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daltons model

john dalton thought the atom was just a tiny solid unbreakable ball

thompsons model

jj thompson discovered negative electrons . he thought the atom was a ball of positive ions with negative ions embedded inside

rutherford model

realised the atom is mostly empty . found a hard positive stone in the middle (nucleus) and said electrons float around it

bohr modeln

said electrons circle the nucleus in neat fixed tracks (shells) like planets orbiting the sun

chadwick

found the neutron hiding inside the center with protons

during chromatography why should you draw in pencil

ink would dissolve and run up the paper ruining results

why should the solvent level be below the pencil line

spots will just wash off into the beaker

pure in chromatography

leaves one spot

impure in chromatography

leaves multiple spots

why do simple molecules have low boiling points

strong covalent bonds , only break weak intermolecular forces

ionic compounds

cannot conduct electricity when solid but can conduct electricity when molten or dissolved

why is the percentage yield never 100%

reaction can be reversible , some product lost when separating

aqueous

substance is dissolved in water

mole formula

moles = mass / relative atomic mass

atom economy formula

atom economy = relative mass of the product you want / total relative mass of all reactants x 100