the history of the atom

Question 1

the theory of atomic structure has changed over time

Answer 1

• start of the 19th century, John Dalton described atoms as solid spheres and said that different spheres made up the different elements.
• in 1897, J J Thompson concluded from his experiments that atoms weren’t spheres. his measurement of charge and mass showed that an atom must contain even smaller, negatively charged particles- ELECTRONS.
• new theory was called ‘the plum pudding model’.
• the plum pudding model showed the atom as a ball of positive charge with electrons stuck in it.

Question 2

Rutherford showed that the plum pudding model was wrong

Answer 2

• in 1909, Rutherford and his student Marsden conducted the famous alpha particle scattering experiment. They fired positive charged alpha particles at an extremely thing sheet of gold.
• from PPM, they were expecting the particles to pass straight through the sheet or to be slightly defected at most.
• because the positive charge of each atom was though to be very spread out through the ‘pudding’ of atom. most particles did go straight past the sheet but some were deflected more than expected and a small number deflected backwards. PPM was wrong.
• he came up with the idea to explain the new evidence- the nuclear model of the atom. in this, there’s a tiny, positively charged nucleus at the centre, where most of mass is concentrated.
• ‘cloud’ of negative electrons surrounds this nucleus- so most of the atom is empty space.

Question 3

Bohr’s nuclear model explains a lot

Answer 3

• scientists realized that the electrons in a ‘cloud’ around the nucleus of an atom, as R described, would be attracted to the nucleus, causing atom to collapse. Bohr’s nuclear model of the atom suggested that all the electrons were contained in shells.
• Bohr’s proposed that electrons orbit the nucleus in fixed shells and aren’t anywhere in between. Each shell is a fixed distance from the nucleus.
• Bohr’s theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists’ observations at the time.