[chem] chemical bonding & structure Flashcards
structures that a substance can exist in
- giant ionic structure
- simple molecular structure
- giant molecular structure
- giant metallic structure
- macromolecules
properties of ionic compounds
- high melting point
- conductor of electricity in molten and aqueous states
- soluble in water but insoluble in organic solvent
why do ionic compounds have high melting point?
- ionic compounds have a giant ionic structure
- large amount of energy is required to overcome the strong electrostatic forces of attraction between oppositely-charged ions
what are ionic bonds?
ionic bonds are strong electrostatic forces of attractions between oppositely-charged ions
why will ___(magnesium oxide) have a much higher melting point than ___(sodium chloride), although both substances have a giant ionic structure?
- the charges of magnesium oxide and oxide ions are 2+ and 2- respectively
- the charges of sodium ions and chloride ions are 1+ and 1- respectively
- due to the greater charges, more energy is required to overcome the stronger electrostatic forces of attractions between magnesium ions and oxide ions compared to those between sodium ions and chloride ions
why are ionic compounds a conductor of electricity in molten and aqueous states?
- ionic compounds have a giant ionic strucutre
- in the solid state, the oppositely-charged ions are hled in fixed positions by strong electrostatic forces of attraction between oppositely-charged ions. thus, ions are not free-moving and hence cannot conduct electricity
- in the molten or aqueous state, the strong electrostatic forces of attractions between oppositely-charged ions have been overcome. the ions are free-moving and hence can conduct electricity.
why are ionic compounds soluble in water but insoluble in organic solvent? [enrichment]
- water is a polar molecule
- the partial positive charge formed at the hydrogen ends of water molecules will be attracted to anions, and the partial negative charge formed at the oxygen ends of water molecules will be attracted to cations
- water molecules will pull the ions away from the crystal lattice into the solution and the ionic compound will eventually dissolve into the solution
what are examples of organic solvents?
ethanol, trichloromethane, turpentine, benzene
what are some uses of ionic compounds?
- used as refractory materials. refractory materials are heat-resistant
- the melting point of magnesium oxide is 2852degC. it is used to line the inner surface of a high temperature furnace
- ceramic crucible for strong heating
what are covalent bonds
covalent bonds are generally formed between non-metal atoms by the sharing of electrons
what is a non-polar covalent bond?
if the two atoms involved in covalent bond are either the same or have similar electronegativity, a non-polar covalent bond results. the two atoms have equal pull on the shared pair of electrons
what is a polar covalent bond?
if the two atoms involved in covalent bonding have different electronegativity values, the pair of shared electrons are not equally shared between the two atoms and polat covalent bond results
which atom will the shared electrons be closer to in a polar covalent bond?
if atom b is more electronegative than atom a, the bonding electrons are nearer to b in the bond
who gets partial negative charge and who gets a partial positive charge in a polar covalent bond?
- the more electronegative atom acquires a partial negative charge
- the less electronegative atom acquires a partial positive charge
what is the sign for a partial negative/positive charge? (draw out)
upside down 2 with the curve being closed plus either a +/- sign at the right hand side
categories that covalent substances can be categorised into
- simple molecular substance
- giant molecular substance
- macromolecules
what are simple molecular substances made up of?
many simple molecules
what is holding the atoms together in the molecules of simple molecular substances?
strong covalent bonds
what holds the molecules together in simple molecular substances?
weak intermolecular forces of attraction
examples of simple molecular substances
- water
- methane
- carbon dioxide
- ammonia
- iodine
properties of simple molecular substances
- low melting and boiling point
- non-conductor of electricity in any state
- insoluble in water but soluble in organic solvent
why do simple molecular substances have have low melting and boiling points?
- they have a simple molecular structure
- small amount of energy is required to overcome the weak intermolecular forces of attraction
why are simple molecular substances a non conductor of electricity in any state?
- they have a simple molecular structure
- there are no free-moving valence electrons available to conduct electricity
what are some exceptions - which simple molecular substances can conduct electricity and why?
- hydrogen chloride
- sulfur dioxide
- ammonia
because they can dissociate in water to form free-moving ions
can tap water conduct electricity? why?
they might, because impurities that are ionic might be mixed in tap water
uses of simple molecular substances and what is that property they use?
- in perfumes and flavouring
- in room deodorants
- insect repellents [naphtalene]
property: volatile
how is a giant molecular structure formed?
atoms can form a network of covalent bonds throughout the strucutre
examples of substances with giant molecular structure
- diamond
- graphite
- silicon
- silicon dioxide
strucutre of diamond?
- has a giant molecular strucutre
- each carbon atom is covalently bonded to 4 other carbon atoms in a tetrahedral arrangement
properties of diamond
- high melting point
- does not conduct electricity
- insoluble in water and organic solvents
- hard
why does diamond have a high melting point?
- has a giant molecular strucuture
- large amount of energy is required to break strong covalent bonds between carbon atoms
why does diamond not conduct electricity?
- has a giant molecular structure
- all 4 valence electrons of each carbon atom are used for covalent bonding, so there are no free-moving valence electrons present to conduct electricity
why is diamond hard?
- all the carbon atoms are held by strong covalent bonds throughout the structure, hence diamond is hard
uses of diamond and the property it uses
- drill tips
- polishing tools
property: hard
strucutre of graphite
- giant molecular layered structure
- made up of many layers of hexagonal rings of carbon atoms
- each carbon atom forms strong covalent bonds with 3 other carbon atoms in hexagonal arrangement in layers
- the carbon atoms are held together by strong covalent bonds but the layers are held together by weak intermolecular forces of attraction
properties of graphite
- high melting point
- can conduct electricity
- insoluble in water and organic solvents
- soft and slippery
why does graphite have a high melting point?
- has a giant molecular layered structure
- large amount of energy is required to break the strong covalent bonds between the carbon atoms
why can graphite conduct electricity?
each carbon atom uses only 3 out of 4 valence electrons in bonding. the valence electrons not used in bonding are free-moving to conduct electricity
why is graphite soft and slippery?
- has a giant molecular layered structure
- the layers of carbon atoms in graphite are held together by weak intermolecular forces of attraction. the layers of atoms are able to slide over one another easily when a force is applied
uses of graphite and properties it uses
- soft and slippery
- in pencil lead [it is soft, layers of carbon atoms slide off the pencil onto the paper easily]
- solid lubricant to reduce friction in machinery - conductor of electricity
- in brushes for electric motors and as inert electrodes
strucutre of silicon
giant molecular structure
properties of silicon
- high melting and boiling point
- non-conductor of electricity
- hard solid
- insoluble in water and organic solvent
structure of silicon dioxide
giant molecular structure
properties of silicon dioxide
- high melting and boiling point
- non-conductor of electricity
- hard solid
- insoluble in water and organic solvent
what makes up a macromolecule?
many simple covalent molecules/simple molecular strucuture joined together into chains of much larger molecules
bonds between molecules in macromolecules
strong covalent bonds between non-metal atoms within each large molecule
examples of natural polymers
- silk
- wool
- starch
- rubber
example of man-made polymers
1.poly(ethene)
2. nylon
3. polyester
4. polystyrene
5. many other plastics
properties of macromolecules
- no fixed melting/boiling point
- usually solids at r.t.p.
- insoluble in water but soluble in organic solvents
- non-conductor of electricity in any states
why are most macromolecules/polymers solids at r.t.p.?
- they have a large molecule size
- weak intermolecular forces of attraction are strong enough for it to be a solid at r.t.p.
why will they soften over a range of low temperatures when heated?
the weaker intermolecular forces of attraction between the large molecules are overcome by molecular vibrations with higher kinetic energy
why are most macromolecules a non-conductor of electricity?
they do not have mobile ions/electrons
arrangement of atoms in pure metals
metal atoms are closely packed together in the solid state to form a giant metallic [lattice] structure
what is metallic bond?
the strong electrostatic forces of attraction between the positive metal ions and the ‘sea’ of free-moving valence electrons
why is there a ‘sea’ of free moving valence electrons in pure metals?
the metal atoms in the giant metallic structure lose their valence electrons to become positive ions. hence, the valence electrons are delocalised and can move freely between the metal ions
why metallic bonding and not ionic bonding in metals?
all metal ions form positive ions, so ionic bonding cannot occur as there are no negative ions for electrostatic attraction between oppositely-charged ions
do metallic bonding exist within all metals?
yes
what are alloys?
a type of solid-solid mixture of a metal with one or more other elements
what is bronze made up of? (the 2 metals and their percentage)
90% copper
10% tin
why are the properties between alloys and pure metals different?
in alloys, the regular arrangement of metal atoms is disrupted by the addition of other elements with atoms of different sizes, which are randomly distributed. this disrupts the orderly arrangement of atoms in pure metals
properties of pure metals
- high melting point
- can conduct electricity in all states
- insoluble in water and organic solvents
- ductile and malleable
why do metals have high melting point?
- has a giant metallic structure
- large amount of energy is needed to overcome the strong electrostatic forces of attraction between the positive metal ions and the ‘sea’ of free-moving valence electrons
exceptions of metals with high melting point
mercury, which is a liquid at room temperature
why can pure metals conduct electricity in all states?
- has a giant metallic structure
- there is a ‘sea’ of free-moving valence electrons present to conduct electricity
why are pure metals ductile and malleable?
- has a giant metallic structure
- metal ions are arranged regularly in layers. when a force is applied, the layers of metal ions can slide over one another easily
- therefore, metals are malleable and ductile
properties of alloys
- melts over a range of temperatures
- can conduct electricity in all states
- insoluble in water and organic solvents
- harder and stronger than metals
why can alloys conduct electricity in all states?
there is a ‘sea’ of free-moving valence electrons present in alloys
why are alloys harder and stronger than pure metals?
- in an alloy, the metal atoms and atoms of the added element are of different sizes
- as this disrupts the orderly arrangement of atoms in pure metal, it is much harder for the atoms to slide over each other when a force is applied
- hence, alloys are harder and stronger than the pure metals they are made from, but they tend to be less malleable and less ductile
uses of metals and the property it uses
- good conductor of electricity, ductile
- used in electric wires and in electric goods [most common metal used in copper] - high melting and boiling points
- tungsten metal has a high melting point of 3410degC. it is used to make the filament of light bulbs
uses of alloys and the properties it uses
- increased hardness and strength of alloys
- bronze and brass alloys are used in making more durable items [e.g. bell]
- steel is used to make items that can withstand heavy loads [e.g. warehouse shelves]
what are allotropes?
allotropes are different forms of the same element
examples of well-known allotropes
diamond and graphite are well-known allotropes of carbon
