pr1 chem: history of atoms

atom

smallest particle of an element that retains the element’s properties

Democritus (460-370 BCE)

• “matter is made of tiny, solid indivisible particles (atoms)

• differences in matter are due to the differences in the size, shape, and movement of atoms

John Dalton

• all atoms of one element are identical

• atoms of a given element are different from those of any other element

• atoms are indivisible; cannot be created nor destroyed

• atoms of one element combine with atoms of another to form compounds

Which of Dalton’s theories are wrong, and why?

• all atoms of one element are identical

  • isotopes

• atoms are indivisible; cannot be created nor destroyed

  • atoms are made of subatomic particles; therefore they ARE divisible

JJ Thomson

• discovered the electron and its negative charge

  • “plum pudding” model

cathode ray tubes

glass tube in which electrons flow due to opposite charges at each end

Ernest Rutherford

• discovered the nucleus via the “Gold Foil Experiment”

  • discovered the proton

Gold Foil Experiment

• alpha particles produced from the radioactive decay of polonium streamed toward a gold foil sheet and bounced off

  • the bouncing off showed they were hitting something large and dense, which Rutherford called the nucleus

James Chadwick

discovered the neutron

Niehls Bohr

developed the Bohr model of the atom

Bohr model

• electrons are restricted to specific energies and follow paths called orbits a fixed distance away from the nucleus

What is wrong with the Bohr model?

electrons do not follow a fixed orbit (electron cloud)

atomic number

number of protons/electrons in a neutral atom

atomic mass

• protons + neutrons

• all compared to C-12

isotopes

• atoms of the same element but with different # of neutrons

• most elements have at least 1 naturally occurring isotope

current model of the atom

quantum mechanical model