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Beginning of atomic theory
Democritus (460-370 B.C), a greek philosopher questioned whether there was a limit to how much matter could be broken down to. He theorised that there would eventually be a limit to how small matter could become, and he named the smallest piece "atomos", meaning "cannot be divided -
The Dalton Model
John Dalton (1766-1844) expanded on Democritus's theory and stated that:
1.All matter is made up of atoms. Atoms are indivisible and indestructable.
2.All atoms of a given element are identical in mass and properties.
3.Compounds are formed by a combination of two or more different kinds of atoms.
4.A chemical reaction is a rearrangement of atoms.
5.Atoms can neither destroyed nor created.
His model of an atom looked like a simple, solid sphere -
The Plum Pudding Model
Joseph John Thompson (1856-1940) discovered the electron, which totally changed the view of the atom, as the atom was thought to be the smallest piece of matter. He proposed a new model, showing the atom being made up of negative electrons floating around a positive charge, much like how raisins are dispersed throughout a plum pudding, thus naming it the plum pudding model -
The Planetary Model
Ernest Rutherford (1871-1937) discovered a property of atoms while he was shooting alpha particles through a thin sheet of gold foil. Amazingly, some of the particles passed through the gold foil without bouncing back. This prompted Rutherford to change the Plum Pudding model. Rutherford's new model showed the atom as a tiny, dense positively charged nucleus core with lighter, negative electrons orbiting it, much like how the planets orbit the Sun. -
The Bohr Model
Neil Bohr (1885-1962) refined Rutherford's model by proposing that electrons:
-orbit the nucleus without losing energy
-could move only in fixed orbits with specific energies
-with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus
He created a new atomic model, which depicted the atom as a small, positive nucleus surrounded by electrons held in cirular orbits through electrostatic forces, rather than gravity. -
The Cloud Model
Erwin Shrodinger( (1887-1961) took the Bohr Model one step further, and proposed that electrons were actually waves., and that their exact position in the atom cannot be confirmed, but the general area of the electron can be discovered through probability. The Scrodinger model shows the nucleus while the abundance of dots, or the cloud, around it shows the history of where the electron has been and where it probably will go next
