Chapter 2 The Atom

Matter

All matter is composed of particles, can be in the form of atoms, ions or molecules

What is an atom

Is the smallest part of an element that shows the element's properties

The Greeks

Proposed matter was made up of small individual particles (‘Atomos')

John Dalton

Carried out a number of experiment and proposed the atomic theory

Assumptions of Dalton's theory

Matter is made up of small particles called atoms

Atoms are indivisible (cannot be broken down)

Atoms of the same element have an identical atomic mass

William Crooke

Discovered cathode rays

Discovered cathode rays must consist of some kind of very small particles

Crooke's first experiment

Discovering cathode rays

How: passed electric current through a vacuum tube at low pressure

Observed: fluorescence on the glass at the far end of the tube

A shadow of the metal cross (anode) was observed at the for end of the tube

Concluded: radiation was coming from the cathode that was being blocked by the anode metal cross

Called these rays cathode rays

Crooke's second experiment

Discovering cathode rays must consist of some kind of very small particles

Passed electric current through a vacuum tube at low pressure with a paddle wheel in the centre

Observed: the paddle wheel moved and always away from the cathode

Concluded: the cathode rays must consist of some kind of very small particles

George stone

Irish physicist

Named the electron

JJ Thompson

Discovered the election must consist of small subatomic particles

Properties of electron Thompson discovered

Discovered that electrons are negatively changed

Calculated the ratio of the size of the charge on an electron to its mass (e/m ratio)

How: Passed cathode rays through a small hole in the anode to obtain a narrow beam

Passed this narrow cathode may beam between two parallel metal plates

Observed: if the parallel plates were not charged the cathode rays were passed straight through and hit the centre of screen at the end of the tube

If the parallel plates were charged, the cathode rays were deflected towards the positively charged plate and hit the screen off centre

Concluded: cathode rays consisted of negatively changed subatomic particles which were called electrons

Thompson's plum pudding model

Atoms were spheres of positive charge with negative electrons randomly imbedded

Cathode rays

Cathode rays are beams of negatively charged electrons

Properties: consist of negatively charged electrons

Can be deflected by electric and magnetic fields

Can be detected by fluorescence on a glass tube

Can cause a small paddle wheel to move

Robert Millikan

Determined the magnitude of the charge on an electron

Known as Milikan's oil drop experiment

Milikan's oil drop experiment

Ernest Rutherford

Discovered the nucleus

Discovered positively charged protons within the nucleus

How: bombarded a thin gold foil with alpha particles

Observed:

Expected: Most of the alpha particles went straight through the gold foil undeflected or with very slight deflection

Not expected: some alpha particles DEFLECTED at large angles

Completely not expected: a tiny number of alpha particles REFLECTED completely along their original paths

Concluded:

1. Atoms are mostly empty space - if most alpha particles passed through undeflected on only slightly

2. There must be an area in the centre of the atom that is:

   • Very small - if mort alpha particles passed through undeflected or only slightly deflected

   • Positively charged - of some positive alpha particles that came close were deflected at large angles, they were being repelled

   • Dense - if the tiny number of alpha particles that collided were reflected completely back only their original path

3. Electrons must reside in an electron cloud surrounding the nucleus

James Chadwick

Discovered neutrons in the nucleus which have no charge

How: bombarded a sample of beryllium with alpha particles

Observed: particles were emitted that had the same mass as protons but had no charge

Concluded: there was a third sub -atomic particle he called this the nether