 Ne 10

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

4. Ions • More than one electron may be gained/lost but >3 electrons not common. O  O2- N  N3- Mg  Mg2+ Al  Al3+ 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-metalstend to form anions • Metals tend to form cations

8. Ions 1+ 2- 1- 2+ 1+, 2+ or 3+

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

10. 2- O + O 2+ + Mg Mg Ions Ne Examples: O, Mg • Mg Mg2+ + 2 e- • O + 2 e-O2-

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

12. - + Na Cl Ionic Bonding • The electrostatic attraction of cations and anions results in ionic bonds being formed. 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 2- 2+ + Mg O Mg + O  Mg2+ + O2- One Mg2+ for every one O2- so compound is MgO

15. + + Na Na O 2- + + + + Na Na O 2Na + O  2Na+ + O2- Two Na+ for every one O2- so compound is Na2O

16. Al Al O O O 3+ 3+ Al Al 2- 2- 2- O O O 2Al + 3O  2Al3+ + 3O2- Two Al3+ for every three O2- so compound is Al2O3

17. Ionic Bonding • Produces a solid - crystal lattice. • Cations and anions are packed so as to maximise the attractive forces and minimiserepulsion. • 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

19. 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….)

20. Questions to complete for the next lecture: • Predict the formula of the ionic compound formed between the following pairs: • Li and Br • Li and S • Li and N • Mg and Br • Mg and S • Mg and N • Al and F • Al and O • Al and N