3.Bonding Flashcards
What is ionic bonding
The electrostatic forces between oppositely charged ions in a giant lattice
Name properties of ionically bonded compounds
- solid at room temperature
- high melting point as they are giant structures as electrostatic forces extends throughout the compound
- conduct when molten or dissolved
- brittle
What is a covalent bond
A shared pair of electrons
How does sharing electrons hold atoms together
Atoms within a covalent bond are held together by the electrostatic attraction between the nuclei and shared electrons
What are properties of covalently bonded molecules
- low melting point due to weak intermolecular forces
- don’t conduct electricity
What is dative covalent bonding
Covalent bonding in which both electrons come from one atom in the bond.
Which are stronger covalent or dative
Neither, they are the same strength
What is metallic bonding
The electrostatic attraction between lattice of positive ions and a sea of delocalised electrons
What does metallic bonding’s strength depend on
- charge of the ion, greater the positive charge, greater the number of delocalised electrons, stronger the electrostatic attraction
- size if the ion, the smaller the ion the closer the delocalised electrons are to the positive nucleus
What kind of structure is metal
Giant lattice structure
Describe properties of metals
- can conduct electricity due to delocalised electrons
- good conductors of heat due to sea of electrons
- malleable and ductile
- high melting points as they are giant structures
Define electronegativity
The power of an atom to attract the electron density in a covalent bond towards itself
What does electronegativity rely on
- nuclear charge
- the distance between the nucleus and the outer shell electrons
- the shielding
Describe the trend in electronegativity up a group
- electronegativity increases
- there is less shielding by electrons in inner shells
Describe the trend in electronegativity across a period
- electronegativity increases
- nuclear attraction increases for the same amount of shielding
What is polarity
Unequal sharing of electrons between atoms that are bonded covalently
When atoms in a covalent bond have a large difference in electronegativity the bond is said to be
Polar
Name the 3 intermolecular forces in order of strength
- van der Waals
- Dipole-dipole
- Hydrogen bonding
What is a dipole-dipole force
An intermolecular force that results from the attraction between molecules with permanent dipoles
What is a dipole moment
Sums up the effect of the polarity of all the bonds in the molecule
What are van der Waals forces
An intermolecular force of attraction between instantaneous dipole and induced dipole and acts between all molecules
How are instantaneous dipoles caused
The dipole is caused by the changing position of an electron cloud
Why are van der Waals sometimes called instantaneous dipole-induced dipole forces
As the electron distribution of the original atom changes it will induce new dipoles in the atoms around it
What causes a larger van der Waal force
The number of electrons present so atoms or molecules with large atomic or molecular masses produce stronger van der Waals
Using the idea of van der Waal forces explain why the boiling point of noble gases increase down the group
- as you go down the group the atomic numbers of the noble gases increase
- this is because there are more electrons present
- this causes larger instantaneous dipoles
- larger van der Waal forces
Using the idea of van der Waal forces explain why the boiling points of hydrocarbons increase with chain length
- as hydrocarbons increase in chain length
- larger surface area so more points of contact with adjacent molecules
- this means more van der Waals forces are instantaneously formed
What is hydrogen bonding
An intermolecular force in which a H d+ atom interacts with a more electronegative atom with a d- charge
When do hydrogen bonds form
- a hydrogen atom that is bonded to a very electronegative atom. This produces a strong partial charge
- a very electronegative atom with a lone pair of electrons. These will be attracted to the partially charged H atom.
What are the only atoms that are electronegative enough to form hydrogen bonds
- O
- N
- F
Why is water denser than ice
- in water molecules can move around
- in ice molecules can no longer move around, the hydrogen bonds hold a fixed position
- this results in a 3D structure
- to fit into this structure molecules are slightly less packed
- this means ice is less dense
Using the idea of intermolecular forces describe how ironing works
- iron provides heat to break hydrogen bonds
- pressure forces molecules to be flat
- when you remove iron hydrogen bonds reform and hold the molecules in the new positions
What is electron pair repulsion theory
- explains the shape of molecules
- each pair of electron around an atom will repel all other electrons
- the pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion
Shape of molecule: two pairs of electrons
Linear. 180°
Shape of molecule: three pairs of electrons
Trigonal planar. 120°
Shape of molecule: four pairs of electrons
Tetrahedron. 109.5°
Shape of molecule: five pairs of electrons
Trigonal bipyramid. 90° (same plane) 120° on different.
Shape of molecule: six pairs of electrons
Octahedral. 90°
What is the effect of a lone pair of electrons.
Reduces the bond angle by 2.5°
Why is the bond angle reduced by lone pairs
- lone pairs are only attracted by one nucleus
- therefore are pulled closer to the nucleus and other electrons
- electron repulsion theory, paired electrons are repelled further reducing angle by 2.5
What is the order of electron pair repulsion strength.
bp-bp< bp lp< lp lp
What is enthalpy change of melting/vaporisation
Energy required to weaken the forces holding a element in a solid (liquid) state resulting in a liquid (gas)
What are crystals
Solids with a regular arrangement of particles held together by forces (covalent, ionic or metallic or weaker IMF)
Name the 4 basic types of crystals
- ionic
- metallic
- molecular
- macromolecular
Which two types of compounds exist as crystals
Ionic and metallic (lattices)
Molecular crystals
- held together by IMF
- low melting points as IMFs are weak
Name 3 macromolecular crystals
- diamond
- graphite
- buckminsterfullerene
Diamond
- each electron in carbon atoms outer shell is bonded with another carbon atom
- bond angles of 109.5
- forms a giant 3D lattice of covalent bonds
- very hard
- very high melting
- does not conduct electricity
Graphite
- each carbon forms 3 covalent bonds
- bond angle 120
- 1 delocalised electron for each carbon free to move within graphene layer
- layers of graphene held by weak van der Waals
- soft (weak IMF)
- high melting (strong covalent bonds in giant structure)
- conducts electricity along layers (delocalised electron)
