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The Atomic Theory

John Dalton
 English chemist
 Early 1800’s
 He revived the theory of the atom
as the smallest piece of matter.
 Was able to explain how chemical
substances behave with one
another.

Five main points of Dalton’s theory:
 Elements are made of tiny particles called atoms.
 All atoms of a given element are identical.
 The atoms of a given element are different from those of any
other element; the atoms of different elements can be
distinguished from one another by their respective relative
weights.
 Atoms of one element can combine with atoms of other
elements to form chemical compounds; a given compound
always has the same relative numbers of types of atoms.
 Atoms cannot be created, divided into smaller particles, nor
destroyed in the chemical process; a chemical reaction simply
changes the way atoms are grouped together

J.J. Thomson
 English physicist
 Believed in the atomic theory
 Discovered the electron in
1897
 Nobel prize in 1906 for physics
 Developed the muffin or raisin
bun model of the atom

Ernest Rutherford
 Student of Thomson
 Discovered the nucleus in
1911
 Later discovered the proton
▪ This was with the alpha
scattering experiment using
alpha particles and gold foil
 Nobel prize in 1908

Neils Bohr
 Danish physicist
 Nobel prize in 1922
 Discovered that electrons
travel in orbits around the
nucleus (shells)
 Later worked on the
Manhattan Project

James Chadwick
 Discovered the neutron
in 1932
 Nobel prize for physics
in 1935
} Nucleus
Shell

Atomic Number
 Refers to the number of protons in an atom
 Refers to the number of electrons also, if the atom
is neutral
 Each element has its own atomic number

Atomic Mass
 The total number of all neutrons and protons in
the nucleus of the atom
 i.e. Carbon = 12 amu (atomic mass units)

Isotopes:
 Atoms of the same
element that have
different masses and
different numbers of
neutrons