Atomic Theory and Models in Chemistry

John Dalton's Atomic Theory

• John Dalton proposed the Atomic Theory in 1803.
• His theory stated that all matter is composed of atoms, which are indivisible and indestructible particles.
• Dalton's theory explained why elements always combine in simple whole-number ratios to form chemical compounds.
• Each element has its own unique type of atom with a specific weight.

J.J. Thomson's "Plum Pudding" Model

• In 1897, J.J. Thomson discovered the electron.
• He used magnets and electric plates to deflect cathode rays, which are made up of negatively charged particles.
• The electron is a subatomic particle that is over 1000 times lighter than the smallest atom, hydrogen.
• Thomson's model proposed a positively charged sphere with negatively charged electrons embedded in it, resembling plum pudding.

Ernest Rutherford's Nuclear Model

• In 1909, Rutherford and his colleagues bombarded a thin gold foil with alpha particles.
• Most alpha particles passed straight through, but some were deflected at large angles.
• This experimental evidence suggested that the atom has a small, dense, positively charged center called the nucleus, which contains most of the atom's mass.
• Rutherford's model proposed a mostly empty space, with electrons orbiting the nucleus.

Niels Bohr's Planetary Model

• Bohr revised Rutherford's model by proposing that electrons orbit the nucleus in fixed energy levels or orbits, similar to the planets orbiting the sun.
• The electrons are at fixed distances from the nucleus, with the lowest energy level closest to the nucleus.
• Electrons can jump between energy levels by gaining or losing a discrete amount of energy, called a quantum of energy, which explains the concept of a quantum leap.
• The Bohr model explained why each element has specific chemical properties due to the number of electrons available for reactions, which are the valence electrons in the outermost orbits.

Quantum Mechanical Model

• The quantum mechanical model is the most recent and accurate model of the atom.
• It's based on the principles of quantum mechanics, which proposes that electrons behave as both particles and waves.
• The quantum mechanical model describes the probability of finding electrons in specific regions around the nucleus called orbitals.
• It's a more complex model than Bohr's, but it's able to explain the behavior of atoms in more detail.