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.

Theories of the Atom

Democritus thought that the atom was indivisible.The word atom means “cannot be cut”

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.

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

Theories of the Atom

The atom is a tiny, solid,

indestructible sphere

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.

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.

Theories of the Atom

• 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

Theories of the Atom

Ernest Rutherford: The Nucleus and the Proton (1909)

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.

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.

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.

Theories of the Atom

Niels Bohr: Electron Orbits (1913)

–studied the

hydrogen atom

and the light it

produces when

excited (heated,

electrified)

Theories of the Atom

Niels Bohr: Electron Orbits (1913)

Theories of the Atom

Niels Bohr: Electron Orbits (1913)Bohr revised the atomic theory to include these points:

Theories of the Atom

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.

Electrons can jump from orbit to orbit. They release energy as light when they jump from higher to lower orbits.

Each orbit can hold a certain maximum number of electrons: 2 in the first orbit, 8 in the second, and 8 in the third.

Homework:

• Worksheet: Development of the Atomic Theory

• pg. 233 #2, 3, 5