ATOMIC THEORY TIMELINE

DEMOCRITUS - 460-370 B.C.

• "Nothing exists except atoms and empty space; everything else is opinion."
• Democritus was the first person to say that matter was infinitely divisible.
• No one listened to him because he was only a philosopher, he did not have any proof.

JOHN DALTON - 1803

• Dalton's Atomic Theory:
• 1. Each element is composed of tiny, indestructible particles called atoms.
• 2. All atoms are identical.
• 3. Atoms combine in simple, whole-number ratios to form molecules and compounds.
• Dalton pictured atoms as tiny, indestructible atoms.

J.J. THOMSON - 1897

• Thomson proposed that the atom is breakable.
• He proposed that the atom's structure has electrons suspended in a positively charged electric field.
• Because electrons are negative, the atom must have a positive charge to balance it out.
• He created the "Plum Pudding Model".

HANTARO NAGAOKA - 1904

• Nagaoka suggested that an atom has a central nucleus.
• He proposed that electrons move in orbits like the rings around Saturn.

ERNEST RUTHERFORD - 1911

• Rutherford devised the gold-foil experiment.
• He shot a beam of alpha particles and some reflected off of the foil.
• He discovered that the atom is only mostly empty.
• Rutherford find that an atom has a small, dense, positively charged nucleus.

NIELS BOHR - 1913

• In the Bohr model of the atom, electrons travel in orbits around the nucleus.
• The farther the electron is from the nucleus, the more energy it has.
• Only specific orbits with specific distances are allowed.
• Bohr proposed that electrons move in a circular orbit at fixed distances from the nucleus.
• Bohr's model only works with atoms with one electron, which is only hydrogen.

LOUIS DE BROGLIE - 1923

• Louis de Broglie proposed that moving particles like electrons have some properties of waves.

ERWIN SCHRÖDINGER - 1926

• He discovered an equation that predicts the probability of finding an electron
• with a particular amount of energy at a particular location in the atom.
• His equations lead to the quantum mechanical model.
• In this atom, the positively charged nucleus contains protons and neutrons.
• Their is also an electron cloud that shows the probable locations of electrons in an atom.

JAMES CHADWICK - 1932

• James Chadwick confirmed the existence of neutrons, which have no charge.
• Atomic nuclei contain neutrons and positively charged protons.