Bonding

Ionic bonding

• Metal loses an electron

• Non- metal gains an electron and becomes a negative ion

• These two ions are attracted to each other by an electrostatic force of attraction

• Ionic compounds form a lattice structure

Properties of ionic compounds

• High MP/BP-Lots of energy is required to overcome the strong electrostatic force of attraction

• Soluble in polar solvents- Water molecules surround the ionic compounds because they have a partial positive and negative charge. Overcoming the electrostatic force of attraction between the positive and negative ions

• Don’t conduct electricity when solid-as ions are not free to move so thus unable to carry charge. Only conduct electricity when molten or dissolve.

• Brittle

Why are ionic compounds brittle?

• In an ionic lattice, positive and negative ions are arranged in a repeating pattern to minimise repulsion

• When a force is applied, the layers of ions shift causing like charged ions to align

• This causes repulsion and the crystal breaks

Covalent bonding

• Non metals share electrons

• A single covalent bonds contains a pair of electrons

• The shared electrons are attracted to the two nuclei of the atoms forming the bond.

What is a dative/coordinate bond?

• A dative covalent bond contains a shared pair of electrons, with both electrons supplied by one atom

• Dative bond is shown by an arrow

Example of a dative bond

• NH4⁺

• NH3 has a lone pair of electrons which bond to a hydrogen ion

Metallic bonding

• Involves attraction between delocalised electrons and positive ions arranged in a lattice

Properties of metallic bonding

• High conductivity- Free electrons move easily and can carry charge

• Malleable-Layers of metal atoms can slide over each other

• Sonorous

What factors affect the strength of a metallic bond?

• Number of protons/strength of nuclear charge- More protons, stronger the bond

• Number of delocalised electrons- more electrons, stronger the bond

• Size of the ion- smaller the stronger the bond. Smaller ions have higher charge density because the charge is more concentrated and less spread out in a smaller ion.

4 types of crystal structure

• Ionic

• Metallic

• Macromolecular (giant covalent)

• Molecular (simple covalent)

Why do simple molecular substance have a low MP/BP

• weak intermolecular forces that require little energy to break

What happens to all simple molecular substances if you cool them below their melting point?

• Becomes solid

• Forming a simple molecular lattice

Why does H2O have a high MP/BP (despite being a simple molecular substance)

• In ice H2O molecules are held together by intermolecular forces

• But one of the IM forces is hydrogen bonding which require lots of energy to break

What does the solubility of a simple molecular substance depend on?

• Whether or not the substance is polar or non polar

• Non polar substance dissolve well in non polar solvents

• Polar substances dissolve in polar solvents

Why do simple molecular substances not conduct electricity?

• They don’t carry mobile charged particles that are able to carry charge

Define electronegativity

• The ability of an atom to attract the pair of electrons in a covalent bond

What does electronegativity depend on?

• The size of the positive charge on the nucleus

• Atomic radius- Smaller the atom the more electronegative

• Shielding- Less shells more electronegative

What is a pure covalent?

• Pair of electrons is midway between both nuclei of the atoms

• Occurs when both atoms have the same electronegativity e.g. cl-cl (cl2)

What is a dipole ?

• Separation of charge

What is the electron pair repulsion theory?

• Shape of a molecule is determined by the electron pairs surrounding the central atom

• The electron pairs will move away from each other as far as possible

Linear (shape of molecules)

• 2 bonding pairs (surrounding the central atom)

• Bond angles is 180 degrees

• If the central atom has 2 bonding pairs its linear but this is not the case if central atom has lone pair of electrons

Trigonal planar

• Central atom has 3 bonding pairs

• Bond angle 120 degrees

Tetrahedral

• Central atom has 4 bonding pairs

• Bond angle is 109.5 degrees

Trigonal bipyramidal

• 5 bonding pairs

• Bond angles 90 and 120 degrees

octahedral

• 6 bonding pairs

• Bond angle 90 degrees

What does the dashed and solid wedge mean?

• Dashed= atom is projecting behind the plane

• Solid= atom projecting outwards

What does it mean if atoms are lying on the same plane?

• They all lie flat rather than being above or below each other

How do lone pairs effect bond shape of molecule

• Lone pairs repel more strongly than bonding pairs.

• (There’s extra repulsion)

What does the extra repulsion do to bond angles

• Decreases bond angles by 2.5 degrees

Describe the trigonal pyramidal shape

• Has 3 bonding pairs and 1 lone pair of electrons on the central atom

• BA = 107

Describe the bent shape

• Has 2 bonding pairs and 2 lone pairs of electrons on the central atom

• BA is 104.5 degrees

What is a polar molecule?

• A molecule with an uneven distribution of electrons which results in a permanent dipole

Why may a polar bond not have a dipole?

• A molecule may contain polar bonds, but not be a polar molecule if the shape of the molecule is symmetrical (if the arrows towards the more electronegative elements face away from each other)

• This causes the dipoles to cancel each other out

What can break a covalent bond?

• Covalent bonds are extremely strong and can only be broken via a chemical reaction

• Physical changes like boiling only overcomes IM forces between molecules NOT covalent bonds

Explain Vander Waals forces

• Random movement of electrons shifts all electrons in atom 1.

• This repels electrons in atom 2 causing an induced dipole

What does the strength of Vander Waals forces depend on ?

• The number of electrons

• More electrons= High strength

Explain permanent dipole dipole forces

• This force occurs between polar bonds that contain a permanent dipole

Explain hydrogen bonding

• Hydrogen interacts with a significantly more electronegative atom F,O,N

• The F,O,N atoms must have an available lone pair of electrons

Why is ice less dense than liquid water ?

• In liquid, the h2o molecules are moving randomly, forming and breaking hydrogen bonds

• When you freeze water, the H2o molecules arrange themselves in an ordered structure, stabilised by hydrogen bonding.

• Therefore the H2o molecules are further apart in ice than liquid