Bonding And Structure Flashcards
London dispersion forces (induced dipole-dipole)
-Electrons are constantly moving, and they move randomly
-If by chance all the electrons are on one side of the atom, one side of the atom will become charged with a negative dipole
-This is called an instantaneous dipole
-This instantaneous dipole will now repel the electrons on another atom, pushing them to the other side inducing a dipole onto an atom next to the instantaneous dipole
-This newly polar atom is called an Induced dipole
-These dipoles now create bonds between each other called London forces
-These bonds are weak and require little energy to overcome
-They are caused by random electron movement, so every single atom or molecule will experience London forces
-Also, the strength of these forces is dependent on the number of electrons, where the number of electrons is proportional to boiling point
Permanent Dipole-dipole interactions
-Interactions between polar molecules with permanent dipoles
-May or may not be stronger than London forces depending on the amount of electrons, as example, a molecule may not have a permanent dipole, but because it has more electrons, the London forces may require more energy to overcome than another molecule with a permanent dipole
Hydrogen bonding
-Hydrogen bonding is the strongest of the intermolecular forces, and requires two conditions:
• It requires a Hydrogen atom bonded to a strongly electronegative element ; N, O and F
• The electronegative atom must have at least one lone pair of electrons
• Therefore, hydrogen bonding only occurs with Fluorine, Nitrogen and Oxygen
-In a diagram of Hydrogen bonding, a dotted line must be drawn directly from the H to the lone pair
Hydrogen bonding in Water
-Hydrogen bonds are very strong, so leads to a very high boiling point, requiring lots of energy to overcome
-The solid form of water, ice, is less dense than liquid water due to Hydrogen bonds
-In liquid water, water molecules are constantly moving and hydrogen bonds are constantly being made and broken
-However, when we reach the freezing point of water, the hydrogen bonds forms a regular structure where the water molecules are further apart than in liquid water, thus being less dense
-This is useful for example water living organisms, for it insulates the water below
Electronegativity
-A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
-Electronegativity is the measure of attraction of a bonded atom for the pair of electrons in the covalent bond
-Electronegativity is measured on the Pauling scale
Trends in electronegativity
-Electronegativity generally increases across a period and up a group
-This is because the higher number of protons in the nucleus across a period increases the attraction to the outer electrons
-Therefore the bonding pair of electrons are attracted more strongly
-Electronegativity increases up a group also for there are less shells so therefore the distance between the electrons and nucleus decreases
-So the bonding pairs of electrons are attracted more strongly
Polar bonds
-The difference in electronegativity creates what’s called a permanent dipole, where the charge difference is always present
-If two bonded atoms are different, their attraction for the shared pair of electrons is unequal
-The atom with a greater attraction for the shared pair of electrons is more electronegative
Polar molecules
-Polar molecule; a difference in charge exists across the molecule
-The symmetry of the molecule means that the effect of any permanent dipoles is cancelled out and there is no difference in charge across the molecule
Electron pair repulsion theory
-This theory states that the shape of a molecule is determined by the electron pairs surrounding the central atom (only referring to the outer shell)
-This is because electrons repel all other electron pairs, so more as far apart as equally from each other to minimise repulsion
-A Covalent bond = Pair of Electrons
Lone pairs (VSEPR)
-Lone pairs of elecctrons not involved in bonding
-They repel other pairs more than bonding pairs because they are closer to the nucleus, more electron dense
-Each lone pair reduces the bond angle by 2.5 degrees
Define allotrope
-An Allotrope is a different structural form of the same element, in the same physical state
-For example, Carbon can take the structures of diamond, graphite, graphene and fullerenes in its solid state
Diamond and graphite (Allotropes of carbon)
-Both diamond and graphite are giant covalent structures with a regular lattice, so are very strong
-In Diamond each carbon atom is bonded to 4 other carbon atoms, producing a regular 3d pattern/structure
-In graphite, the carbons are bonded to only three other carbons, leaving one free delocalised electron, allowing graphite to conduct electricity
-Furthermore, the atoms are arranged in a hexagonal structure which form large flat sheets
-These sheets form layers, which are only held together by weak electrostatic attraction
-Graohite also still has a high melting point due to giant covalent structure
Graphene (Allotropes of carbon)
-These sheets of graphene and like structures can be modified by scientists into tubes or spheres (called fullerenes)
-They have many uses:
• Scientists can form spheres around other molecules
• Large surface area to volume ration so may be used as industrial catalysts
• Tubes can be used to strengthen other materials due to high length to diameter ratio
• Tubes can be used in nanotechnology for they conduct electricity
What is a Metallic bond?
A Metallic bond is the strong electrostatic attraction between the positive ions and the negatively charged delocalised electrons
-Metals have a giant metallic structure, fixed lattice of positive ions surrounded by a sea of delocalised electrons that are free to move
What is a covalent bond
-Reaction between a non-metal and a non-metal
-When covalent bonds are formed, the orbitals of the electrons overlap, and this forms a shared pair of electrons
-A Covalent bond is a bond formed between the sharing of electrons between two non-metals, held together between the electrostatic attraction between the nuclei and this shared pair of electrons
-Elements in period three and above have a d subshell, so all of their electrons can bond to have more than eight electrons in the outer shell, this is called the expansion of the octet
Dative Covalent bonding
-A dative bond is when an atom uses a lone pair of electrons to form a covalent bond
-A dative bond is represented by an arrow pointing away from the element providing the lone pair
-In the case of dative bonding, both of the electrons come from one atom, e.g:
