Atomic Theory and Structure of the Atom

• first to suggest the existence of atoms

• believed atoms were small indivisible particles

• Atom: smallest particle of an element that retains all properties of the element.

A. Democritus (460 B.C. – 370 B.C)

B. Dalton• Atomic theory composed of 4

postulates– Elements are composed of atoms– Atoms of the same element are

identical, each element is unique– Compounds are composed of atoms

of more than 1 element, atoms combine in fixed ratios to form compounds

– Chemical rxn involves separation, combination, or rearrangement of atoms

Name of theoryBillard Ball

Model

C. Electron1. Symbol: e−

– Charge −1

**after Dalton, there was a big hiatus in the theory of the atom. Everyone accepted Daltons theory and went on to study radiation for a while, and this is when the electron was discovered

D. J.J. Thomson• An atom is thought of as a uniform positive

sphere with electrons evenly distributed through out (like plum pudding or a chocolate chip cookie)

Name of theoryPlum Pudding

Model

Cathode Ray Tube

Battery

Evacuated tube

CathodeAnode

− +

E. MillikanMillikan Oil Drop

• Found the mass of an

electron to be 9.10×10−28 g

F. Rutherford• Conducted gold foil experiment

What did Rutherford expect?

Gold Foil Experiment Animation and Discussion

Click here for video!

What did Rutherford actually see?

F. Rutherford

• Concluded all positive charge and mass is concentrated in small region called nucleus and the electrons are outside the nucleus

• Proposed atom is mostly empty space

Name of theoryNuclear Model

Proton– p+

– Charge: +1– Mass:1.673×10−24 g

G. Bohr• Expanded on the nuclear atom• Electrons travel in circular orbits around

the nucleus

Name of theoryPlanetary Model

H. Schrödinger• model allowed the electron to occupy

three-dimensional space like an electron “cloud”

Name of theoryQuantum Mechanical

Model or Electron Cloud Model

I. ChadwickThe NeutronSymbol: n0

Charge: noneMass:1.675×10−24 g

**The neutron was found by comparing the mass of hydrogen to helium, since He increased by 1proton, the mass should be double, but it was four times, leading to a subatomic particle that had mass but no charge.

Fun Facts

• The volume of a hydrogen nucleus is a trillion times smaller than the volume of a hydrogen atom, yet the nucleus contains most of the mass.

• If the nucleus (proton) of a hydrogen atom were as large as the width of a human thumb, the electron would be on the average about one kilometer away in a great expanse of empty space.