Models of an Atom

John Dalton (1766 – 1844) Experiment/Observations: He studied partial pressures of the atmosphere He noticed that the ratios in which elements combine remain in fixed, simple proportions.

Theory All matter is made up of individual particles called atoms, which cannot be divided: The four postulates: All elements are made of atoms All atoms of the same element have the same mass, and atoms of different elements have different masses Compounds contain atoms of more than one element In a particular compound, atoms of different elements always combine in the same way

Billiard Ball Model

Thomson’s (1856 – 1940) Model of the Atom Experiment/Observations: Used a cathode ray tube – a sealed glass tube with metal disks at each end. Wires are connected as a source of electric current so that one disk is positively charged and one is negatively charged. A beam appears between the disks. He hypothesized that the beam was made of negative particles

He placed two plates (one positive and one negative) on either side of the beam. Evidence that his hypotheses was true: No matter the type of of metal used as disks, the beam bent towards the positive plate and the particles had about 1/2000 of hydrogen atom.

Theory Atoms are made of subatomic particles – the plum pudding model.

Plum pudding Model

Rutherford’s (1871 – 1937) Atomic Theory Experiment/Observations He hypothesized that if alpha particles (positively charged particles) were shot at a sheet of gold foil, they would pass through in a straight path based on Thomson’s model.

About one out of every 20,000 alpha particles were deflected more than 90 degrees – they must have come close to a charged object. The closer to this charge – the greater the deflection.

Theory The positive charge of an atom is concentrated in it’s nucleus – the nuclear model.

The Nuclear Model

Neils Bohr (1885 – 1962) Experiment/Observations: In Rutherford’s model, the electrons would be constantly emitting energy according to classical physics, and the atom would be very unstable. To fix this, Bohr theorized that electrons must have a fixed speed, and fixed energy. Using Planck’s constant for light emissions, Bohr obtained the accurate formulas for energy levels of hydrogen atom.

Theory Electrons orbit the nucleus at fixed spends and fixed energy. Electrons may change energy levels by gaining or losing energy.

Bohr Model

Current – Electron Cloud Model Schrodinger developed probability function of where an electron may be found – we cannot say with certainty where an electron actually is at any point.

Electron Cloud Model