Atomic Theory

460 BC Democritus develops the idea of atoms

He pounded up materials in his pestle and

mortar until he had reduced them to

smaller and smaller particles which he

called

ATOMA

(Greek for indivisible)

Scientist What He Discovered Date

John Dalton

J.J. Thomson

Ernest Rutherford

Niels Bohr

James Chadwick

Modern Electron Cloud Model

John Dalton – 1600’s

Elements are composed of one TYPE of atom

An atom of one element cannot be changed into an atom of a different element

Atoms can only be re-arranged,notdestroyed or created

Atoms from different elements form compounds.

J.J. Thomson – 1897

Atoms have subatomic particles within them

Atoms contain negatively charged particles so must have positive charges also

Model looked like a solid mass with negatively charges embedded in it – called electrons

Ernest Rutherford - 1910

Used the Gold Foil Experiment – Page 104

There is a nucleus in the atom Nucleus make up most of atom’s mass

and electrons move around it Positive particles exist and named

protons

Niels Bohr - 1913

Student of Thomson and Rutherford Electrons could only have specific

amounts of energy in energy levels Electrons must move in orbits

James Chadwick - 1932

• New particle discovered with about the same mass as proton called neutrons

• Neutrons have no electrical charge

Electron Cloud Model – 1920’s

Electrons form a “cloud” as they move around the nucleus

An electron’s movement is related to its energy level

• Electrons are arranged in Energy Levels or Shells around the nucleus of an atom.

• first shell a maximum of 2 electrons

• second shell a maximum of 8 electrons

• third shell a maximum of 18 electrons

•Fourth shell a maximum of 32 electrons

In Other Words, It’s All About The Electrons!!!

Use Graphic Organizer

Atoms

• Smallest unit of matter

•Have subatomic particles

Structure of Atoms

• Nucleus – contains most of the mass of atom (protons and neutrons)

• Energy shells – contains electrons

Atomic Charges

• Protons – have positive charge

• Neutrons – have no charge

• Electrons – have negative charge

• Isotopes – have extra neutrons

Electrons• Very small, little mass

• Have negative charge

• Forms a “cloud around the nucleus of atom

Electrons orbit nucleus

Energy shells hold electrons

valance electrons are in outer shell hat interact with other atomic electrons

Protons

• Makes up part of mass in nucleus

• Has positive charge

• Is called the atomic number

Neutrons

• Makes up part of mass in nucleus

• Has no charge (neutral)

Electrons

O Each electron shell can hold

a certain number of

electrons

O Electron shells are filled

from the inside out

O Noble gases have full outer

electron shells

O All other elements have

partially filled outer electron

shells

ElectronsO Electrons are always

moving. They spin very quickly around the nucleus of an atom.

O As the electrons zip around, they can move in any direction, as long as they stay in their shell.

Atomic Mass and Number

Size and Scale

• Atoms are mostly empty space

• The tiniest speck of dust may contain 10 billion atoms.

Size and Scale of Atoms

• To give you an idea of the scale of an atom, picture an empty baseball stadium, like Turner Field

• If the nucleus (the size of a pencil eraser) is on the pitcher’s mound, where would the electrons be?

Size and Scale of Atoms

• In the top row of seats!

Atomic Number• Every atom of an element has the same

number of protons

• The atomic number identifies the element and its placement on the table.

• number of protons in the nucleus of an atom

• Is equal to number of electrons of an atom

Isotopes and Atomic Mass• Atoms with the same number of protons, but

a different number of neutrons are called isotopes.

• Isotopes are identified by its mass number

• Atomic mass is the sum of protons and neutrons in the nucleus

• Examples

of Isotopes: