GSCI 163

Lecture 2

Atomic theory

• What are things made of?

Imagine dividing an object into smaller and smaller units. Is there a limit to how much we can divide?

Atom

• Means “uncuttable”. Democritus (460-362 BC)

John Dalton (1766 – 1844)

• All matter is made of atoms• Atoms cannot be created or destroyed.• All atoms of a given element are identical in

mass and properties. Different elements differ from each other because they are made of different atoms.

• Compounds are formed by the combination of two or more kinds of atoms in precise ratios.

• A chemical reaction is a rearrangement of atoms.

Is the atom indivisible?

• J.J. Thompson – discovery of the electron 1897.

The cathode ray tube

What is the electron?

• Robert A. Millikan in 1906 measured the charge of a single electron

The model of the atom at this point

• Positive and negative charges mixed like raisin in jello

Discovery of the nucleus

• Ernest Rutherford in 1907 found that the atom has a nucleus made of positive particles.

• 100,000 smaller than the orbit of the electron

How is the nucleus held together?

• We need a glue to overcome electrical repulsion!

• New particle = neutron

• James Chadwick in 1932

Dmitri Mendeleev

Next class

Atomic states

• Read handout day 3• Quiz on these topics (first 10-15 min of class)