Atomic Theories

Dalton’s atomic theory

• He lived from 1766 - 1844
• He was an English chemist
• His ideas were:
• all elements were made from atoms
• atoms from the same element were alike
• atoms from different elements were different
• atoms from different elements combine to form compounds(in whole number ratios)

His model was called the ‘solid ball model’

JJ Thompson

• Used a cathode ray to prove that Dalton’s model could be broken into smaller particles
• He is known for discovering electrons
• He discovered them using a gas discharge tube
• Cathode rays are made of electrons

His model was known as the ‘plum pudding model’

Rutherford

• He added to Thomsons plum pudding model

• He used the gold foil experiment to discover the existence of:

• Atomic nucleus

• Protons

• He directed a narrow beam of alpha particles(+ charges) towards a thin piece of gold foil

• He predicted the (+)charges would go through however he found out every once in a while the particle was deflected/bounced back (about 1% of the time)

• this happened because the charge hit a central mass of positive charge and was repelled(like charges repel)

• His conclusions were:

• the atom contains a positively found nucleus

• the nucleus contains almost all the mass of the atom but occupies a very small volume of the atom

• the negatively charged electrons occupied most of the volume of the atom

• the atom is mostly empty space

• he created a new model

the electrons orbit around the nucleus like the planets orbit around the sun

Bohr

• worked in Rutherford’s lab

• experimented with light and it’s interaction with matter to develop a new model

• he proposed the following:

• protons and neutrons are in the nucleus

• electrons can only be in a certain distance from the nucleus

• the energy levels orbit the nucleus at fixed energy levels