History of atomic models

• In Bohr's model, electrons orbit around the nucleus in discrete energy levels called shells or orbitals.
• Leucippus is described as the founder of atomism, the theory that all things are made up of atoms.
• Leucippus' theory stated that matter is homogeneous but consists of an infinity of small indivisible particles.
• Leucippus' student Democritus Atomos came up with a Greek word which means indivisible or uncuttable.
• Democritus theorized that everything is composed of indivisible particles that are always in motion and between these particles lie an empty space.
• John Dalton stated that matter is divided by tiny indivisible particle, which he called atoms, and his model is known as the solid sphere model.
• John Dalton also proposed that atoms of different elements combine in fixed, compounds whole-number ratios when forming compounds.
• Joseph John Thomson discovered that atoms consist of negatively charged particles known as electrons through his cathode ray tube experiment and called his model the plum pudding model.
• Eugene Goldstein discovered the positively-charged particles called protons.
• Ernest Rutherford discovered the central structure of the atom, which he called nucleus, through the gold foil experiment and observed some of the alpha rays deflect or bounce back when it hits the gold foil.
• James Chadwick discovered that atoms consisted not only of protons and electrons but also neutrons, a neutral subatomic particle that has approximately the same mass as a proton.
• Niels Bohr proposed that electrons travel in orbits around the nucleus in well-defined paths called orbits and called his model the planetary model because it resembles our very own solar system.
• In 1926, an Austrian physicist, Erwin Schrödinger, came up with his own model, which he called quantum mechanical and proposed that, rather than the electrons moving in fixed orbits or shells, these electrons behave as waves.
• The theory of electrons behaving as waves was backed up by Werner Karl Heisenberg stating that since electrons are very small particles in an atom, its location cannot be exactly known and how it is moving.
• This theory is known as the uncertainty principle, wherein the nucleus is surrounded by clouds of electron density or electron ecloud.
• In these clouds, there are regions called electron orbital where electrons are likely to be found.
• This model is also based on the wave-particle duality, that just like light, electrons can behave as wave and particle at the same time, which was proposed by Louis de Broglie.
• ISOTOPES Forms of an element having the same number of protons but different number of neutrons

  NUCLEONS a collective term for protons and neutrons.

  PROTONS AND NEUTRONS these are found in the nucleus of an atom.

  MASS NUMBER refers to the total number of protons and neutrons in an atom.

  NEUTRAL ATOMS atom with the same number of protons and electrons

  ATOMIC MASS the average mass of an atorm, with all its naturally occurring isotopes