3.1-3.7 Flashcards
(34 cards)
how many elements were discovered before 1815
45
what is the law of triads and who created it
Johann Dobereiner. when elements are put into groups of three (with similar chemical properties) it states that the atomic mass of the second element falls halfway between the first and third.
which elements did Johann Dobereiner notice had a pattern
calcium, strontium, barium
and
chlorine, bromine, iodine
what did John Newlands do?
he arranged the 62 elements known at his time by atomic mass. he noticed a repeating pattern every 8 elements called the law of octaves. this pattern broke when he reached the transition metals.
what did Dmitri Mendeleev do?
he ordered the elements by atomic mass. when certain elements were put in columns a pattern of properties emerged. he left gaps and predicted properties of unknown elements. he switched the positions of some elements to better group them by properties
what did Henrey Moseley do?
he noticed that when ordered by increasing atomic number the errors in Mendeleev’s table were corrected.
what is periodicity
a repeating trend in the properties of the elements across each periods in the periodic table
what are the properties that repeat in the periodic table
- atomic radii
- 1st ionisation energy
- electronegativity
- conductivity
- M.P and B.P
explain the trend in atomic radius across periods
it decreases. shielding stays the same but nuclear charge increases. the attraction is stronger so outer electrons are pulled closer.
explain the trend in 1st ionisation energy across periods
increases. shielding stays the same but nuclear charge increases and atomic radius decreases. stronger attraction between the nucleus and outer electron
explain the trend in electronegativity across periods
it increases. shielding stays the same. atomic radius decreases, nuclear charge increases. causes stronger attraction between nucleus and bonding pair
explain the trend in conductivity across periods
first three elements: increases. the electron cloud gets denser,
others: non-conductive cause there’s no free charge.
explain the trend in melting points across periods
first three elements: increases, electron cloud gets denser, atomic radius decreases, charge increases.
group 4: larger M.P than metals. strong covalent bonds in the giant covalent lattice
group 5-8: much lower M.P’s. intermolecular forces are weak so low energy requirement.
what is oxidation
the gain of oxygen or the loss of electrons
what is reduction
the loss of oxygen or the gain of electrons
whats a redox reaction
when oxidation and reduction take place
what are oxidising and reducing agents
oxidising agent: the species that accepts the electrons
reducing agent: the species that transfers the electrons
what are oxidation numbers
the hypothetical charge on an element in an ion or molecule if the bonding is considered 100% ionic.
rules of assigning ON’s.
- always 0 in elements or diatomic molecules
- the most electronegative element has a negative ON
- in simple ions the ON is the same as the charge
4, ON’s add to zero in neutral molecules - ON’s add to equal the charge of molecular ions
which elements usually have the same ON’s and what are there exceptions
Fluorine: always -1 but 0 in the diatomic molecule (F2)
oxygen: -2 but 0 in O2. -1 in H2O2. +2 in F2O
Hydrogen: +1 but 0 in the diatomic molecule (H2)
chlorine: -1 but O in Cl2. +1 to +7 with oxygen and fluorine
how are molecule names
put the ON in roman numerals in brackets next to its name. ie, Lead(II) oxide. or sulphate(VI)
how can you identify a redox reaction
assign ON’s to each element. any changes to the ON’s of the reactants indicate the loss or gain of electrons. this shows you what was oxidised and reduced.
what is disproportionation
when the same species is oxidised and reduced in a reaction.
how can half equations be combined to give a full redox equation
makes the number of electrons equal by multiplying everything in the reaction. combine the reactants and products to write a full equation. cancel out any electrons, hydrogens and waters if there on both sides.
