1. 1 Chem 400 Chapter 3 Brief Intro to Chemical Bonding

2. 2 When atoms react with one another in rxns, the nucleus remains unchanged. However, the electrons of the atoms interact, forming chemical bonds to create compounds. In other words, electrons are responsible for the chemical properties of substances. There are several types of chemical bonds, with the 2 major types being ionic bonds and covalent bonds. Chemical Bonding

3. 3 A covalent bond is a chemical bond where the electrons of 2 atoms share electrons between the 2 nuclei. The electron clouds overlap, forming the bond. In general, covalent bonds form between 2 or more nonmetal atoms or metalloid atoms. Chemical substances held together by covalent bonds are called molecules. Covalent Bonding

4. 4 Water is a molecule with 2 H atoms covalently bonded to an oxygen atom. We can represent a water molecule with a ball and stick model or a space-filling model. We can also use structural formulas to represent or draw molecules. Covalent Bonding

5. 5 Representations of Molecules Space-Filling Model

6. 6 Some elements are actually molecules: H 2, N 2, O 2, I 2, F 2, Cl 2, and Br 2 are all diatomic molecules or diatomic elements. Memorize these 7! Know their formulas! Notice that although they are molecules, they are elements, and NOT chemical compounds. (Remember, chemical compounds contain 2 or more DIFFERENT elements.) Sulfur exists as S 8 while phosphorus exists as P 4 Covalent Bonding

7. 7 Ionic bonds are formed when electrons are transferred between 2 atoms. 1 atom will give up or lose 1 or more electrons, while the 2nd atom will gain 1 or more electrons. Ionic bonds commonly form between metals and nonmetals. Ions and Ionic Bonding

8. 8 Atoms which have gained or lost electrons are no longer neutral but have a charge, and so are called ions. A cation is a positively charged ion (lost electrons) An anion is a negatively charged ion (gained electrons) We can draw a picture and then write a chemical equation for the formation of ions: Ions and Ionic Bonding

9. 9 Ion Formation

10. 10 Once the ions are formed, can you see how the ionic bond is formed? What do opposite charges do? Equation for formation of NaF, an ionic solid: Na + + F -  NaF Ions and Ionic Bonding

11. 11 Unlike molecules which are individual discrete compounds, ionic compounds (solids at room temperature) are actually not just 1 cation bonded to 1 anion, but a huge network of cations and anions. For example, sodium chloride is NaCl, or table salt. Based on the chemical formula, you might think that there is a unit of 1 Na cation bonded to 1 Cl anion. In reality, each Na+ cation is surrounded by 6 Cl- anions, and each Cl- anion is surrounded by 6 Na+ cations. These cations and anions are packed close together in what is called a crystal lattice. Ions and Ionic Bonding

12. 12 Ions and Ionic Bonding

13. 13 So why does sodium chloride have the formula NaCl and not Na 6 Cl 6 or as Na x Cl x ? The rule is that we express ionic compounds as Empirical Formulas, that is the lowest, most reduced formula. Ions and Ionic Bonding

14. 14 Although many ions are simple monatomic ions, composed of just 1 element, there are also many polyatomic ions. Ions and Ionic Bonding

15. 15 Polyatomic ions are ions containing 2 or more elements. Examples include: Polyatomic ions are actually atoms covalently bonded together, but the group of atoms has an overall charge so it is an ion (actually a molecular ion). Ions and Ionic Bonding

16. 16 The hydrogen cation, H +, the hydroxide anion, OH -, and the hydronium ion, H 3 O +, are 3 of the most important ions in chemistry. The reason is that these ions are what defines acid- base chemistry. And H + is the shorthand notation for H 3 O + in water. Acids and Bases

17. 17 The first definition of an acid and base was the Arrhenius Theory: Acid: a substance which produces a proton in water Ex: HCl, HF, HNO 3 Base: a substance which produces the hydroxide ion in water Ex: NaOH, LiOH Acids and Bases

18. 18 Notice that in the above examples, the bases are ionic salts as well as bases. But the acids are molecules which just happen to "ionize" or lose an H + in water. HCl for example stays intact as HCl in the gas phase. HCl readily ionizes to become H + and Cl - when it is dissolved in water. Acids and Bases

19. 19 We show whether an acid or base is dissolved in water or is a gas, liquid, or solid by showing the following notations: HCl(aq) HCl(g) CH 3 CO 2 H(l) NaOH(s) There are also polyprotic acids, like sulfuric acid, H 2 SO 4, which may lose more than 1 proton when added to water: Acids and Bases

20. 20 There are also bases which produce more than 1 OH- anion: Acids and Bases