Chapter 5
Democritus thought atoms were indivisible & indestructible Lacked experimental support 4 th century B.C.
Democritus thought atoms were indivisible & indestructible Lacked experimental support 4 th century B.C.
John Dalton (1766-1844) Dalton s Atomic Theory All elements composed of tiny, indivisible atoms Atoms of same element are identical. Atoms of 1 element are different from another element
Dalton s Atomic Theory (cont) Atoms of different elements can either physically mix or chemically combine in simple, whole number ratios to form compounds
Dalton s Atomic Theory (cont) Chemical Reactions occur when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical reaction.
Dalton s Theory Was tested experimentally Dalton formulated a hypotheses to explain his observations
Dalton s Theory is mostly accepted today Except that we now know atoms CAN be divided - into subatomic particles
JJ Thompson (1856-1940) discovered electrons using a cathode ray tube passed electric current through gases at low pressure
He sealed the gases in a tube fitted at both ends with electrodes (metal disks) Connected to source of high voltage electricity
Anode became positively charged Cathode became negatively charged
A glowing beam formed between the 2 electrodes Called the cathode ray
The cathode ray is attracted to metal plates that have a positive charge Negatively charged plates repelled the ray
Thompson knew like charges repel and opposite charges attract
The ray must be negatively charged particles moving at a high speed He called these particles electrons
Every gas did the same thing so every element must have electrons
JJ Thomson s model of the atom is called the plumpudding model the electrons were randomly placed throughout the atom
Robert Millikan (1868-1953) found quantity of charge in 1 electron (e - ) Also determined the ratio of the charge to the mass of 1 e - calculated the mass of 1 e -
Ernest Rutherford discovered the nucleus He had a thin piece of gold foil & bombarded it with alpha particles (helium nucleus) Most particles passed through the foil like he expected Some were deflected slightly
Other particles bounced back at him! He was as surprised as if he had sent a bowling ball at a tissue and it came back to hit him He figured that there must be a dense positive part of the atom, but most of the atom is empty space http://video.google.com/videosearch?q=rutherford%20experiment&um=1& ie=utf-8&sa=n&tab=wv#
Bohr proposed that electrons have a fixed energy and move in energy levels around the nucleus which is why they don t fall into the nucleus
The energy levels are like the rungs of a ladder electrons cannot be in between levels, and need a specific amount of energy to move from one to another
We haven t always known what the atom looked like There have been many different models throughout the past few hundred years
Dalton thought atoms were solid and indivisible JJ Thomson discovered the electron, & made the plumpudding model
Rutherford discovered the nucleus Bohr proposed electrons in orbitals around nucleus
Nucleus: massive center of the atom, holds the protons and neutrons Proton: positively charged particle
Neutron: nuclear particle, has NO charge! Electron: orbits the nucleus, negatively charged
Energy Level: where an electron can be found (e - orbital, e - shell) Ion: an atom with a charge, more or less e - than a neutral atom
Atomic Number: # of protons Mass Number: # of protons plus number of neutrons Isotope: atom with more or less neutrons than normal
A neutral atom will have the same number of electrons as it does protons
Atomic Mass: the weighted average of all the isotopes of an element
To find the number of neutrons: round the atomic mass to the nearest whole number, then subtract the # of protons