The Atomic Theory Atomic Theory Matching Atom Size Democritus Dalton – Ball Model J.J. Thompson – Raisin Bun Model Rutherford – Gold Foil Experiment Bohr – Energy Level Model

How are atoms studied? Atoms are the building blocks of matter Atoms are too small in size to study easily Size of Earth : soda can = soda can : atom

Sizing up the Atom Materials are able to be subdivided into smaller and smaller particles – these are atoms. If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long

The Size of the Atom Despite their small size, individual atoms are observable with instruments such as scanning electron microscopes In this picture, atoms are being moved by the single atom tip of a Atomic Force Microscope (AFM). Apart from allowing scientist to image atoms, this instrument also allows them to actually move them one at the time.

The Atom is Really, Really Small! Just How Small Is An Atom?

The Greek philosopher Democritus (460 B. C. – 370 B. C The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms He believed that atoms were indivisible and indestructible

Dalton’s Atomic Theory – The Ball Model: 1800s Referred to the atom as a small, hard, indestructible sphere that cannot be subdivided Different atoms have different properties An atom is the smallest particle of an element an element is a substance made up of only 1 type of atom

J.J. Thomson In 1897 J.J. Thomson demonstrated that there are negatively charged particles found within atoms using cathode ray tubes After further investigation he concluded that the same negatively charged particles are found in all atoms These are now called electrons (e-)

Thomson’s Model He knew atoms didn’t have a charge (+ or -) Since his particles were negative, he knew that there must also be positively charged particles within the atom J. J. Thomson

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding” It was called the “Plum Pudding” or “Raisin Bun” model.

Ernest Rutherford’s Gold Foil Experiment - 1911 “+” charged particles (alpha particles) were fired at a thin sheet of gold foil Particles that hit on the detecting screen (blue film) were recorded

What he thought would happen given the “Plum Pudding” Model Some alpha particles would be slightly diverted due to their avoidance of the other positive particles inside the atom

What he actually found Most of the alpha particles slightly deflected and carried on but some bounced backwards Therefore, they must have collided and rebounded off something very dense

Rutherford’s Gold Foil Experiment

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected (1 out of 8000!) Egads! It’s like shooting bullets at tissue paper and watching them bounce back

The Rutherford Atomic Model Based on his experimental evidence he concluded that the atom: Is mostly empty space Contains a small, dense, positively charged nucleus at the center Negatively charged electrons revolved around the positive nucleus

Bohr - 1912 Niels Bohr If there are negatively charged particles around a positively charged nucleus, why don’t they attract and collide (opposites attract) - - + - -

The Bohr-Rutherford Model Central positive nucleus Electrons move in orbits (energy levels) around the nucleus Electrons more or less stay in their orbit unless an increase in energy causes them to move