Atomic Theories: How do we know what atoms are

made of?

Democritus (400 BC) • Ancient Greek philosopher• Believed that everything in

the universe was made of indivisible particles called “atoms” (or “atomos” in Greek)

• Said atoms were constantly moving

• Atoms only differed in shape, position, and arrangement

Democritus Cont’d

John Dalton (1803)• Visualized atoms to look like a pool

ball/bowling ballAll elements are made of atoms.All atoms of the same element are exactly

alike and have same mass.An atom of one element cannot be changed

into an atom of a different element.Atoms cannot be created nor destroyed, only

rearranged.Compounds are made from atoms of

different elements.

John Dalton Cont’d

J.J. Thomson (1897)• Visualized the atom to

look like a chocolate chip muffin or plum pudding

• An atom consists of negative charges scattered throughout a ball of positive charges.

• Proved that an atom could be divided

• Said the atom is neutral.

JJ Thompson Cont’d

Ernest Rutherford (1908)Rutherford was a student of

Thomson.Visualized the atom to look like a

peachPositive charge (protons) is

located in the center of the atom.Center is called the nucleus.Almost all of the atom’s mass is

located in the nucleus.Atom is mostly empty space with

the electrons moving around the nucleus.

Ernest Rutherford Cont’d

Niels Bohr (1913)• Student of Thomson and

Rutherford• Visualized the atom to look

like a sliced red onion & planets in the solar system

• Electrons orbit around the nucleus in a definite orbit

• Electrons could have only specific amounts of energy, leading them to move in certain orbits

Niels Bohr Cont’d

The Wave Model (1920s)

• Based on principles of wave mechanics

• Electrons do not move in a definite path

• It is impossible to determine the exact location of an electron

• The atom has a small, positively charged nucleus, surrounded by the electron cloud

The Wave Model Cont’d