structures Flashcards
coordination number of an ion
Gives number of its nearest neighbours
Crystal coordination number
Number of anions around each cation in an ionic lattice
ionic solids
Giant lattices of positive and negative ions
Sodium chloride and caesium chloride
sodium chloride
Six sodium ions surround each chlorine ion and six chlorine ions around each sodium ion only
Giant structure as we can’t state how many ions there are
caesium chloride
coordination number of 8:8
Cs+ is larger than Na+ ion and more cl- can fit around it
properties of an ionic solid
melting temperature
high
giant lattices are held by strong electrostatic forces between the opposite charged ions
takes a large amount of energy to overcome these forces of attraction
properties of ionic solids
Solubility
Often soluble in water, water molecules are polar- oxygen atoms have a partial negative charge and hydrogen atoms a partial positive charge
in solution the oxygen ends of the water molecule are attracted to the positive ions and the hydrogen ends to the negative ions
properties of ionic structures
Hard but brittle.
When force is applied, layers of ions slide of each other causing ions of the same charge to be next to each other
Ions repel each other and the crystal shatters
properties of ionic structures
Poor electrical conducttivity when solid
But good conductor when Molten or dissolved as electric current will flow if charge particles are free to move when a potential difference is applied
In solid- Ions are fixed in position by strong ionic bonds but in molten or dissolved ions are free to move and will move to the electrodes of opposite sign so will carry a current
giant covalent structures
Consist of networks of covalently bonded atoms that stretch throughout the whole structure
Allotropes
different forms of the same element in the same state
Diamond
Each carbon atom is covalently bonded to four others
The atoms arrange themselves in a tetrahedral shape
Bonding forces are uniform throughout structure
properties of diamond
Very high melting temperature. Energy needed to break the strong covalent bond is very high
Extremely hard due to strength of covalent bond and geometrical rigidity of the structure
Insoluble in water - no ions to attract polar water molecules
Poor conduct of electricity - no free electrons or ions present to carry charge
Graphite
Consists of layers of hexagonal rings
Each carbon is joined to three others by strong covalent bonds
Fourth electron from each carbon is delocalised
Layers are held together by weak van der Waals forces
delocalised
An electron that is not attached to a particular atom- it can move around between atoms
Simple molecular solids
Have covalent bonds within molecules had held together by weak intermolecular forces
properties of simple molecular solids
- low Mt and Bt: Although covalent bonds within molecules are strong the intermolecular forces holding molecules together are weak and do not need much energy to break
- soft: Weak intermolecular forces between molecules are easily broken
- normally insoluble in H2O: no ions to attract polar water molecules but compounds can form hydrogen bonds with water and be soluble
-Poor conductor: No delocalized electrons or ions to move and carry charge
Iodine
Atoms are covalently bonded in pairs to form diatomic molecules
These molecules are held together by weak van der Waals forces and are arranged in regular patterns
ice
in ice molecules are arranged in rings of six held together by hydrogen bonds
In this ordered structure water molecules are further apart than they are in liquid state and structure creates large areas of open space inside the rings and as a result, at 0 degrees, Ice is less dense than liquid water
metals
When metals are close to each other each atom loses control over its outer electrons
These electrons are no longer restricted to a particular metal atom but are delocalised leaving a positive cation
arrangement of metals
Regular arrangement of metal cations (lattice) Surrounded by a sea of delocalised electrons
properties of metals
- High melting temperatures
Large energy is needed to overcome strong forces of attraction between nuclei of metal cations and delocalised electrons
Melting temperature is affected by number of delocalized electron per cation and the size of the cation
properties of metals
Hard
Metallic bond is very strong
properties of metals
Insoluble
No ions to attract polar water molecules- each metal cation represents rest of the atom apart from outer electron
That electron has not been lost and still the in structure
properties of metals
Good conductors
In both solid and molten
Delocalised electrons can carry a current because when a potential difference is applied across the ends of metal, electrons will be attracted to and move towards, to the positive terminal of the cell
Also good thermal conductors because denuclearize electrons can pass kinetic energy to each other
properties of metal
Malleable (shaped)
Ductile (drawn into a wire)
When a forces applied to a metal the layers of cations can slide over each other
but delocalized electrons move with cations and prevent forces of repulsion forming between layers
prop of graphite
melting temp
high
strong covalent bond in hexagon layers
prop of graphite
soft Slippery feel
Weak forces between layers are easily broken so layers can slide over each other
prop of graphite
Insoluble
No ions to attract polar water molecules
prop of graphite
Good conductor of electricity
Delocalise electrons are free to move along the layers (not from 1 layer from next) so an electric current can flow and conduct parallel to layers
prop of graphite
Low density
Relatively large amount of space between layers because length of covalent bonds in the layers is much shorter than length of van der Waals forces between layers
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