TOPIC 4: BONDING 4.2: COVALENT BONDING

Topic 4: Bonding 4.2: Covalent bonding Essential Idea: Covalent compounds form by the sharing of electrons. Nature of Science: Looking for trends and discrepancies – compounds that contain non-metals have different properties from compounds that contain non-metals. (2.5) Use theories to explain natural phenomena – Lewis introduced a class of compounds which share electrons. Pauling used the idea of electronegativity to explain unequal sharing of electrons (2.2)

IB Topic 4: Bonding 4.2: Covalent bonding Understandings: 1. A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. 2. Single, double, and triple covalent bonds involve one, two, and three shared pairs of electrons, respectively. 3. Bond length decreases and bond strength increases as the number of shared electrons increases. 4. Bond polarity results from the difference in electronegativities of the bonded atoms.

IB Topic 4: Bonding 4.2: Covalent bonding Applications and Skills: 1. Deduction of the polar nature of a covalent bond from electronegativity values

IB Topic 4: Bonding Ionic Bond: Transfer of electrons from metal to non-metal; bond formed by electrostatic attraction between oppositely charged ions. E.N. difference >1.8 Covalent Bond: Sharing of electrons between non-metals

4.2 U1 Covalent Bond Covalent bonds occur between nonmetals since both want to gain electrons. They share electrons to achieve a noble gas (stable) configuration (8 valence e-).

4.2 U1 Covalent Bond The electrons in the bond are electrostatically attracted by both nuclei, so that it forms a directional bond between the two atoms.

4.2 U1. Electron sharing Covalent bonds occur between nonmetals since both want to gain electrons. They share electrons to achieve a stable configuration Lewis Dot Diagrams are used to illustrate the sharing of electrons Need 1 electron: H F Cl Br I Need 2 electrons:OSSeTe Need 3 electrons:NPAs Need 4 electrons:CSi (spread out e- if 4)

Key Terms: Lone pairs: electrons on a dot diagram that are already paired (also called non-bonding pairs) Shared pairs (Bonded pair): electrons that are shared in a covalent bond H S H 4.2 U1. Electron sharing Lone pair Shared pair

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? FF + 7e - FF 8e - F F F F Lewis structure of F 2 lone pairs single covalent bond

4.2 U2. Bond Types Single covalent bonds Sharing 1 pair of electrons Examples: H 2 H H H H HClH Cl H Cl CCl 4 Double covalent bonds Sharing 2 pair of electrons Examples: O2O2 H 2 CO

4.2 U2. Bond Types Triple covalent bonds Sharing 3 pair of electrons Examples: N 2 CH 2 N N

8e - H H O ++ O HH O HHor 2e - Lewis structure of water Double bond – two atoms share two pairs of electrons single covalent bonds O C O or O C O double bonds Triple bond – two atoms share three pairs of electrons N N N N triple bond or

Modeling Covalent Bonds Molecular model sets: modeling covalent bonding. Create the 8 molecules listed on the next slide and complete a data table similar to the following: Formula (given) Line DiagramDot Diagram

Modeling Covalent Bonds 1 bond = 2 shared electrons Red = oxygen Black = carbon White = hydrogen Blue = nitrogen (3 bonding sites + 1 “hat”) SilvergensGreen & Silver = halogens H 2 O● H 2 C 2 H 4 ● O 2 CO 2 ● C 6 H 12 NH 3 ● N 2

4.2 U3. Bond lengths and Strength The more pairs of electrons that are shared between two atoms in a bond then the stronger the bond and the shorter the bond length. Length nm Strength (kj mol-1) C-O C-C C=O C=C C

4.2 U3. Bond lengths and Strength Bond Lengths Triple bond < Double Bond < Single Bond shortestshorter longer

4.2 U4. Electronegativity Electronegativity Difference Nonpolar covalent bonds have electronegativity difference of 0 Polar covalent bonds have electronegativity differences above 0 less than or equal to 1.8 Ionic bonds have electronegativity differences greater than 1.8

H F F H Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms. One atom pulls the electron being shared closer, creating an unequal share. Usually based on electronegativity of the atoms electron rich region electron poor region e - riche - poor ++ --

Covalent share e - Polar Covalent Unequal sharing of e - Ionic transfer e - Increasing difference in electronegativity Classification of bonds by difference in electronegativity DifferenceBond Type ≤ 1.8Covalent > 1.8 Ionic 0 (Nonpolar) Pure Covalent 0.1 to less than 1.8Polar Covalent

Molecular (Covalent) Naming for two nonmetals Prefix System (binary compounds) 1.Less electronegative atom comes first. 2. Add prefixes to indicate # of atoms. Omit mono- prefix on the 1st element. Mono- is REQUIRED on the 2nd element. 3.Change the ending of the 2nd element to -ide.

Molecular Naming Prefixes PREFIX mono- di- tri- tetra- penta- hexa- hepta- octa- nona- deca- NUMBER

ELEMENTS THAT EXIST AS DIATOMIC MOLECULES These elements only exist as PAIRS. Note that when they combine to make compounds, they are no longer elements so they are no longer in pairs!

CCl 4 N 2 O SF 6 carbon tetrachloride dinitrogen monoxide sulfur hexafluoride Molecular Naming: Examples

arsenic trichloride dinitrogen pentoxide tetraphosphorus decoxide AsCl 3 N 2 O 5 P 4 O 10 Molecular Formula Examples

Practice 1 Fill in the blanks to complete the following names of covalent compounds. CO carbon ______oxide CO 2 carbon _______________ PCl 3 phosphorus _______chloride CCl 4 carbon ________chloride N 2 O_____nitrogen _____oxide

Practice 1 Fill in the blanks to complete the following names of covalent compounds. CO carbon monoxide CO 2 carbon dioxide PCl 3 phosphorus trichloride CCl 4 carbon tetrachloride N 2 Odinitrogen monoxide

Practice 2 1.P 2 O 5 a) phosphorus oxide b) phosphorus pentoxide c) diphosphorus pentoxide 2.Cl 2 O 7 a) dichlorine heptoxide b) dichlorine oxide c) chlorine heptoxide 3. Cl 2 a) chlorine b) dichlorine c) dichloride

Learning Check 1.P 2 O 5 a) phosphorus oxide b) phosphorus pentoxide c) diphosphorus pentoxide 2.Cl 2 O 7 a) dichlorine heptoxide b) dichlorine oxide c) chlorine heptoxide 3. Cl 2 a) chlorine b) dichlorine c) dichloride

Covalent Bond Physical Properties Covalent Compounds: MP: low BP: low Volatility: may be volatile Conductivity: do not conduct electricity Solubility: typically insoluble in water Form molecules