4. Chemical Bonding & Structure Flashcards
Are metals cations or anions?
Cations
Describe the four properties of ionic compounds.
- High melting point- lots of energy required to break strong electrostatic forces.
- Crystalline- ions have a regular arrangement in their lattice.
- Conduct electricity when molten or solid- ions are free to move.
- Brittle- small displacement of lattice causes like ions to align and repel.
Define ‘ionic bond’.
The electrostatic force of attraction between oppositely charged ions.
What type of ions will have the greatest force of electrostatic attraction?
Small ions with a large charge.
Define ‘metallic bond’.
Positive metal cations are attracted to delocalised electrons by strong electrostatic forces.
What is the structure of a metallic structure?
Giant 3D crystal lattice
Describe the four properties of metallic bonds.
- High melting point- cations are held to electrons by strong electrostatic forces.
- Malleable and ductile- ions are glued into new positions by delocalised electrons.
- Conduct electricity- delocalised electrons are free to move in a given direction.
- Conductor of heat- ions are closely packed, so vibrations transfer easily.
Define ‘covalent bond’.
A pair of shared electrons.
What is a co-ordinate bond?
- A type of bond formed when both electrons come from the same atom.
- The lone pair of electrons help to fill the other atom’s outer shell.
How is a co-ordinate bond represented?
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What is the electronegativity difference for a polar bond?
At least 0.5
What are the three types of intermolecular force?
- London dispersion forces
- Dipole-dipole forces.
- Hydrogen bonding.
Describe the features of London dispersion forces.
- Present between all molecules and atoms.
- Instantaneous dipole forms
- Very weak
Describe how an instantaneous dipole forms.
The electrons are constantly moving, causing one side of the molecule to be slightly charged. This causes the other molecule to be instantaneously attracted.
Describe the features of dipole-dipole forces.
- Some unsymmetrical molecules have permanent dipoles.
- The permanently negative end of one molecule is attracted to the permanently positive end of the other molecule.
- Approximately 1% the strength of a covalent bond.
Describe the features of hydrogen bonding.
- A type of permanent dipoles due to permanent dipole force.
- About 10% strength of a covalent bond.
- H must be bonded to N, O or F.
Describe the features of a giant molecular structure.
- High melting point- many covalent bonds needed to break.
- Don’t conduct electricity (except graphite)- no free ions/ electrons.
- Hard- exists in a tetrahedral structure.
Describe the features of diamond
- Very high melting point- covalent bonds must be broken.
- Very strong- each carbon is joined to 4 other carbons.
- Doesn’t conduct electricity- no free electrons.
Describe the features of graphite.
- Very high melting point.
- Soft- layers are held by weak London forces which can slide over each other.
- Can conduct electricity- delocalised electron as only 3 carbon bonds used.
Describe the features of silica.
- Very high melting point.
- Strong
- Each silicon atom is joined to four oxygens.
- Each oxygen is bonded to 2 silicons
- Doesn’t conduct electricity.
What are resonance structures?
Sets of Lewis structures that describe the delocalisation of electrons in a polyatomic ion or molecule.
What are delocalised electrons?
Electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond.
What must a molecule have to be able to exhibit a resonance hybrid?
A pi bond that can move from one part of the molecule to another.
Why are oxygen and ozone dissociated by different wavelengths of light?
- The chemical bonds between the oxygen in ozone are weaker than in oxygen. As a result, less energetic photons meaning those with longer wavelengths can break apart ozone molecules.
- Oxygen molecules have stronger bonds, so they can only absorb a small range of higher energy, shorter wavelength UV light.
- Ozone have weaker bonds, so they can absorb a broader range of wavelengths, including lower energy, longer wavelength UV light.
Formal Charge= ?
Number of valence electrons- 0.5 (number of bonding electrons)- number of non-bonding electrons.
Which Lewis structure is preferred?
- The difference in FC of the atoms is closest to zero.
- Has negative charges located on the most electronegative atoms.
Describe the features of a sigma bond.
- Sigma is always a single bond.
- Looks like a sphere on the bond axis
- As long as the overlap is on the bond axis, there will be a single bond.
- Sigma always exists (depends on which plane viewed from).
Describe the features of a pi bond.
- Comes from two neighbouring p-orbitals
- Electron density is above or below the bond axis.
- All double and triple bonds are pi bonds.
What is hybridisation?
The concept of mixing atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory.
Describe how hybridisation happens in a carbon atom.
- If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s2, 2s1, 2p3.
- The process is favourable because of the arrangement of electrons- four unpaired and with less repulsion is more stable.
- The four orbitals (a s and 3 p’s)combine or hybridise to give four new orbitals. All four orbitals are equivalent.
Why do alkanes have a tetrahedral arrangement?
The four sp3 orbitals repel each other into a tetrahedral arrangement.
Why do alkenes have a planar arrangement?
The three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them.
What is the arrangement of sp3?
Tetrahedral
What is the arrangement of sp2?
Trigonal planar
What is the arrangement for sp?
Linear
What is the bond angle of a linear molecule?
180°
What is the bond angle of trigonal planar?
120°
What is the bond angle of a bent molecule?
118°
What is the bond angle of a tetrahedral molecule?
109.5°
What is the bond angle of a pyramidal molecule?
107°
What are the bond angles of trigonal bipyramidal?
120° and 90°
What are the bond angles of seesaw?
118° and 90°
What is the bond angle of octahedral?
90°
What is the bond angle of square pyramid?
87°
What is the bond angle of T shaped?
87°
What is the bond angle of square planar?
90°
