Atomic Structure

Democritus (400 B.C.) Believed that matter was composed of tiny particles called atoms His ideas were rejected by others because he had no evidence

John Dalton (1800’s) Dalton’s Atomic Theory All matter is composed of atoms All atoms of a given element are identical Atoms cannot be created, divided or destroyed Different atoms combine to form compounds In chemical reactions atoms are rearranged.

Thompson (1890’s) Identified the electron through use of a cathode ray tube. Determined that electrons carry a negative charge and have an extremely small mass Created the Plum Pudding Model (chocolate chip cookie dough model) Electrons: charge = -1 symbol = e-

Plum Pudding Model Negatively charged electrons were distributed throughout a uniform positive charge

Rutherford (1900’s) Gold Foil Experiment Shot alpha particles (like a laser beam) at a thin piece of gold foil Expected the alpha particles to go through foil, but found that some particles bounced back

Gold Foil Experiment

Rutherford (cont’d) Discovered the nucleus - tiny dense region which contains all of the atom’s positive charge and almost all its mass Developed the Nuclear Model – nucleus in center and electrons found in empty space surrounding nucleus

Nuclear Model Plum Pudding Model Because some of the alpha particles bounced back – they must have hit something with a positive charge

Inside the nucleus Protons – subatomic particle with a charge equal and opposite to an electron Discovered by Rutherford Proton: charge = +1 symbol = p+ Neutrons – subatomic particle with a mass almost equal to that of a proton but has no charge (neutral) Discovered by Chadwick Neutron: charge = 0 symbol = n0

Bohr (1900’s) Discovered that e- have energy of their own that keep them from being drawn into the nucleus Believed that e- were found in paths (orbits) around the nucleus Developed the Planetary Model

Schrodinger (1920’s) Found that e- were not in orbits, but instead were found in 3D clouds (shells) Developed the Modern Model of the atom

MODERN ATOMIC MODEL

How Atoms Differ The number of protons determine the identity of the element Atomic Number = number of p+ Atomic Number = number of e- (in a neutral atom) Mass Number = sum of p+ and no

Nuclear Symbol (Symbolic Notation)

Examples p+ = e- = no = p+ = e- = no =

ISOTOPES Atoms of the same element that have the same number of protons, but a different number of neutrons. Same atomic number, different mass number Carbon-12 The number after the name Carbon-13 is the mass number Carbon-14

Using the Periodic Table

Atomic Mass Weighted average of the masses for all isotopes of an element If you round the atomic mass on the periodic table to the nearest whole number, you will get the mass number of the most abundant (common) isotope

Calculating Atomic Mass Example