Unit 4 - Bonding Part 3 - Covalent Compounds

Molecules Covalent bond – a bond formed when atoms share one or more pairs of electrons Covalent bond – a bond formed when atoms share one or more pairs of electrons Molecule – neutral group of atoms held together by covalent bonds (another name for covalent compound) Molecule – neutral group of atoms held together by covalent bonds (another name for covalent compound) Properties – relatively low melting and boiling points; generally poor conductors Properties – relatively low melting and boiling points; generally poor conductors

Molecular Formulas Molecular formula – chemical formula that shows how many atoms of each element a molecule contains. Molecular formula – chemical formula that shows how many atoms of each element a molecule contains. Examples: H 2 O, CO 2, C 2 H 6 O, (NH 4 ) 2 SO 4 Examples: H 2 O, CO 2, C 2 H 6 O, (NH 4 ) 2 SO 4

Diatomic Elements Diatomic molecule – molecule that has two of the same atoms in it Diatomic molecule – molecule that has two of the same atoms in it The following elements are ALWAYS present in this form. They do not exist as one atom alone. The following elements are ALWAYS present in this form. They do not exist as one atom alone. H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 You need to know these. You need to know these.

The Covalent Bond The octet rule still rules! The octet rule still rules! Atoms share electrons to fulfill the octet rule. Atoms share electrons to fulfill the octet rule. Usually between 2 nonmetals Usually between 2 nonmetals Single bond – share 1 pair of electrons Single bond – share 1 pair of electrons Double bond – share 2 pair of electrons Double bond – share 2 pair of electrons Triple bond – share 3 pair of electrons Triple bond – share 3 pair of electrons

Lewis Electron-Dot Structures The Lewis Dot system is for keeping track on valence electrons; not for showing actual location of electrons The Lewis Dot system is for keeping track on valence electrons; not for showing actual location of electrons Valence electrons are represented by dots Valence electrons are represented by dots Octet rule is the driving factor; but is sometimes broken Octet rule is the driving factor; but is sometimes broken

Lewis Dot cont’d Unshared pair or lone pair – nonbonding pair of electrons in the valence shell of an atom; represented by two dots Unshared pair or lone pair – nonbonding pair of electrons in the valence shell of an atom; represented by two dots Single bond – 1 dash (2 electrons) Single bond – 1 dash (2 electrons) Double bond –2 dashes (4 electrons) Double bond –2 dashes (4 electrons) Triple bond – 3 dashed (6 electrons) Triple bond – 3 dashed (6 electrons)

Polyatomic Ions When counting up valence electrons, subtract one electron for each + charge and add one electron for each – charge When counting up valence electrons, subtract one electron for each + charge and add one electron for each – charge Charge is indicated with brackets around the Lewis dot structure and the charge written as superscript outside the bracket Charge is indicated with brackets around the Lewis dot structure and the charge written as superscript outside the bracket

Naming Covalent Compounds Prefixes indicate number of atoms in the molecule; no ions, so no counting charges! Prefixes indicate number of atoms in the molecule; no ions, so no counting charges! Example: P 2 S 5 Example: P 2 S 5 diphosphorus pentasulfide diphosphorus pentasulfide Prefix (if more than 1) + Name of least electronegative atom Prefix that show the # of atoms + Root name of more electronegative atom + “ide”

Some Naming Rules If the first element named has only one atom, the –mono is dropped. If the first element named has only one atom, the –mono is dropped. The vowels a and o are dropped from a prefix that is added to a word beginning with a vowel. For example: CO is carbon monoxide and N 2 O 4 is dinitrogen tetroxide. The vowels a and o are dropped from a prefix that is added to a word beginning with a vowel. For example: CO is carbon monoxide and N 2 O 4 is dinitrogen tetroxide.

Prefixes – Know These Mono-1Hexa-6 Di-2Hepta-7 Tri-3Octa-8 Tetra-4Nona-9 Penta-5Deca-10

Examples CO carbon monoxide CeB 6 cerium hexaboride SiO 2 silicon dioxide IF 7 iodine heptafluoride SO 3 silicon trioxide Np 3 O 8 trineptunium octoxide SCl 4 sulfur tetrachloride I4O9I4O9I4O9I4O9 tetraiodine nonoxide SbCl 5 antimony pentachloride S 2 F 10 disulfur decafluoride

Which naming method?? How do you know when to name your compound with the ionic naming rules or the covalent naming rules? How do you know when to name your compound with the ionic naming rules or the covalent naming rules? 1. Determine if it is an ionic compound or covalent compound (electronegativity difference) 2. Two non-metals are always covalent