Periodic Table
Scientists’ discoveries/theories:
Davy: used electrochemical techniques for breaking down compounds into elements (Barium,
Strontium, Calcium, Magnesium)
Mosely: discovered positive charge in nucleus (atomic number)
Dobreiner: suggested a Law of Triads, group of three elements of similar properties, where the
weight of the 2nd is the average of the weights of the 1st and 3rd.
Newlands: suggested a Law of Octaves; where elements are arranged in order of their atomic
weight, properties repeated every 8th element.
Robert Boyle: suggested that an element cannot be broken down into anything simpler
Mendeleev: arranged elements in order of atomic weight, left gaps for undiscovered items, swapped
iodine and tellurium because of their properties. Didn’t include noble gases.
John Dalton: suggested that atoms are small and indivisible and make up all matter; atoms of given
element are identical, atoms cannot be created nor destroyed in a chemical reaction
William Crookes: accelerated cathode rays (beams of electrons with negligible mass and negative
charge) in discharge tube. They cast a shadow by anode and moved the wheel.
J.J Thomson: discovered electrons, measured e/m and suggested Plum Pudding Model: positive
sphere with electrons embedded in it
Johnstone Stoney: named the electron.
Millikan: carried out oil drop experiment to determine charge on electron
James Chadwick: discovered neutrons
Modern atomic theory shows that electrons exhibit both wave and particle properties
Rutherford: Gold Foil Experiment (observation  conclusion)
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Bombarded gold foil with alpha particles (helium nuclei with 2 protons and 2 neutrons)
A very large number of alpha particles were not deflected  mostly empty space
Many deflected at small angles  positive mass repelling
A few bounced straight back  they collided with dense nucleus
Isotopes: atoms with same number of protons but different number of neutrons
Relative atomic mass: average mass of atoms of element relative to 1/12th of Carbon-12
Mass spectrometer processes in order: vaporisation, ionisation, acceleration, separation, detection.
Principle of mass spectrometer: positively charged ions are separated according to their mass when
moving in a magnetic field