Historical Development of the

Atomic Theory

Democritus

Ancient Greek philosopher

(circa 400 BC)

Matter is made of tiny particles that we can’t see

Atomos means indivisibleThought these spheres of matter

were solid and could not be broken down further

John Dalton (1766 – 1844)

English scientist and teacher

Performed experiments

4 points to his atomic theory:

Dalton’s atomic theory:

1. All elements are made of tiny indivisible atoms

2. Atoms of the same element are the same as each other and different from other elements

3. Atoms of different elements can mix together in whole-number ratios

4. Chemical reactions occur when atoms change how they are arranged.

J.J. Thompson (1856 – 1940)

English Physicist

Passes electric current through a cathode ray tube

The cathode ray is a stream of negatively charged electrons

Electrons are a sub-atomic particle

Must be a corresponding positive part to the atom

“discovers” the electron

Credited with the “Plum Pudding” Model

Ernest Rutherford (1871 – 1937)

New Zealander who worked in England

Gold Foil experiment

Diagram the gold foil experiment:

An alpha particle (α)is a Helium nucleus (2 protons, 2 neutrons)

What was learned from the gold foil experiment:

Small, massive center called the nucleus

Nucleus has a positive charge (+)

Electrons (-) are outside the nucleus

Most of atom is empty space

Neils Bohr (1885 – 1962)

Danish Physicist

Used Spectral Line Data from hydrogen

Organized electrons into discrete shells or orbitals.

The Bohr Model is what is used to explain electron configurations in Regents Chemistry

Modern Atomic Theory

All matter is composed of atoms

An element is composed of only one type of atom

Compounds are different kinds of elements bonded together in definite proportions

• Atoms can be broken down into smaller units called subatomic particles:• Proton• Neutron• Electron

Proton

positively charged

Mass of 1 amu

Found in the nucleus

Atomic # = protons

Finding Protons:

Drag out periodic table

Neutron

No charge (electrically neutral)

Mass of 1 amu

Also in the nucleus

Atomic mass – atomic #

Calculating neutrons:

Electron

Negatively charged

Mass of 0 (zero) amu

Found outside the nucleus

“Shells” or energy levels

Usually = protons

Wave-Mechanical Model

AKA (Also Known As):

Quantum Mechanical Model

Electron Cloud

Deals with the probability of finding an electron in a certain area of the atom outside the nucleus.

Calculating Electrons:

Electron Shells(or orbitals or energy levels):

(crack out your Periodic Table)

Row # = # of electron shells.

Valence Electrons

electrons in the outermost energy level

Column # = # of valence electrons sort of…

• Column 1 = 1 valence electron• Column 2 = 2 Valence electrons• Columns 3 – 12 there is no

obvious pattern

• Columns 13 -18 = 10 fewer valence electrons than column number

• e.g.• Column 14 = 4 Valence

electrons• Columns 15 = 5 valence

electrons• Exception, He has 2 valence

electrons.