Chemistry - test 3 - covalent molecules

Molecular substances

molecules are usually made up of only non-metals (opposed to an ionic compound)

properties of molecular substances

• have low melting and boiling points

• are non-conductive

• have low hardness

Covalent bonds

as non-metals have similarly high electronegativities, they ‘share’ electrons.

a covalent bond is the result of simultaneous attraction of the two positive nuclei to the same shared pair of electrons.

results in each atom gaining a full octet in its valence shell.

INTRAmolecular forces

exist between the atoms in a molecule.

strong, due to electrostatic attraction between nucleus and electrons.

broken with large amounts of energy.

INTERmolecular forces

between molecules of a covalent substance.

generally weaker than intramolecular forces.

broken with small amounts of energy.

Bonding vs lone pairs

bonding pairs involve shared electrons

a lone pair has no shared electrons

VSEPR theory

Valence Shell Electron Pair Repulsion theory- states that electron pairs repel each other as they are negatively charged

leads to predictable shapes with consistent and specific bond angles

Linear molecules

have one single bond or two double bonds

Bent molecules

two single covalent bonds pointing outwards next to each other

pyramidal molecules

have three bonding pairs and one lone pair

tetrahedral molecules

no lone pairs, all single bonds

trigonal planar molecules

one double bond, two lone pairs

molecular polarity

atoms with higher electronegativities pull electrons closer, meaning they have a slight negative charge.

polar bonds occur when the difference in polarity of atoms is between 0.5 and 2

polarity and symmetry

if a molecule with polar bonds is symmetrical, the pull is equal in both directions, so it will be non-polar overall

determining polarity

1. identify atoms involved in the molecule

2. using electronegativity values on the periodic table, determine electronegativity difference

3. use VSEPR theory to draw shape

4. if polar, add dipole symbols (δ+ or δ-)

5. if the charges are not symmetric, the molecule is polar
   

Intermolecular forces: Hydrogen bonding

strongest of intermolecular forces

ONLY occurs when hydrogen is bonded to flourine, oxygen or nitrogen due to electronegativities

Dipole-Dipole interactions

same as hydrogen bonding, but for molecules not including F, O, N

positive dipoles of polar molecules are attracted to negative dipoles of other polar molecules

Dispersion forces

Weakest of intermolecular forces

instantaneous dipoles cause a slight charge on one side

all covalent molecular substances have dispersion forces

boiling and melting points increase as molecules become larger

Allotropes of carbon - diamond

carbon atoms are bonded to four other carbon atoms in a tetrahedral, covalent network lattice

non-conductive but high thermal conductivity

very high sublimation point

graphite

layer lattice of each carbon atom bonded to three others

fourth valence electron is delocalised

conducts heat and electricity

hardness depends on directionality- can be soft and slippery

high sublimation point

chromatography

used to separate and thus identify components of a mixture by polarity, weight and size

involves a stationary phase and a mobile phase; one is polar, one is non polar

a sample is added to the origin of the stationary phase

the mobile phase moves along stationary phase

different compounds in the sample will desorb (up) into the mobile phase at different rates

• polar compounds move further in a polar solvent

• non-polar compounds will not move as far

retention factors

the ratio of a compound’s distance travelled to the distance travelled by the solvent

both are measured from the origin