Transcript 3.1.1 Fundamental Particles Introduction - Chemistry

Adapted
from:
Atomic Structure
A level at
The Sixth Form College
Colchester
An example of How Science Works:
Development of the Model of
Atomic Structure
Early Ideas of Matter
Polystyrene foam?
Early Ideas of Atoms
• Two Ancient Greeks (5th Century BC):
Look at matter on smaller and smaller scales:
Ultimately you will see individual atoms objects that cannot be divided further (the
definition of atom or atomos).
• Galileo and Newton both believed in atoms
• Lavoisier, Dalton and Avogadro found
support for atoms in Chemistry
Billiard Ball Atoms
• The theory of atoms only became widely
recognised when scientists could measure
accurately the formulas of compounds.
• Knowing that water was formed from gases:
• 2 volumes of hydrogen and 1 volume of
oxygen
• suggested that H2O was the formula and
lead Dalton to hypothesise that water was
made from 2 atoms of hydrogen joined to
one atom of oxygen.
Atomic Structure
• – 1808 state of atomic knowledge (Dalton)
– Matter made of atoms like billiard balls
– Atoms are electrically neutral
– Atoms react in simple whole number ratios
e.g. H2O CO2 MgSO4
Electrical Matter
• Davy and Faraday realised that matter is
electrical in nature as a result of
experiments involving electrolysis
• Now the billiard ball did not look so good
• Where were the electrical charges?
• More investigations were carried out to find
out what was electrically charged
Crookes 1875
• Crookes passed an electric current through
an almost-evacuated tube.
• He found that a beam travelled from the
cathode towards the anode and made the
glass fluoresce
JJ Thompson - 1897
• He found that the beam of cathode rays was
deflected towards the positive electrode
when passed through an electric field.
• The cathode rays must be:
• Negatively charged.
• He used the term “electron” (first used a
few years earlier) for these negative
particles
Thompson’s experiment:
Plum Pudding
• Thompson (Nobel Prize 1906) came up
with the plum pudding atomic model:
Uniform sphere of
+ve charge with
electrons embedded
inside
Goldstein - 1886
• Discovered that positive rays were emitted
behind the cathode in the cathode ray tube.
Positive Rays
• The mass of the particles in the positive
rays depended on which element was in the
Crookes tube
• The smallest positive rays were present
when hydrogen was the initial gas.
• This smallest positive particle was
eventually named the PROTON by
Rutherford in 1914
Becquerel- Radioactivity 1896
• One type of radiation discovered by
Becquerel was Alpha particles
• These were found to be particles much
smaller than atoms with a 2+ charge
• Rutherford and his co-workers decided to
use a stream of alpha particles like bullets to
probe the inner structure of atoms
Rutherford Scattering
• Alpha particle scattering
– Rutherford aimed alpha particles at
a thin foil
– He expected all to go straight
through.
– But noted that some were deflected
• Manchester 1909
– Experiment performed just after
Thomson (Rutherford’s old boss)
published his “Plum Pudding” paper
Rutherford Scattering
Geiger and Marsden
• Continued with Rutherford’s work
• Scattered alpha particles with heavy metal
foils, particularly gold
Geiger and Marsden
• Found that:
• Most alpha particles went straight
through with very little deflection
• A few were deflected by large angles
• About 1 in 8000 was reflected
This image is taken from a Java Applet at:
http://www.scri.fsu.edu/~jac/Java/rutherford.html
Geiger Marsden Experiment
Disproof of the Pudding
• Rutherford calculated from the results –
1911:
• To reflect alpha particles the +ve charge
(and most of the mass) has to be in a very
small diameter
• About 1 x 10-15 m compared to 1 x 10-10 m
for the diameter of the atom
Disproof of the Pudding
New Model:
• To explain the large size of the atom and the
very dense nucleus the next model had both
protons and electrons in the central nucleus
and orbiting electrons.
Chadwick 1932
• It took Chadwick’s discovery of the neutron
to produce the more modern version of the
atom:
• A nucleus containing protons and neutrons
with electrons orbiting in shells.
• Even this is not the full story ………
Solar System Model
• Positively charged nucleus at centre
• Negatively charged electrons in orbit
• Problem –
– Orbiting electrons are accelerating –
– Will give off energy –
– Will spiral in to centre
• Model not stable
Bohr Atom
• 1913 Bohr presented his theory
(Nobel Prize 1922)
• Electrons in atoms can exist ONLY in certain
discrete orbits, and they do not radiate energy
• When an electron jumps from one orbit to another
its energy is exactly equal to the energy difference
between the orbits
• Quantum theory was then the focus of research to
explain the structure in more accurate detail
Atomic Structure
• Atoms are approx 1 x 10-10 m in diameter
• Atoms consist of a positively charged
nucleus surrounded by orbiting electrons
• The nucleus is approx 1 x 10-15 m in
diameter
• Most of the atom is empty space
Nuclear Structure
• The nucleus is made up of two particles
(collectively called NUCLEONS)
• Protons and neutrons
– Protons are +ve in charge
– Neutrons are neutral
Atomic Particles
Relative Charge
Electron -1
Relative Mass
1/1840
Proton
1
+1
Neutron No charge
1
Describing the Nucleus
• We describe the nucleus by noting its
Chemical Symbol along with
– The number of protons Z
– The number of nucleons A
• Each chemical symbol always has the same
number of protons
– Hydrogen – 1
– Helium -- 4
– Carbon – 6
Describing the Nucleus
12
6
7
3
C
Li
indicates a carbon NUCLEUS with its
usual 6 protons and a total of 12
nucleons (6 neutrons)
indicates a lithium NUCLEUS with its
usual 3 protons and a total of 7
nucleons (4 neutrons)
Isotopes
12
6
C
14
6
C
• These two represent isotopes of carbon
– Chemically they would behave identically (if
they are combined with 6 electrons to make
atoms)
– To a physicist they are different
• Different masses
• Different behaviour
– They can be separated by physics, not by
chemistry