early 19th century less than 40 elements discovered
new ones discovered regularly
scientists noticed patterns in the way
they reacted- the properties
eg atomic weight
John Newlands
order of atomic mass
law of octaves
similar properties every 8th element
Problems
some groups have a mix of metals
and non metals
only worked for first few
elements (broke after calcium)
elements still being
discovered
Demitri Mendeleev
left gaps for undiscovered elements-
so elements could have similar
properties to others in the group
predicted atmoic masses and properties for
undiscovered elements
After Mendeleev
noble gases discovered
electrons discovered
atomic structure discovered
in accuracies that still
existed solved by placement
in order of atmic number
Modern periodic table ---
groups and periods
Modern Periodic Table
Now by atmoic number
period number is number of outermost shell
group number is the number of electrons on outermost shell
Group 1 - Alkali Metals
low density
react with non mdetals to form ionic compounds
react with water to release hydrogen
form hydroxides
that dissolvve in
water to give
alkaline solutions
futher down the group -
more reactive the
element + lower melting
& boiling point
soft
conduct heat and electricity
low density-float on water
react with water and oxygen in air (which is why it tarnishes quick)
stopped by being stored in oil
when they react lose electron an form +1 ions
alkali metal+water= metal hydroxide +hydrogen
going down the group ----( high up the group)
outer electron closer to nucleus so stronger
attraction to nucleus so less screening by inner
electrons thus electron lost less easily.......lower
down the outer electron is futher from nuclues so
nuclear attraction weaker so more screening by
inner electron thus electron lost more easily
higher energy level of outer
electron - the more easily the
electrons are lost and the less
easily the electrons are gained
Group 7 - Halogens
poor conductors
poisonous non
metals - coloured
vapours
react with metals to form ionic compounds
ion produced called halide and has charge of -1
further down --- less reactive.....higher
melting & boiling points due to weak molecules
A more reactive halogen can displace
less reactive halogen from an aqueous
solution of its salts ( dissolved in water)
eg Cl 2 + 2KBr = 2KCl + Br 2
dont conduct heat or electricity
diatomic molecules
going down the group----( high up) outer
electron closer to nucleus so nuclear attraction
stronger so less screening by inner electrons
thus electron gainedd more easily (lower
down) outermost electron futher from nucleus
so weaker nuclear attraction so more
screeening by inner electrons thus electron
gained less easily
higher energy level of
outer electron ----
electron more easily
lost....electron less
easily gained
Transition Metals
higher melting and boiling points (except mercury a liquid at room temp)
harder and stronger -
good for structrual
material eg construction,wiring