History of the Development of the Modern Model of the Atom
Dalton’s Atomic Theory (early 1800’s) All elements are composed of indestructible particles called atoms All atoms of an element are exactly alike Atoms of one element (ex. C) are different from atoms of other elements (ex. U) Compounds are formed by joining two or more different atoms in constant whole number ratios.
What did Dalton think the atom looked like?
Dalton was not quite correct. atoms are NOT indestructible particles All atoms of an element are NOT exactly alike
Thomson and the Cathode Ray Tube (late 1800’s) Thomson concluded that the atom was made of smaller particles. Discovered particles with negative charge Electron No matter what metals were used they all produced a negative beam of light Therefore atoms of all elements contain electrons
Video Cathode Ray Tube Animation (53 sec) Demo (2:48)
Thomson theorized a new model of the atom. Very dense with negative particles and positive space between.
Rutherford’s Gold Foil Experiment
OBSERVATIONS most particles went straight through few particles were deflected or bounced back CONCLUSIONS atoms are mostly empty space few particles came close to or hit a positive nucleus in the atom
Rutherford’s Model Electrons randomly orbit a large, positive nucleus
Bohr Model of the Atom (early 1900’s) Electrons have definite, fixed orbits Electrons have a definite energy orbits farther from the nucleus have greater energy
How many e- in each? Energy Level 2 Energy Level 1
Electron-cloud model Modern Model electrons are most likely found in regions outside the nucleus called orbitals and do NOT travel in fixed paths
orbital shapes
Atoms & Isotopes
Atoms: Review
Mystery Atoms
You will need your Periodic Table The bags represent Atomic Nuclei Protons Neutrons Fill in data table to determine the Mystery Atom’s identity. We’ll do A together
A
We’ll talk more about this column after Protons blue Neutrons green Mystery Atom atomic # # neutrons # electrons Atomic Mass Isotopic Symbol EXAMPLE A 7 8 7 15 N B C
contain the same number of protons but different numbers of neutrons. There may be several isotopes of one element.
Isotopic Notation 14 6 C Mass number Atomic number a.k.a. C-14
The most abundant isotope of an element is the whole number mass on the PT. A weighted average of the element (all isotopes) is on the PT.
Weighted Averages & isotopes of Pe
Steps to Calculate a Weighted Average You’ll calculate % (or be given) Multiply the % by the mass (for as many isotopes as you have) Add them together.
The End