development of the periodic table

Dalton’s model

• all elements made up of atoms

• atoms of the same element are exactly the same

• atoms cannot be destroyed, created or split

• atoms cannot be broken down into anything simpler or smaller

J.J Thompson

• improved Dalton’s theory - 1897 , he discovered the electron

• proposed the “plum pudding model” - a ball of positive charge with electrons embedded in the atom

Ernest Rutherford ( Gold foil experiment )

Fired alpha particles at the gold foil and observed their movement

What would have happened to the gold foil if Thompson was correct?

The alpha particles would have passed straight through and not deflected

Ernest Rutherford’s nuclear model ( 1911 )

The positive charge and mass in the atom are concentrated in a small volume in the centre of the atom. The rest of the atom is a vast, empty space

Bohr model

• improved Rutherford’s model - Rutherford did not look at the position of the electrons

• Bohr placed electrons into electron shells as electrons could only exist in electron shells

What happened at the Gold foil experiment

• Most went straight through - The atom is mostly empty space (disproving the old "plum pudding" model which claimed the positive charge filled the whole atom)

• Some were deflected -The centre of the atom has a positive charge (which repelled the positive alpha particles as like charges repel

• A tiny few bounced straight back - The center (nucleus) is tiny, dense, and contains almost all the mass of the atom