history of periodic table Flashcards
what were atoms believed to be at the start of the 19th century? who described them as this?
at the start of the 19th century John Dalton described atoms as solid spheres, and said that different spheres made up the different elements
what was the first subatomic particle to be discovered? who discovered it?
JJ Thomson discovered electrons
when was the ‘plum pudding’ model first introduced?
1897
what was the plum pudding model?
it was a model that showed the atom as a ball of positive charge with negative electrons embedded in it
what was the expected outcome of the alpha particle scattering experiment and why?
they were expecting the particles to pass straight through, or be very slightly deflected at most. this was because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom.
what were the results of the alpha scattering experiment?
while most of the particles did go straight through the gold sheet, some were deflected more than expected, and a small number were deflected backwards.
Who did the alpha particle scattering experiment? when did they do it?
Ernest Rutherford and his student Ernest Marsden in 1909
what did Ernest Rutherford do in the alpha scattering experiment?
fire positively charged alpha particles at an extremely thin sheet of gold
what was Rutherfords new idea for the model of the atom based on the Alpha particle scattering experiment?
He came up with the nuclear model of the atom - a tiny, positively charged nucleus at the centre, where most of the mass is concentrated. A ‘cloud’ of negative electrons surrounds this nucleus, so most of the atom is empty space.
what was the problem with Rutherford’s nuclear model of the atom?
electrons in a ‘cloud’ around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse
what was Niels Bohr’s solution to the problems with Rutherford’s nuclear model?
He suggested that all the electrons orbited the nucleus in fixed shells, each one a set distance from the nucleus
who discovered neutrons?
James Chadwick
how were elements arranged in the early 1800s?
atomic mass
when did Dmitri Mendeleev create his Table of Elements?
1869
How did Dmitri Mendeleev organise the elements?
mainly in order of atomic mass, but he did switch the order if the properties meant it should be changed
