Evolution of the Atomic Theory

The Idea of the Atom is Born!

•  Democritus theorized about an indivisible piece of matter

•  He used the latin term “Atomos” to describe his idea

•  The latin word has been changed to the current term atom

John Dalton’s Contributions: 1808

  Elements are made of atoms  Atoms of the same element are the same  Atoms of different elements are

different   Compounds are two or more elements

combined

JJ Thompson’s Experiment

•  J.J. Thomson conducted an experiment and came to the conclusion that the atom must have a negatively charged particle.

•  Discovered the electron!

JJ’s Model: 1897 •  Knowing that the atom

has a negative part, “electrons,” JJ developed a model that incorporates this new knowledge

•  This model is often called the “Plum Pudding Model”

•  These negative parts were scattered throughout like plums in plum pudding

Nagaoka Model: 1904 •  Japanese physicist

Nagaoka proposed a modle of ahe atom that had a large sphere in the center with a positve charge.

•  His model showed electrons revolving around the center like planets around the sun.

Rutherford’s Experiment •  Ernest Rutherford conducted an experiment

that provided evidence that atoms have a small dense positively charged center

Rutherford’s Model: 1911 •  Rutherford concluded the

positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source.

•  Like charges repel each other (two negatives for example will repel).

Niels Bohr •  Bohr’s model concluded that the atom is

mostly empty space. •  The nucleus is the center of an atom,

containing protons (positively charged) and neutrons.

•  Electrons orbit in certain fixed energy levels (or shells).

•  The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

Bohr’s Model: 1913

The Modern Model •  The orbitals,

associated with energy, determines where the electrons are located.

Present Atomic Theory •  The electrons

located near the nucleus of the atom are the most stable.

•  As outer levels of electrons increase, the atoms become less stable.

Overview •  Democritus developed the idea of the atom about 460 BC

•  John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century. (1808)

•  In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.

•  Japanese physicist Nagaoka proposed a model of the atom (1904) that had a large sphere in the center with a positve charge.

•  In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.

•  In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.