1 Chapter 4 Formation of Compounds Objectives Distinguish between elements and propertiesDistinguish between elements and properties Describe molecular and ionic compounds and differentiate between them.Describe molecular and ionic compounds and differentiate between them. Write electron dot diagrams of simple molecular compoundsWrite electron dot diagrams of simple molecular compounds

2 physically separable chemically separable, table salt Remember?

3 Elements and Compounds Elements are made from one type of atoms Compounds are made from more than one type of atoms. Compounds can be separated into simpler substances only by chemical means. Properties of a compound are usually different from those of the elements composing it. Each element is represented by a one- or two-letter chemical symbol, used to write chemical formulas of compounds. Elements are always present in the same ratio in a given compound.

4 Sodium chloride occurs naturally in large, solid, underground deposits throughout the world and is dissolved in the world’s oceans. Salt can be obtained by mining these solid deposits, and by the evaporation of seawater. Salt: A Familiar Compound

5 sodium chloride, table salt, NaCl sodium, Na chlorine, Cl 2 Chlorine gas and sodium metal are highly reactive, sodium chloride is much more stable (less reactive)

6 Noble Gas Elements Noble Gas Elements, Are the only elements that exist as isolated atoms in nature. They are monoatomic. They are extremely unreactive (inert gases), only very few compounds are known, none in nature.

7

8 Octet rule Atoms can become stable by having eight electrons in their outer energy level, (or two electrons in the case of some of the smallest atoms), also called a noble gas configuration. Elements become stable by achieving the same configuration of valence electrons as the closest noble gas.

9 Less stable atoms try to achieve noble gas configuration by a)gaining or losing, or b)sharing valence electrons Exception

10 Electrons Can Be Transferred Sodium lost one electron, it now has one more proton than electrons and therefore a positive charge. Chlorine now has an extra electron, and therefore a negative charge.

11 Ionic Compounds are composed of positive and negative ions, typically metal + nonmetal Nucleus 11 p + Nucleus 17 p + 11 e - 17 e - 10 e - 18 e - Nucleus 11 p + Nucleus 17 p + lose one electron -1e - gain one electron +1e - Sodium atom (Na) Sodium ion (Na + ) Chloride ion (Cl - ) Chlorine atom (Cl) Note that only the arrangement of electrons has changed. The atom’s nucleus remains unchanged.

12 NaCl formation

13 ionic compoundsAlthough composed of ions, ionic compounds are electrically neutral, because the total positive charge of the cations is equal to the total negative charge of the anions. (+ and – cancel out) Chloride anion Sodium cation 18 e - 17 p + 10 e - 11 p + Arrangement of Na + and Cl - ions in crystals of NaCl Crystals of sodium chloride Ionic compounds are usually formed from metallic with nonmetallic elements and they are crystalline solids at room temp.

14 Anion: A N egative ION

15 More physical properties of ionic compounds Tendency to dissolve in water Electrolytes compound that conducts electricity when melted or dissolved in water. In order to conduct electricity, ions must be free to move because they must take on or give up electrons.

16 Formation of Water by Electron Sharing In a water molecule the eight valence electrons (six from oxygen and one each from two hydrogens) are distributed among the three atoms. By sharing an electron pair with the oxygen, each hydrogen claims two electrons in its outer level. The oxygen, by sharing two electrons with two hydrogens, claims a stable octet in its outer level. Thus, each atom achieves a stable noble gas configuration.

17 Electron Sharing Produces Molecules molecule A molecule is an uncharged group of two or more atoms held together by covalent bonds. Ethanol, also known as ethyl alcohol, is a typical covalent compound, as is carbon dioxide.

18 A single covalent bond is a shared pair of valence electrons Nonmetal atoms form covalent bonds when they share electrons to form an octet. Covalent bond Attraction of two atoms for a shared pair of electrons (no ions)

19 Pairs of valence electrons that are not shared between atoms are called unshared, free, lone or nonbonding electron pairs.

20 Sometimes two or three pairs of electrons may be shared to give double or triple covalent bonds

21 NaCl formations and H2O formation

22 Compounds are substances composed of two or more different elements chemically combined Compounds are substances composed of two or more different elements chemically combined. Two types… They can be either molecular compounds or ionic compounds Ionic Compounds Made from ions Metal + nonmetals Compounds Molecular Compounds Made from molecules all nonmetals

23 Differences Between Ionic and Molecular Compounds

24 Practice Problems… Remember: metal + nonmetal = ionic all nonmetals = molecular Will these elements form ionic or molecular compounds? a.Na and Brb. C and H c. Al and Od. Ti and Oe. N and O IonicMolecular Ionic Molecular

25 Molecular Formula Perspective Drawing Structural Formula Ball-and Stick Molecular Model Space-filled Molecular Model Different ways to display molecular formulas

26 water H 2 O 2 hydrogen atoms and 1 oxygen atom Carbon dioxide CO 2 1 carbon atom and 2 oxygen atom2 Examples of molecular compounds