1. Democritus: The Atom (around 400 BCE)
Theories of the Atom
Democritus: The Atom (around 400 BCE)
Democritus thought matter could be divided into smaller and smaller pieces until a single indivisible particle is reached
He named this particle the atom
He proposed that atoms are of different sizes, in constant motion, and separated by empty spaces.
Democritus thought that the atom was indivisible.
The word atom means “cannot be cut”

2. Aristotle: Earth, Water, Air, and Fire (around 450 BCE)
Theories of the Atom
Aristotle: Earth, Water, Air, and Fire (around 450 BCE)
Aristotle rejected the idea of the atom. He proposed four basic elements: earth, water, air, and fire.

3. The atom is a tiny, solid, indestructible sphere
Theories of the Atom
John Dalton: The Billiard Ball Model (1807)
all matter is made up of tiny, indivisible particles called atoms
• all atoms of an element are identical
• atoms of different elements are
different
atoms are rearranged to form new substances in chemical reactions but they are never created or destroyed
The atom is a tiny, solid, indestructible sphere

4. J. J. Thomson: The Plum Pudding Model (1897)
Theories of the Atom
J. J. Thomson: The Plum Pudding Model (1897)
Thomson used a device called a cathode ray tube to conduct his experiments
The particles he detected were attracted to the positive end of the circuit, so they had to be negatively charged.

5. J. J. Thomson: The Plum Pudding Model (1897)
Theories of the Atom
J. J. Thomson: The Plum Pudding Model (1897)
Thomson theorized that atoms contain negatively charged particles called electrons.
He believed that since atoms are neutral, the rest of the atom is a positively charged sphere.

6. Ernest Rutherford: The Nucleus and the Proton (1909)
Theories of the Atom
Ernest Rutherford: The Nucleus and the Proton (1909)
Rutherford expected the -particles to pass undeflected through the atoms
However, a small fraction of the -particles were deflected
To deflect the energetic -particles, the nucleus must be dense, with positive charge

7. Ernest Rutherford: The Nucleus and the Proton (1909)
Theories of the Atom
Ernest Rutherford: The Nucleus and the Proton (1909)
Rutherford theorized that the centre, or nucleus, of the atom consists of positively charge particles called protons.

8. Ernest Rutherford: The Nucleus and the Proton (1909)
Theories of the Atom
Ernest Rutherford: The Nucleus and the Proton (1909)
the positively charged nucleus is surrounded by a cloud of negatively charged electrons, and the atom is mostly empty space.

9. James Chadwick: The Neutron (1932)
Theories of the Atom
James Chadwick: The Neutron (1932)
Chadwick revised Rutherford’s theory, and proposed that the nucleus contains positively charged protons and neutral particles called neutrons.
In the planetary model of the atom, electrons orbit the nucleus the way planets orbit the Sun in our solar system.

10. Niels Bohr: Electron Orbits (1913)
Theories of the Atom
Niels Bohr: Electron Orbits (1913)
studied the hydrogen atom and the light it produces when excited (heated, electrified)

11. Theories of the Atom
Niels Bohr: Electron Orbits (1913)

12. Niels Bohr: Electron Orbits (1913)
Theories of the Atom
Niels Bohr: Electron Orbits (1913)
Bohr revised the atomic theory to include these points:
Electrons are located in specific orbits (energy levels, shells).
Each electron in an orbit has a specific amount of energy. The farther the electron is from the nucleus, the higher its energy.
Each orbit can hold a certain maximum number of electrons: 2 in the first orbit, 8 in the second, and 8 in the third.
Electrons can jump from orbit to orbit. They release energy as light when they jump from higher to lower orbits.

14. Homework:
Worksheet: Development of the Atomic Theory
pg. 233 #2, 3, 5