Chem history Flashcards
what did John Dalton do and what are his 5 postulates
1803, came up with Dalton’s atomic theory
- all matter is made up of tiny particles called atoms
- atoms cannot be subdivided, created or destroyed
- atoms of the same element all have the same mass, size and other properties
- able to combine with atoms of other elements in variable integer ratios to form chemical compounds
- in chemical reactions, atoms are rearranged, combined and separated
why was Dalton wrong
- protons, neutrons, electrons
- isotopes
what is a cathode ray tube?
- sealed glass tubes from which most of the air has been evacuated
- high voltage applied across 2 electrodes at 1 end of the tube
- beam of particles (aka cathode ray) flows from cathode to anode
what did JJ Thomson discover?
1897, atoms consist of negatively charged and positively charged particles through cathode ray experiment, plum pudding model
what are JJ Thomsons 3 ideas?
- cathode ray consist of negatively charged particles
- negatively charged particles have a mass of about 1/2000 of hydrogen atom, therefore must exist within the atom
- these subatomic particles are found within all atoms
how did JJ Thomson prove that - particles exist?
- put 2 oppositely charged electric plates around the cathode ray
- cathode ray deflected from - to + electric plate
- cathode ray composed of - charged particles
how did JJ Thomson prove that they exist as part of the atom?
- 2 magnets on either side of cathode ray, deflected again
- determined mass to charge ratio of these particles, discovered mass smaller than any known atom, mass 1/2000 hydrogen atom
- must exist as subatomic particles
how did JJ Thomson prove that these particles exist for all atoms?
- repeated experiment with different electrode metals
- properties of cathode ray remained constant no matter what cathode material they originated from
how was plum pudding model created?
- since overall charge of atom is neutral, positively charged particles also exist
- numerous electrons embedded within a much larger, uniform, positively charged sphere, accounts for most of its mass
what did Earnest Rutherford discover?
1911, postively charged nucleus, gold foil experiment
Describe gold foil experiment
- fired a beam of alpha particles through a thin sheet of gold foil
- expected alpha particles to pass through the gold foil with little to no deflection
- small number of alpha particles were deflected very significantly, with some even turning back on its original direction
why were alpha particles expected to pass through gold foil?
- even though alpha particles had a positive charge, positive charge in PPM assumed to be spread throughout the entire volume of atom
- electric field from the positive charge of atom too weak to deflect the relatively larger and fast moving alpha particles
What was nuclear model?
- atoms consisted of mostly empty space occupied by only the low mass electrons
- all of the atoms mass and postive charge is located in a tiny volume of the atom, the nucleus
- positive charge being carried by protons
Limitations of Rutherfords
- protons and electrons alone could not account for all the atoms mass
- did not explain emission and absorption spectra
What did Niels Bohr discover
1913, existence of shells, electrons occupy specific energy levels in the atom, proved by investigating hydrogen emission and absorption spectra
describe AS relative to Niels Bohr
continuous emission spectrum from a black body is passed through a cool sample of gas, some spectral lines were subtracted from continuous ES
define random error
an error caused when 1 measurement differs slightly from the next, comes from unpredictable changes that happen during the experiment
define systematic error
always cause measured error to be above or below the true value, consistent and will occur again if the investigation is repeated in the same way
define accuracy
ability to obtain the correct measurement
define precision
ability to consistently obtain the same measurement
how to improve flame test (validity reliability etc)
low precision: different perceptions of colour
- perform flame test in dark room, use non-luminous flame, use AES
low accuracy: some metal ions produce similar colours
Limitations of Bohr model
Failed to explain the Zeeman effect, effect of magnetic field on the spectra of atoms
Failed to explain the stark effect, electric
What did James Chadwick discover?
1932, Existence of neutrons in the nucleus, when he bombarded Beryllium with alpha particles, an unknown radiation was produced
Chadwick interpreted the radiation being composed of particles with a neutral charge and the approximate mass of a proton
Why did Chadwick take so long to discover neutrons?
prior experiments utilised the fact that charged particles reacted with an electric or magnetic field, facilitated their earlier discovery
neutrons uncharged, forced a later discovery
