Early models of the
Atom
• Democritus of Abdera,
• 4 th century BC
• Atoms are indivisible and indestructible
• Coined the term “atom”

John Dalton
1766- 1844
• All elements are composed of tiny indivisible particles called atoms
• Atoms of one element are identical. The atoms of any one element is different from atoms of any other element.

Lavoisier: (1743-1795)
• When a chemical reaction occurs mass is neither created or destroyed
– The mass of the reactants always equals the mass of the products
– Law of conservation of mass

John Dalton
• Atoms of different elements can chemically combine with one another in simple whole number ratios to form compounds
• Chemical reactions occur when bonds between atoms break, and new bonds form.
Atoms are never changed into atoms of another element

J.J. Thomson1856-1940
An electrical current passed through a gas at low pressure
“bent” the cathode ray toward a (+) charged metal plate
Atoms have Electrons lectdemoscrtube

• Rutherford 1871-1937
• Gold foil experiment
• Atoms are mostly empty space surrounding a dense nuclei
Chemistry Movie

• The number of protons in the nucleus
• The number of positive charges in the nucleus
• Equal to the number of electrons
• A WHOLE NUMBER

• Number of protons + the number of neutrons
• For an element, the mass number is a whole number

• Atoms that all have the same number of protons, but have different numbers of neutrons are called isotopes
• Isotopes have different atomic mass – because they have different numbers of neutrons

Isotope examples
• Hydrogen-1 has 1 proton, no neutrons
• Hydrogen-2 has 1 proton, 1 neutron
• Hydrogen-3 has 1 proton, 2 neutrons
• All isotopes are identical in terms of chemical behavior
• For a population of atoms, the Average atomic mass is a decimal number – and depends on the relative proportions of isotope

Average Atomic Mass
• This is not a whole number
• Multiply the relative atomic mass for each isotope by the relative abundance.
• Add the products together
• Do not divide by the number of isotopes!!

Average atomic mass examples

Practice problems
• Pg 120 14-17
• General Chemistry I

Periodic Table
• Dimitr Mendeleyev (1834-1907) listed elements in columns in order of increasing atomic mass
• He then arranged the elements according to similarities in their properties
• Mendeleyev left spaces for elements that he predicted would be found

Henry Moseley 1887-1915
• Arranged elements in a table based on atomic number instead of atomic mass http:// www.webelements.com/webelements/elements/text/p eriodic-table/key.html

The modern Periodic table
• Periods – rows
• Families: columns
• Group A elements are representative elements
• Group B elements are transition metals
• At the bottom, are the inner transition metals

Basic structure- you should know
• Metals on the left
• Nonmetals on the right
• Semimetals or metalloids separate the metals from the non metals

Common families
• Group IA – Alkaline metals
• Group IIA- The Alkaline earth metals
• Group VII A – The halogens
• Group 0 or IIX – the noble gases