Atomic Structure A level at The Sixth Form College Colchester Adapted from: An example of How Science Works: Development of the Model of Atomic Structure.

Atomic Structure

A level atThe Sixth Form College

Colchester

Adapted from:

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 ).

• 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 Relative Mass

Electron -1 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

C126

indicates a lithium NUCLEUS with its usual 3 protons and a total of 7 nucleons (4 neutrons)

indicates a carbon NUCLEUS with its usual 6 protons and a total of 12 nucleons (6 neutrons)

Li73

Isotopes

C146

C126

• 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