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.
Top Related