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Revision of assumed knowledge

All isolated systems will move spontaneously to the lowest energy state

available to them.

An ELEMENT is a pure material made up of one type of atom.

An ATOM is the smallest part of an element that maintains the

characteristics of the element.

A MOLECLE is a chemical combination of two or more atoms.

A COM!N" is a pure substance with the smallest part of which that

maintains the characteristics of the compound is the molecule.

Atom is to element that a molecule is to acompound

A solid has a #$ed shape

 The atoms and % or molecules are in #$ed positions.

A li&uid ta'es the shape of its container.

 The atoms of which it (s made are free to move around each other.

A gas #lls the volume that contains it.

 The atoms move around in the container colliding with each other and thewalls of the container.

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Rutherford model of the atom

)ith the discovery of radiation *nuclear+, -utherford was able to e$plore

the atom. The accepted view was the Thomason plum pudding model/

i.e. atoms come in various si0es but were of uniform density and

bac'ground material, impregnated with something else li'e raisins in a

plum pudding. Electrostatic charging was e$plained by having too many or

too few raisins.

Rutherford

-utherford #red alpha particles i.e. helium nuclei through a gold foil sheet

and determined their path by counting impacts on a scintillation detector.

1e found that most particles passed straight through, some were

de2ected slightly and a small number re2ected bac'. -utherford said this

was li'e #ring a cannon ball at tissue paper and having it bounce bac'.

 These results could be e$plained if most of matter i.e. the atoms were

located in a positive central core, the positive charges balanced by

orbiting negative particles and most matter being empty space.

3oldAlpha !articles

Alpha

-adioactive source

6cintillation counter

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-utherford atom

Nuclear radii are 7 89 :8; m

Atomic radii are 7 89 :89 m

4rom the bohr model we can say that electrons associated with atoms

e$ist in discrete, speci#c energy levels. These levels may be thought of as

 

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1 1e

Li ?e ?

C N O 4 Ne

Na Mg Al

6i ! 6

Cl Ar @ Ca

  Atomic Mass Number of protons 5 Neutrons

  B  Al Aluminium

  8

  Atomic Number number of protons