Atomic Structure

Order of scientists and the atomic model

1. Democritus - all matter was made up of little identical lumps called atomos

2. John Dalton - agreed that matter is made up of atoms that could be broken down (smallest unit)

3. J.J Thomson - electrons are able to be removed from atoms - plum pudding model

4. Rutherford - atomic model of today - alpha scattering experiment

How was Rutherford alpha scattering experiment conducted?

• Stream of alpha particles from a radiative source fired at a very thin gold foil

• When alpha particles strike a fluorescent screen visible flashes can be seen

• Flashes were recorded and the number of alpha particles scattered at different angles

What were the results of the alpha scattering experiment?

• Most particles went straight through - atom is mostly empty space

• Some scattered at large angles - centre of the alpha particle must be very positively charged to repel them (nucleus)

• Some scattered at angles larger than 90 - the mass of the nucleus must be greater than the made of the alpha particle

• Only very few scattered at angles larger than 90 - the diameter of the nucleus must be much smaller than the diameter of the atom

• Nucleus is dense, small positive

What is the atomic number?

same as the proton number - number of proton in the atoms nucleus (same number as the electrons)

What is the nucleon number?

Same as the mass number (total number of protons and neutrons)

= mass (in atomic mass units)

In standard notation where is the atomic number and the mass number?

Mass number on top

Atomic number beneath