Heating and Freezing

Salting

Fermentation

The process of heating copper before it is hammered into shapes, to make it stronger and less brittle, is called annealing.

The process of separating metals from compounds by melting them is called smelting.

John Dalton(1766 - 1844)

The “billiard ball” model

Atoms are tiny, indivisible particles of elements.

All elements are composed of atoms.

Atoms of the same element are identical.

Atoms combine in fixed ratios to form compounds.

J.J. Thomson(1856 – 1940)

The “plum pudding” model

An atom consists of one large positive charge and many small negative charges embedded in it.

Thomson’s experiments showed the existence of positive and negative subatomic particles.

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? --which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science."

- an excerpt from a speech he made in 1934J.J. Thomson(1856 – 1940)

Ernest Rutherford(1871 –

1937)

The “planetary” or “nuclear” model

Every atom has a tiny, extremely dense positive core which he called the nucleus.

The negative electrons orbit the nucleus like planets around the sun.

Rutherford was surprised when all the particles did not go straight through the gold foil. He realized that each atom must have a dense core of positive charge.

Neils Bohr(1885 –

1962)

Electrons are arranged around the nucleus in very specific orbits or energy levels.

Bohr used the light emitted from excited hydrogen atoms to form his ideas about the structure of the atom.

The hydrogen emission spectrum

Light is emitted when an electron falls from a higher energy level to a lower one. A different colour is emitted for each different transition.

Every different element emits its own characteristic emission spectrum.

This model is very difficult to visualize and is beyond the scope of this course.

The modern model of the atom describes the positions of electrons in an atom in terms of mathematical probabilities.

An electron can potentially be found at any distance from the nucleus, but tends to exist more frequently in certain regions around the nucleus than others, depending on its energy level. The “electron cloud” model

read pages 18 – 25

A1.3 Check and Reflect page 25 #’s 1 – 11

A Letter to Thomson