SCH4U U2A1A1 History of the atom

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.

Quantum theory of the atom Quantum theory specifies where electrons might be and the paths they travel in.