ATOMS, ATOMIC THEORY, ELEMENTS, INTRO TO SYMBOLS AND FORMULAS

Lecture for 10/5/07

Early ideas of the composition of matter

Democritus in 450 BC Aristotle and other well-known Greek

philosophers didn’t believe Democritus

Atoms are defined in terms of elements An atom is the smallest particle of an

element that retains the chemical identity of that element

17th -18th century – new ideas emerge

1. Antoine Lavoisier2. Joseph Louis Proust: “Law of Constant

Composition”(1799) Compounds always contained the same ratio by weight

of their elements Concluded that a given compound always has the same

composition, no matter where it came from

3. John Dalton: Developed an atomic theory of matter in 1803 (called Dalton’s atomic theory)

1. Each element is composed of extremely small particles called atoms.

2. All atoms of a given element are identical, but they differ from those of any other element.

3. Atoms are neither created nor destroyed in any chemical reaction.

4. A given compound always has the same relative number and kinds of atoms.

How can we be sure that atoms exist? Evidence that atoms exist is

overwhelming and conclusive Scanning tunneling microscope

provides pictures of atoms Chemistry is the study of two

worlds – macroscopic and submicroscopic Observations are made in the macroscopic

world, however, to understand this world one must try to understand the atoms that compose it.

The Elements

Around 114 known elements and 92 are naturally occurring

The term “element” can mean different things:1. Microscopic form

2. Macroscopic form

3. Elements comprised of molecules Diatomic molecules: BrINClHOF

Symbols and Formulas

Symbols for elements: Names are derived from Greek, Latin, and German Symbol is usually the first letter or first two letters of

the element name. Rule: first letter is capitalized, second is not

Formulas for compounds: Rules for writing formulas:

1. Each atom present is represented by its element symbol.2. The number of each type of atom is indicated by a subscript

written to the right of the element symbol.3. When only one atom of a given type is present, the subscript 1 is

not written.

hydrogen Hnitrogen Nboron Bargon Arcalcium Ca

nickel Ni

silver Aggold Au

tungsten Wcopper Cu

potassium Ksodium Nalead Pb