Atomic Theory Democritus 400 BCE Greek Philosopher Democritus

Atomic Theory

Democritus • 400 BCE, Greek Philosopher Democritus – developed theory that if you continuously cut an object, you would eventually end up with a particle that could not be cut. • He called the particle an atom – from the Greek work atomos meaning not able to be divided.

Dalton • Published his atomic theory in 1803: – All substances are made of atoms that cannot be created, divided, or destroyed – Atoms of the same element are exactly alike – Atoms join with other atoms to make new substances

Thomson • Discovered that there are small particles inside the atom • Experimented with a cathode-ray tube, which had charged metal plates. He found the ray bent toward the positive plate, which proved that there are negatively charged particles present in an atom – called electrons • Designed the “plum-pudding” model, which placed electrons mixed in with the solid atom

Thomson’s Plum Pudding Model

Rutherford • Aimed a beam of small, positively charged particles at a thin sheet of gold foil. • He discovered that atoms are made up of mostly empty space, with a solid, positively charged nucleus.

Bohr • 1913 – Studied the way atoms react to light. • He proposed that electrons move around the nucleus in certain paths, or energy levels. • He believed electrons could jump between levels, but must follow a path in the level.

James Chadwick • In 1932, James Chadwick discovered the neutron. • In his experiments, he discovered that the atomic mass of an atom was double what it should be based on the number of protons. • However, the overall charge of the atom was not affected by this extra mass, leading him to discover the neutron which has a neutral charge.

Electron Cloud • The modern atomic theory – electrons do not travel in definite paths • The exact path of an electron cannot be predicted, but there are regions inside the atoms where the electrons are likely to be found. This region is called the “electron cloud”