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

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

17 th -18 th century – new ideas emerge Antoine Lavoisier 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 John Dalton : Developed an atomic theory of matter in 1803 (called Dalton’s atomic theory ) Each element is composed of extremely small particles called atoms. All atoms of a given element are identical, but they differ from those of any other element. Atoms are neither created nor destroyed in any chemical reaction. A given compound always has the same relative number and kinds of atoms.

Antoine Lavoisier

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

John Dalton : Developed an atomic theory of

matter in 1803 (called Dalton’s atomic theory )

Each element is composed of extremely small particles called atoms.

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

Atoms are neither created nor destroyed in any chemical reaction.

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.

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: Microscopic form Macroscopic form Elements comprised of molecules Diatomic molecules: Br I NClHOF

Around 114 known elements and 92 are naturally occurring

The term “element” can mean different things:

Microscopic form

Macroscopic form

Elements comprised of molecules

Diatomic molecules: Br I NClHOF

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: Each atom present is represented by its element symbol. The number of each type of atom is indicated by a subscript written to the right of the element symbol. When only one atom of a given type is present, the subscript 1 is not written. hydrogen H nitrogen N boron B argon Ar calcium Ca nickel Ni silver Ag gold Au tungsten W copper Cu potassium K sodium Na lead Pb

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:

Each atom present is represented by its element symbol.

The number of each type of atom is indicated by a subscript written to the right of the element symbol.

When only one atom of a given type is present, the subscript 1 is not written.