Topic 4: Chemical bonding and structure Flashcards
How are ions formed
Ions are formed when one or more electrons are transferred from one atom to another
Why do ions form
Ions form in order to gain a full outer shell (so have to lose or gain electrons)
Cations and anions
Cations are positively charged (metals)
Anions are negatively charged (non-metals)
Structure of ionic compounds
Ionic compounds are typically solids and have lattice-type structures that consist of repeating cations and anions
Physical properties of ionic compounds
- Ionic compounds have strong electrostatic forces therefore have low volatility
- Ionic compounds in their solid state cannot conduct electricity (because the ions cannot move so cant carry a charge). However, when they are molten or aqueous they can
- Ionic compounds dissolve in polar solvents such as water. THis is because the polar solvents are attracted to the ions in the lattice (ions are pulled from the lattice)
How are covalent bonds formed
Covalent bonds are formed between two non metals (formed by electrostatic attraction between a shared pair of electrons and positive nuclei)
They share electrons in order to achieve full outer shell
Single, double, triple covalent bond
Covalent bonds can take the form of single bonds, double bonds, or triple bonds
Single bonds can be seen in F2
Double bonds in O2
Triple bonds in N2
Covalent bond strength and length
Strength= T>D>S
Length= S>D>T
Octet rule
States that elements tend to lose electrons, or gain electrons, or share electrons in order to acquire a full outer shell
VSEPR theory
Since electrons are negatively charged subatomic particles, pairs of electrons repel one another to be as far apart as possible in space
Electron domain geometry
Based on the total number of electron domains (every pair of electrons occupy an electron domain)
Molecular geometry
Gives the shape of a molecule
VSEPR: 2 electron domains
Linear 180 degress
VSEPR: 3 electron domains
Trigonal planar 120 degrees
Bent Less than 120 degrees
VSEPR: 4 electron domains
Tetrahedral 109.5 degrees
Trigonal pyramidal less than 109.5 degrees
Bent less than 109.5 degrees
Resonance structures
This occurs when there is more than one possible arrangement for a double bond in a molecule.
These multiple possible arrangements contribute to an overall structure called resonance forms.
The actual electronic structure of the species is called a resonance hybrid of these forms
In order to show the idea of resonance, the contributing resonance forms are linked via a double arrow
Example of resonance is benzene C6H6 which forms a delocalized benzene ring
Molecular polarity
If there are non-polar bonds, the molecule is non polar
If there are polar bonds and molecule is symmetrical, molecule is non polar
If there are polar bonds and molecule is asymmetrical, molecule is polar
Allotropes
Allotropes are structures of the same element that can vary in both chemical and physical properties
Carbon has 4 allotropesL graphite, diamond graphene, c60 fullerene
Silicon dioxide
Most commonly in the form of quartz. An example of a three-dimensional covalent network solid
Coordinate covalent bonding
A covalent bond in which the pair of electrons comes from only 1 of the atoms (e.g [NH4]+ )
Simple covalent structures
These contain only a few atoms held together by strong covalent bonds (e.g carbon dioxide)
These have low boiling points and are usually liquid and gases, due to the weak intermolecular forces
Non conductive because they do not have any free electrons or an overall electric charge
Giant covalent structures
Contain a lot of non-metal atoms each joined to adjacent atoms by covalent bonds - e.g silicon dioxide
These are very strong because there are many bonds involved
High melting points because a lot of strong covalent bonds must be broken
Some are conductive as some have free electrons whilst others do not
Graphite
Graphite has a hexagonal layer structure. It has covalent bonds between the carbon atoms however has weak van der wall forces between the layers themselves- allowing the bonds to be overcome very easily. This means that they layers can slide over eachother.
Graphite has good conductivity as it contains delocalized electrons between its layers (with the electrons moving through the layers)
Diamond
In diamond. each carbon atom is joined to four other carbon atoms in a tetrahedral arrangement. Diamond is very hard and has a high melting point. Diamond is a bad conductor as there are no delocalized electrons as the outer shell electrons are used for covalent bonds
C60 fullerenes
60 carbon atoms form a sphere in which each atom is covalently bonded to three others. Fullerene is a semiconductor that is very strong and light. It is used in the medical field for the transportation of medicines in the body.
NOT A LATTICE
Silicon
A member of group 4 and so is able to form 4 covalent bonds to other silicone atoms to form network covalent structure
Intermolecular forces
The forces that exist between molecules
There are three main types of intermolecular forces
Intramolecular forces
Forces that exist between ATOMS (ionic and covalent)
London dispersion forces
The weakest intermolecular force.
A temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.
London dispersion forces are the attractive forces that cause non-polar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently
Dipole-dipole
A dipole is a molecule that has both positive and negative regions
A dipole dipole force is when the positive side of a polar molecule attracts the negative side of another polar molecule
The strength of this attraction with depend on the distance and orientation of the dipoles
The molecules need to be very close to eachother like a liquid in order to work
Hydrogen bonds
A hydrogen bond is an attraction between a slightly positive hydrogen on one molecule and a slightly negative atom on another molecule
Hydrogen bonds are a type of dipole-dipole force
The large electronegative difference between hydrogen atoms and fluorine, oxygen, and nitrogen causes hydrogen bonds to be the strongest of all intermolecular forces
Strengths of intermolecular forces
London dispersion forces < dipole-dipole forces < hydrogen bonds
Metallic bonding
A metallic bond is the electrostatic attraction between positive metal ions and delocalized electrons
Properties of metals
- Metals conduct electricity because they have a sea of delocalized electrons. This means that the electrons can move freely and therefore can carry a charge.
- Metals are malleable (change shape when a force is applied) and ductile (can be drawn into wires). This is because the layers can slide over eachother.
- Shiny
Alloys
Alloys are the result of melting and mixing different metals together in order to create a material with enhanced properties.
This is done by adding mixing amounts of various metals in order to achieve certain characteristics.
The production of alloys is possible because of the non-directional nature of the delocalized electrons, and the fact that the lattice can accommodate ions of different sizes.
Alloys are usually more stronger than regular metals. This is because if different atoms are present, the regular network of positive ions will be disturbed. The atoms of a different size also makes it harder for layers of positive ions to slide over eachother and therefore prevent bending or denting of the metal.