Molecules and Covalent Compounds The Octet Rule, Covalent Compounds and Their Names Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Valence Electrons Valence electrons Determine the chemical properties of the elements. Are the electrons in the highest energy level. Are related to the Group number of the element. Example: Phosphorus has 5 valence electrons 5 valence electrons P Group 5A(15) 2, 8, 5

Groups and Valence Electrons All the elements in a group have the same number of valence electrons. Example: Elements in Group 2A(2) have two (2) valence electrons. Be 2, 2 Mg 2, 8, 2 Ca 2, 8, 8, 2 Sr 2, 8, 18, 8, 2

Periodic Table and Valence Electrons Representative Elements Group Numbers 1 2 13 14 15 16 17 18 H He 1 2 Li Be Al C N O F Ne 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8 Li Mg Ge Si P S Cl Ar 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8

Learning Check State the number of valence electrons for each: A. O 1) 4 2) 6 3) 8 B. Al 1) 13 2) 3 3) 1 C. Cl 1) 2 2) 5 3) 7

Solution State the number of valence electrons for each. A. O 2) 6 2) 6 B. Al 2) 3 C. Cl 3) 7

Octet Rule Atoms lose, gain or share valence electrons in order to achieve noble gas electron configurations (an octet in the outer shell). Exception is He that is stable with 2 valence electrons (duet). valence electrons He 2 2 Ne 2, 8 8 Ar 2, 8, 8 8 Kr 2, 8, 18, 8 8

Forming Octets Atoms acquire octets By forming compounds. To become more stable. By losing, gaining, or sharing valence electrons. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Ionic and Covalent Bonds Ionic bonds involve Loss of electrons by a metal. Gain of electrons by a nonmetal. Covalent bonds involve A sharing of electrons. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Forming a H2 Molecule Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

H2, A Covalent Molecule In a hydrogen (H2) molecule Two hydrogen atoms share electrons to form a covalent single bond. Each H atom acquires two (2) electrons. Each H becomes stable like helium (He). Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Diatomic Elements These elements share electrons to form diatomic, covalent molecules. Table 5.9 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check What is the name of each of the following diatomic molecules? H2 ______________________ N2 _______________ Cl2 _______________ O2 _______________ I2 _______________

Solution What are the names of each of the following diatomic molecules? H2 hydrogen N2 nitrogen Cl2 chlorine O2 oxygen I2 iodine

Number of Covalent Bonds The number of covalent bonds can be determined from the number of electrons needed to complete an octet. Table 10.1 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Electron-Dot Formulas Electron-dot formulas show The order of bonded atoms in a covalent compound. The bonding pairs of electrons between atoms. The unshared (lone) valence electrons. A central atom with an octet. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Guide to Writing Electron-Dot Formulas STEP 1 Determine the arrangement of atoms. STEP 2 Add the valence electrons from all the atoms. STEP 3 Attach the central atom to each bonded atom using one pair of electrons. STEP 4 Add remaining electrons as lone pairs to complete octets (2 for H atoms). STEP 5 If octets are not complete, form one or more multiple bonds.

Electron-Dot Formulas and Models of Some Covalent Compounds Table 5.10 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Names of Covalent Compounds Prefixes are used In the names of covalent compounds. Because two nonmetals can form two or more different compounds. Examples of compounds of N and O: NO nitrogen oxide NO2 nitrogen dioxide N2O dinitrogen oxide N2O4 dinitrogen tetroxide N2O5 dinitrogen pentoxide

Naming Covalent Compounds STEP 1 Name the first nonmetal as the element. STEP 2 End the name of the second nonmetal with -ide STEP 3 Use prefixes to show the number of atoms (subscripts). Mono is usually omitted. Table 5.11 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Naming Covalent Compounds What is the name of SO3? STEP 1 The first nonmetal is S sulfur. STEP 2 The second nonmetal is O named oxide. STEP 3 The subscript 3 of O is shown as the prefix tri. SO3 → sulfur trioxide The subscript 1(for S) or mono is understood.

Naming Covalent Compounds Name P4S3 STEP 1 The first nonmetal P is phosphorus. STEP 2 The second nonmetal S is sulfide. STEP 3 The subscript 4 of P is shown as tetra. The subscript 3 of O is shown as tri. P4S3 → tetraphosphorus trisulfide

Formulas and Names of Some Covalent Compounds TABLE 4.13 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Select the correct name for each compound. A. SiCl4 1) silicon chloride 2) tetrasilicon chloride 3) silicon tetrachloride B. P2O5 1) phosphorus oxide 2) phosphorus pentoxide 3) diphosphorus pentoxide C. Cl2O7 1) dichlorine heptoxide 2) dichlorine oxide 3) chlorine heptoxide

Solution Select the correct name for each compound. A. SiCl4 3) silicon tetrachloride B. P2O5 3) diphosphorus pentoxide C. Cl2O7 1) dichlorine heptoxide

Learning Check Write the name of each covalent compound: PCl3 _____________________ CCl4 _____________________ N2O _____________________

Solution Write the name of each covalent compound: CO carbon monoxide CO2 carbon dioxide PCl3 phosphorus trichloride CCl4 carbon tetrachloride N2O dinitrogen oxide

Guide to Writing Formulas STEP 1 Write the symbols in the order of the elements in the name. STEP 2 Write any prefixes as subscripts. Example: Write the formula for carbon disulfide. STEP 1 Elements are C and S STEP 2 No prefix for carbon means 1 C Prefix di = 2 Formula: CS2

Learning Check Write the correct formula for each of the following: A. phosphorus pentachloride B. dinitrogen trioxide C. sulfur hexafluoride

Solution Write the correct formula for each of the following: A. phosphorus pentachloride 1 P penta = 5 Cl PCl5 B. dinitrogen trioxide di = 2 N tri = 3 O N2O3 C. sulfur hexafluoride 1 S hexa = 6 F SF6