Atomic Structure

A level

AtThe Sixth Form College

Colchester

Early Ideas of Atoms

• Two fifth-century BC Greeks, – Said look at matter on smaller and smaller

scales ultimately you would see individual atoms - objects that could not be divided further (the definition of atom).

• Galileo and Newton both believed in atoms

• Lavoisier, Dalton and Avogadro found support for atoms in Chemistry

Atomic Structure

• 1900 – state of atomic knowledge– Matter made of atoms– Atoms are electrically neutral– Atoms contain -ve electrons -- which were

identified by JJ Thomson in 1897– Atoms must contain +ve charge

Atomic Model

• Plum Pudding– Put forward by

Kelvin in 1903 and supported by JJ Thomson

– Uniform sphere of +ve charge with electrons embedded inside

Rutherford Scattering

• Alpha particle scattering– Rutherford aimed alpha particles at

a thin foil and 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 while most alpha 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 the +ve charge (and most of the mass) has to be in a very small diameter

• About 10-15 m compared to 10-10 m for the diameter of the atom

Disproof of the Pudding

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 it emits a single photon, its energy is exactly equal to the energy difference between the orbits

Atomic Structure

• Atoms are approx 10-10 m in diameter

• Atoms consist of a positively charged nucleus surrounded by orbiting electrons

• The nucleus is approx 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

Charge Mass

Electron -e

-1.6 x 10-19 C

9.11 x 10-31 kg

Proton +e

+1.6 x 10-19 C

1836 me

Neutron No charge 1839 me

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 helium NUCLEUS with its usual 2 protons and a total of 4 nucleons (2 neutrons)

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

He42

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