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what is an element
a pure substance that cannot be broken down into two or more simpler substances by chemical methods
what is a compound
a pure substance containing 2 or more elements that are chemically combined in a fixed ratio
what are the differences between compounds and mixtures
compounds has a fixed ratio while a mixture has no fixed ratio
compounds cannot be separated into its constituent elements easily but mixtures can
compounds have different properties from its constituent elements but mixtures usually have similar properties to its constituent elements
mixtures boil over a range of temperatures while compounds have a fixed melting and boiling point
what are the structural properties of ionic compounds
ionic compounds have giant ionic crystal lattice structures
it consists of large numbers of alternating positive and negative ions held together by electrostatic forces of attraction
what are the physical properties of ionic compounds
high melting and boiling points
because the mutual electrostatic attraction between oppositely charged forces are strong, a large amount of energy is needed to overcome these forces
hard but brittle
they are hard as the strong attractive forces cause ions to resist motion
if enough force is applied, ions move away from their lattice positions and ions of the same charge approach each other. repulsive forces between ions of the same charge become larger than the attractive forces and lattice structure shatters, hence ionic compounds are hard but brittle
soluble in water and insoluble in organic solvents
dissolved in water - aqueous state
can conduct electricity in the aqueous state
when in aqueous state, the ions are mobile and can conduct electricity
what are the structural properties of covalent substances
atoms in the molecule are held by strong covalent bonds, but the molecules are held together by weak intermolecular forces of attraction
what are the physical properties of covalent substances
low melting and boiling points
the weak intermolecular forces of attraction can be easily overcome with a small amount of energy
exists mostly as gases and liquids at room temperature
as the molecule gets larger, the intermolecular forces of attraction are stronger, so the melting and boiling points are higher
eg: iodine is a solid at room temp, bromine is a liquid at room temp (iodine higher molar mass)
most are insoluble in water and soluble in organic solvents
cannot conduct electricity
have no mobile charge carriers to conduct electricity as they typically exist as neutral molecules
some can disassociate in water to dissolve into its individual ions which are mobile and can conduct electricity as a solution (eg: HCL hydrogen chloride)
what is an allotrope
different forms of the same element with different structural arrangements of atoms
what are examples of giant covalent structures
diamond (carbon allotrope)
graphite (carbon allotrope)
silicon dioxide
what is the structural of diamond
each carbon atom forms strong covalent bonds with four other carbon atoms
this gives diamond a three dimensional network structure in a tetrahedral arrangement
what is the structural properties of graphite
each carbon atom forms covalent bonds with three other carbon atoms
gives a layered structure consisting of hexagonal rings of carbon atoms
strong covalent bonds are found within each layer, but weak intermolecular forces of attraction exist between the layers
what are the physical properties of diamond
hard
made up of only strong covalent bonds
large amount of energy is needed to break the structure apart
very high melting and boiling points
due to the large number of bonds in the network
very large amount of energy is required to break the strong covalent bonds
insoluble in water and in organic solvents
do not conduct electricity
has no mobile electrons
what is the physical properties of graphite
soft and slippery
only a small amount of energy needed to overcome the weak intermolecular forces of attraction between the layers of carbon atoms
high melting and boiling points
due to the large number of bonds in the network
a large amount of energy is required to break the strong covalent bonds between the atoms
insoluble in water and organic solvents
conducts electricity
each carbon atom is bonded to three other carbon atoms
this leaves one unbonded electron per carbon atom, which is free and mobile to conduct electricity
why are carbon nanotubes such a good conductor of electricity
the delocalised mobile electrons are trapped within the cylinder. this means that electricity can be conducted up and down the nanotube
why is fullerene (the soccer ball shaped carbon allotrope) a good carrier of substances
they have a large surface area which would make them good catalysts
what is the metallic bond
the mutual electrostatic attraction between positively charged metal ions and the “sea of delocalised electrons”
what is an alloy
is a mixture of a metal with one or more other elements
what are the structural properties of metals and alloys
metals:
have a regular structure
they are malleable and ductile
sea of delocalised electrons will hold the metal ions together until a much greater force is applied
alloys:
atoms are of different sizes, so it has a irregular arrangement
a larger force is needed to make the layers slide over each other
thus, alloys tend to be less malleable and ductile than the pure metals they are made from
alloys are harder and stronger than pure metals
what are the physical properties of metals and alloys
high melting and boiling points
atoms are held together by strong electrostatic forces of attraction between positively charged metal ions and sea of delocalised electrons
alloys melt over a range of temperatures and metals have fixed mp and bp
good electrical conductors
the sea of delocalised electrons found in metals and alloys make them good electrical conductors
