Stability and Ionic Bonding

1. Stability and Ionic Bonding Section 20.1 & 20.2

2. SC Standards Covered • Standard PS-4.1 • Explain the role of bonding in achieving chemical stability. • Standard PS-2.5 • Predict the charge that a representative element will acquire according to the arrangement of electrons in its outer energy level. • Standard PS-4.6 • Predict the ratio by which the representative elements combine to form binary ionic compounds, and represent that ratio in a chemical formula. • Standard PS-4.4 • Classify compounds as crystalline (containing ionic bonds) or molecular (containing covalent bonds) based on whether their outer electrons are transferred or shared.

3. Combined Elements • Most elements are not found by themselves in nature • Usually they are found combined with other elements • Two or more elements chemically combined is called a compound • The properties of a compound are very different from the properties of the elements that make up the compound

4. Consider Table Salt…

5. Chemical Formulas • Chemical formula – tells what elements and how many of each element are in a unit of that compound • H2O – water; contains two atoms of hydrogen and one atom of oxygen • NH3 – ammonia; contains one atom of nitrogen and 3 atoms of hydrogen H H O N H H H

6. Mg(OH)2 – milk of magnesia – contains one atom of magnesium, two atoms of oxygen, and two atoms of hydrogen Mg O O H H

7. Atomic Stability Why do Elements Form Compounds? - the noble gases are extremely stable atoms that do not combine with other elements. - it is the eight valence electrons that make the noble gas atoms stable

8. An atom is chemically stable when its outermost energy level is complete • 1st level – 2 electrons • All other levels – 8 electrons • All elements combine with other elements by losing, gaining or sharing electrons so that they will have eight valence electrons!!

9. - When atoms begin to lose, share, or gain electrons they begin to get attracted to other atoms and form chemical bonds - a force that holds atoms together in a substance. x5

10. Ionic Bonding • Atoms that gain or lose electrons become charged forming an ion. What ion will Sodium form? Since sodium has 1 valence electron it can lose that electron and then have eight valence elect. 1 e- 8 e- 11 p+ 12no 2 e-

11. This will give the sodium atom a +1 charge • 11p+ + 10e- = +1Na+1 Superscript – written above line; gives the charge of ion Group 1 → 1 valence e- → +1 Group 2 → 2 valence e- → +2 Group 13 → 3 valence e- → +3

12. What ion will fluorine form? 7e- Fluorine needs to gain 1e- in order to become stable 2e- 9p+ 10n0 - This will give fluorine a -1 charge F-1 Group 17 → 7 valence e-→ -1 Group 16 → 6 valence e- → -2 Group 15 → 5 valence e- → -3 x6

13. Using electron dot structure to show ionic bonds Explain the formation of the ionic bond between sodium and chlorine Na Cl Na+[ Cl ]-1 NaCl Electron dot struct. Formula unit

14. Steps to draw electron dot structures for ionic compounds • Draw the electron dot structures of the elements • transfer electrons from the metal to the nonmetal • add elements as needed • continue transferring until all atoms are stable

15. 5. Write out the chemical formula using subscripts to show how many of each element it took

16. Lets try aluminum and oxygen!! Al O 2 Al+3 [ O ]-2 3 O Al Al2O3 O

17. Your Turn!! Use electron dot structures to write chemical formulas for the following combination of elements: • potassium and fluorine • Magnesium and fluorine • Aluminum and nitrogen • Magnesium and phosphorous