Events
400 BCE
Democritus discovers atoms.
Democritus claims that all matter is made of "atomos".
There was no empirical evidence to support his idea.350 BCE
Aristotle believes all matter is made of air, fire, earth and water.
No attempts to validate any theories were made during this time.1807
John Dalton proposes Dalton's atomic theory.
1. All matter is composed of tiny, invisible particles.
2. Each chemical element is composed of its own kind of particles, called atoms.
3. Atoms of an element are alike and have the same mass.
4. Atoms cannot be created or destroyed during physical or chemical changes.
5. Atoms unite in simple whole number ratios of form compounds (law of definite proportions).
Suggested that all molecules are binary, but had no evidence to support this.1811
Amedeo Avogadro
Avagadro was the first to distinguish molecules from atoms.
(Avagadro's number: 6.02 x 10^23)1869
Dmitri Mendeleev
Mendeleev developed the periodic table of the elements.1879
William Crookes
Crookes discovered the electron. (e-)1886
Eugen Goldstein
Goldstein discovered the proton.1897
J.J. Thomson
Tomson stated that an atom is
spherical and has a positive charge.
Electrons are evenly distributed within the charge.
His model of the atom is known as the "plum-pudding"
(or chocolate chip cookie) model. This is analogous to a
chocolate chip cookie, where the chocolate chips are
distributed randomly within the cookie. In this model,
the cookie represents the atom, the dough represents
the protons and the chocolate chips are the electrons.1909
Robert Millikan
Millikan discovered the charge of the electron using his oil drop experiment.1911
Ernest Rutherford
Rutherford discovered the nucleus using his gold-foil experiment.
He also stated that the atom is mainly empty space, containing an
extremely small, positively charged nucleus where most of the
atom's mass was located, around which the negatively charged
electrons move.1913
Niels Bohr
Bohr discovered atomic orbitals. He proposed that electrons
travelling around the nucleus could only occupy orbits
that had fixed energy values, being multiples of n, where n
represents the energy level. Each value of n is associated with
an orbit of different radius.
Bohr accurately predicted the line spectrum of the hydrogen atom.
This work led to the planetary model of the atom, in which
electrons circle the atom's nucleus in fixed circular paths as
the planets do around the Sun.1932
James Chadwick
Chadwick discovered the neutron.Periods
Quantum theory of the atom
Quantum theory specifies where electrons
might be and the paths they travel in.
Comments