WUP#10 Draw a model of an atom

Atomic Theory and Structure of the Atom

A. Democritus (460 B.C. – 370 B.C) first to suggest the existence of atoms believed atoms were small indivisible particles Atom: smallest particle of an element that retains all properties of the element.

B. Dalton Name of theory Billiard Ball Model Atomic theory composed of 4 postulates Elements are composed of atoms Atoms of the same element are identical, each element is unique Compounds are composed of atoms of more than 1 element, atoms combine in fixed ratios to form compounds Chemical rxn involves separation, combination, or rearrangement of atoms Name of theory Billiard Ball Model

C. Electron Symbol: e− Charge −1 **after Dalton, there was a big hiatus in the theory of the atom. Everyone accepted Daltons theory and went on to study radiation for a while, and this is when the electron was discovered

D. J.J. Thomson Name of theory Plum Pudding Model An atom is thought of as a uniform positive sphere with electrons evenly distributed through out (like plum pudding or a chocolate chip cookie) Name of theory Plum Pudding Model

Cathode Ray Tube Evacuated tube Anode Cathode − + The electric current creates the cathode ray (invisible) which spans the whole length of the tube. Its only visible because of the coating on the inside of the tube J.J. Thompson discovers electrons are negatively charged particles with very little mass using the cathode ray tube Battery

Found the mass of an electron to be 9.10×10−28 g E. Millikan Millikan Oil Drop Found the mass of an electron to be 9.10×10−28 g An atomizer produces fine oil droplets The oil droplets fall through a hole in the first chamber as a stream of tiny droplets X-rays negatively charge the oil droplets An applied voltage on two plates surrounding the oil droplets creates an electric field. The electric force pulls some droplets upward The rate at which the oil droplets are falling and rising between the two charged plates is measured through a microscope

F. Rutherford Conducted gold foil experiment

What did Rutherford expect? Click here for video! Gold Foil Experiment Animation and Discussion

What did Rutherford actually see?

F. Rutherford Concluded all positive charge and mass is concentrated in small region called nucleus and the electrons are outside the nucleus Proposed atom is mostly empty space Name of theory Nuclear Model Proton p+ Charge: +1 Mass:1.673×10−24 g

G. Bohr Expanded on the nuclear atom Electrons travel in circular orbits around the nucleus Name of theory Planetary Model Student of Rutherford’s developed this model patterned after the solar system know as the planetary model

Quantum Mechanical Model or Electron Cloud Model H. Schrödinger model allowed the electron to occupy three-dimensional space like an electron “cloud” Name of theory Quantum Mechanical Model or Electron Cloud Model Shcrodinger, Austrian physicist Took Bohr’s model one step further using mathematical equations to find the likelihood of finding an electron in a certain position, introducing the ideas of sub-energy levels.

I. Chadwick The Neutron Symbol: n0 Charge: none Mass:1.675×10−24 g Prisoner of WWI in Germany, after being released got his PhD under his mentor Rutherford. Discovered the neutron using beryllium atoms with alpha particles and received the Nobel Prize for his discovery. **The neutron was found by comparing the mass of hydrogen to helium, since He increased by 1 proton, the mass should be double, but it was four times, leading to a subatomic particle that had mass but no charge.

Fun Facts The volume of a hydrogen nucleus is a trillion times smaller than the volume of a hydrogen atom, yet the nucleus contains most of the mass. If the nucleus (proton) of a hydrogen atom were as large as the width of a human thumb, the electron would be on the average about one kilometer away in a great expanse of empty space. Protons and neutrons are made of even smaller units called Quarks and they themselves are in turn made of vibrating strings of energy.