A Brief History of the Atom
In the beginning…. the earth was without form and void and
darkness was upon the face of the deep;
How do we make sense of the immense chaos of the matter that
surrounds us? For centuries humans have tried to make sense of it.
Two ways of making sense of the world of matter are seen in the
development of the atom (or if you like Atomic theory) and the
Periodic table.
First the History of the Atom
Aristotle
4 elements?
Democritus
Atoms 400 B.C.
Al Razi
Plaster of Paris
Libau
Alchemia
Boyle
Lavoisier
Dalton
J.J.Thompson
1660 behaviour of gases studied
Oxygen
Defn. element
Electrons in raisin Bun model 1897
Hantaro Nagaoka
Solar system model of the atom 1904
Rutherford
1908 Radioactivity and discovery of the
nucleus
Theory of electron shells
Niels Bohr
James Chadwick
Quantum theory
Nucleus contained positively charged
particles called protons and neutral
particles called neutrons
The atom is a cloud of electrons
surrounding a nucleus
A Brief History of the Atom
Development of the Periodic Table
Earliest chemists
Used planetary symbols for elements
John Dalton
Developed first set of symbols to describe
elements
Jons Jacob
Used letters rather than pictures to describe
each element
Listed elements in A breakthrough way of organizing matter.
increasing atomic
mass
John Newland
Recognized patters when elements were listed
1864
in increasing atomic mass (by comparing it to
the Carbon atom)
Law of Octaves
Properties of elements seemed to repeat at
regular intervals. (Why?)
Dmitri Mendeleev Was able to organize the elements in a way
Russian chemist
that reflected the patterns in the properties of
1869
elements.
Atomic Theory
Element- the building blocks of all matter are elements;
they are defined as pure substances. The idea of a pure
substance is important, because it helps us know that the
matter is always going to act that way. In 1808 John Dalton
stated that each element is composed of a particle called an
atom. All atoms in a particular element are all equal in mass.
Further no two elements have atoms of the same mass. For
example gold atoms have a different mass than tin atoms.
J.J. Thompson was the first person to find out about sub
atomic particles. He used cathode ray tubes. After
experimenting with streams of negatively charged particles he
found there mass was much smaller than even a hydrogen
A Brief History of the Atom
atom. In 1897 he described this model of a positively charged
nucleus mixed with negatively charged electrons.
From 1898-1907 Ernest Rutherford was doing some work and
he shot some positively charged particles through gold foil.
Rutherford proposed that atoms were mostly space. At the
core was a tiny positive charged center. He called this core the
nucleus. The nucleus is only about 1/10,000th the size of an
atom.
Niels Bohr was a Danish researcher. He made the next
major contribution to the atom theory. He said that electrons
A Brief History of the Atom
do not orbit randomly in an atom. Bohr came up with the idea
of the electron shells. For example as an electron gained or lost
energy it would jump between these shells.
James Chadwick discovered the next improvement in the
atomic theory. His idea was that the nucleus contained neutral
particles called neutrons and positively charged particles called
protons. The neutron has about the same mass as the proton.
The electron has on 1/1837th the mass of either a proton or a
neutron.
In conclusion, today we know from quantum mechanics
that the structure of the atom is slightly different. The
quantum mechanical model describes atoms as existing in a
charged cloud around the nucleus.
A Brief History of the Atom