in periods - by periodicity ( repeating trends in physical and chemical properties)
in groups - by similar chemical properties
history of periodic table
before, elements were ordered by atomic mass
Mendeleev created the periodic table - he left gaps for undiscovered elements
he grouped them in similar chemical properties
now the periodic table is ordered by proton number
Factors that affect ionisation energy:
nuclear charge
atomic radius
electron shielding
What is first ionisation energy?
The energy required to remove one electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
trend of first ionisation energy down a group?
decreases down a group, because:
nuclear charge increases - this is not enough to increase ionisation energy
atomic radius increases - attractive force between nucleus and outer electrons is weaker - energy required to remove an electron decreases
electron shielding increases - more shells between nucleus and outer shell - attractive force is weaker - energy required to remove an electron decreases
What do ionisation trends down a group support?
support the Bohr model of the atom
trend of first ionisation energy across period 2 and 3?
increases, because:
nuclear charge increases - attractive force increases - more energy required to remove an electron
atomic radius decreases (due to increased nuclear charge) - attractive force increases - more energy required to remove an electron
electron shielding remains constant
what is the cause of the slight first decrease of ionisation energy across periods 2 and 3?
this happens because of an addition of an electron
the elements sit at a subshell which is slightly further from the nucleus
attractive force decreases - energy required to take an electron decreases
What is the cause of the second slight decrease of the ionisation energies in p2 and p3?
electron repulsion
removing an electron from sulphur/oxygen involves taking it from an orbital with 2 electrons
electrons repel each other - less energy needed to remove an electron from an orbital with 2 than in 1 (in nitrogen and phosphorous)
successive ionisation?
The removal of more than 1 electron from the same atom
what is metallic bonding?
strong electrostatic attraction between cations and delocalised electrons
what structure are metals?
giant metallic lattice structure
Describe graphite
each carbon bonded 3 times, 4th electron delocalised
strong covalent bonds - very high melting point and insoluble
layers far apart - low density
delocalised electrons between layers carry a charge - conduct electricity
weak forces between layers - layers slide over each other easily
describe diamond/silicon ?
each carbon bonded 4 times in tetrahedral shape
very strong covalent bonds - very high melting point, strong, insoluble
compact, rigid structure - thermal conductor
no delocalised electrons - not an electrical conductor
can be cut to make gemstones
desribe graphite?
is 1 layer of hexagonal carbon rings
one layer thick - lightweight, transparent
has delocalised electrons - conductor of electricity, also they strengthen the covalent bonds - high strength
trend in melting points across p2 and p3?
1
What are dicots?
Plants which
produce seeds which contain two cotyledons, which act as food stores for the developing embryo
form the first embryonic leaves when the seed germinates