Unit 4

Chemical bonds connect atoms to each other within a compound◦ Also known as Intramolecular forces of attraction

It requires energy (ie: the addition of heat) to break a bond

Chemical bonds store potential energy

Atoms will gain, lose, or share electrons in order to achieve a full valence shell

Electronegativity differences between atoms help determine what type of bond with form between them:◦ Electrons are always attracted to the atom with the

higher electronegativity

Bonds that involve a transfer of an electron from one atom to another

Bonding due to electromagnetic forces

Formed between a metal and a nonmetal

Electronegativity difference between the atoms is 1.7 and higher

Sodium and Chlorine

Calcium and Iodine

Formed between two nonmetals

Electronegativity difference is less than 1.7

Involve the sharing of electrons

F + F F2

Single bonds

Double bonds

Triple bonds

Each bond represents 2 electrons

Compounds that contain covalent bonds are called molecules

Compounds that contain polyatomic ions contain both ionic and covalent bonds

Valence Shell Electron Pair Repulsion

Applies to molecules only (substances with covalent bonds)

Since “like charges” repel, pairs of electrons will repel other pairs of electrons

Each pair will take a position that will be as far away as possible from other pairs of electrons

Shapes of molecules are influenced by this phenomenon

Polar Covalent bonds

◦ Bonds between atoms that do not share electrons equally, caused by elements having different electronegativities

◦ Electrons are pulled harder by atoms with the higher electronegativity. This creates a “dipole moment”

Polar molecules

◦ Are asymmetrical because there is a “dominant” directional pull of electrons (quasi + & quasi -), although the molecule itself, is neutral

◦ Polar molecules generally have higher boiling and melting points their non-polar counterparts

Non-polar covalent bonds

◦ Bonds between atoms that equally share electrons, (they have the same electronegativity that results in an END of zero)

Non-polar covalent molecules

◦ Are symmetrical without any “dominate” directional pulling of electrons

◦ Can have polar bonds, non-polar bonds, or a combination of both

Coordinate covalent bonds

◦ A bond between atoms in which one of the atoms provides all the shared bonds

◦ Otherwise, not different than any other covalent bond

Coordinate covalent bonds

Metallic bonds

◦ Bonding between metals

◦ “Sea of electrons”: metals are arranged in fixed patterns, but the electrons are free to move. (this often polarizes the metal and makes it magnetic)

◦ Metals are good conductors of heat and electricity because of the mobile electrons

Metallic bonds

Ionic solids

◦ Substances in the solid phase, formed by ionic bonding

◦ Example: NaCl (table salt)

Ionic solids

◦ High melting points

◦ Are crystalline in structure

◦ Do not conduct electricity, except in aqueous solutions (because it dissociates into freely moving ions)(the more ions it breaks into – the more conductive it is)

Molecular substances

◦ May exist as solids, liquids, or gases (depending on the strength of attraction between molecules)

◦ Examples: water, ammonia, sugar, oxygen

Molecular substances

◦ Formed by covalent bonding

◦ Relatively soft (compared to ionic solids)

◦ Poor conductors of heat and electricity (no ions) Sugar dissolves, but does not dissociate

◦ Relatively low melting points

Network solids

◦ Solids that are covalently bonded into “giant networks”

◦ Usually one element bonded to itself many times

◦ Examples: diamonds, graphite, asbestos, plastics

Network solids

◦ Graphite and diamond have the same formula: C

◦ How graphite becomes a diamond

Network solids

◦ Hard

◦ Poor conductors of heat and electricity

◦ High melting points

Attraction between molecules

◦ Dipole-dipole interactions

◦ Hydrogen Bonding

◦ Van der Waals Forces

◦ Molecule-Ion Attraction

Dipole-dipole interaction

◦ Attraction between polar molecules (dipoles)

◦ Molecules arrange themselves so that a positively charged “end” (δ+) lines up with a negatively charged “end” (δ–).

Opposites attract!

Hydrogen “bonding”

◦ Attractive force between H and F, O, N in separate molecules

◦ Causes substances to have higher boiling points because of this additional attractive force

◦ Much stronger than dipole-dipole forces

◦ Usually represented by a dotted line ( - - - - - - )

The music video

van der Waal’s forces

◦ The major force of attraction between non-polar molecules

◦ Act similarly to gravitational forces; VDW forces increase with increasing mass and increasing proximity of the molecule to another molecule

◦ Makes it possible for non-polar substances to exist as solids and liquids

van der Waal’s forces◦ The formation of a diatomic molecule

van der Waal’s forces◦ Instantaneous dipole

van der Waal’s forces◦ Instantaneous dipole

van der Waal’s forces

Diatomic bromine is a liquid Diatomic iodine is a solid

van der Waal’s forces

Iodine has greater intermolecular forces because it is more massive

Molecule-ion attraction◦ Attractive forces between a dipole and ions

Molecule-ion attraction◦ Usually solute / solvent relationships