The Atom By: M. Silverman

Structure

The Proton Location: Nucleus Charge: Positive (+) Responsible for identity of atom and physical properties. +

Neutron Location: Nucleus Charge: Neutral (0) Responsible for nuclear properties (stability)

Electron Location: Electron Cloud Charge: Negative (-) Responsible for chemical properties -

JJ Thomson 1897 Discovered charge to mass ratio of electrons (=1.759 x 108 coulombs/gram) Early cathode ray tube Modern cathode ray tube

Ernest Rutherford 1898 Determined the atom has small, dense nucleus surrounded by electrons and mostly empty space. Nucleus Electron cloud

R.A. Millikan 1909 Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron. 

James Chadwick 1932 Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus, he discovered the neutron. 

Dalton’s Atomic Theory 1) All elements are composed of atoms. 2) Atoms of one element are identical and different from those of any other element. 3) Atoms combine in whole number ratios to form compounds. 4) Chemical reactions rearrange atoms, but do not change them.

Atoms and Elements

Atomic Number (Z) Equals number of protons in the atom. Functions as an ID number. Read directly off the periodic table. What is the atomic number of this Element? What element is it? 56, Barium

Atomic Weight Read directly off periodic table More correctly: Average Atomic Mass (Ar) Read directly off periodic table Rounded off, gives Mass number of most common isotope. What is the atomic mass of this Element? 137.3 amu

Mass Number (A) Found by adding protons + neutrons Is for an individual atom, not an average. What is the mass number of this isotope of Barium? 56 + 81 = 137 amu

Isotopes Varying forms of an element with different numbers of neutrons and thus, different mass numbers. Barium - 137

Many larger Isotopes are Unstable = radioactive Unbalanced ratios of protons to neutrons also make the nucleus unstable. Nuclear Fission

Write the mass notation A way of representing isotopes by putting the mass number to the upper left of the symbol and optionally, the atomic number on the bottom left. Write the mass notation for Carbon - 12 and Carbon – 14. C C Sometimes E is used in place of X.

Summary Rules for Atoms: Number of Protons = Atomic number p+ = Z Electrons = Protons in a neutral atom e- = p+ Average atomic mass rounded off equals mass number of most common isotope AM rounded off = A Neutrons = mass number – atomic number no = A - Z

Bohr Diagrams Named for Niels Bohr. Place protons and neutrons in center. Place electrons around nucleus in shells. Electron energy levels have limits to what they can hold. Electron energy levels maximum numbers: 2 e- 8 e- 18 e- 4-7. 32 e- What element is drawn here? Niels Bohr Who’s hanging out with Niels here??? F

Next: Calculating Average Atomic Mass Nuclear Chemistry Waves, electrons, light, and Quantum Theory!!!