Composition of an Atom

Models of the Atom: A Historical Perspective

Atomic Theory Timeline

400 B.C.Democritus

Democritus In 400 B.C. Democritus believed all existence

consisted of two things: atoms (coined the term atom from the

Greek word ‘atomos’ meaning indivisible) the void (space)

Democritus’ theory: atoms are particles in space (the void) atoms always in motion; make life predictable matter could not be divided indefinitely;

atoms are indivisible all things we see differ because of the shape,

size, and arrangement of atoms

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

Aristotle In 350 B.C. Aristotle and his followers

dismissed the idea of atoms

Instead modified an earlier theory that matter was made up of four ‘elements’:

earth fire water air

Even though Aristotle was wrong his theory persisted for 2000 years!

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

Lavoisier During his lifetime, 1743-1794, Antoine Lavoisier

established the Law of Conservation of Matter Law= in a chemical reaction, the mass of the

reactants is equal to the mass of the products

Theory stated that mass is concrete and measurable, rather than ‘magical’

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

1800

Dalton

Dalton In 1800 John Dalton was examining gases

dissolved in water and decided that matter was made of individual particles with spaces between them

Devised a chemical atomic theory: all matter is made of atoms atoms of an element are identical each element has different atoms atoms of different elements combine in

constant ratios to form compounds atoms are rearranged in reactions

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

1800

Dalton

1900

Thomson

Thomson In 1897, looking at charged particles, J.J

Thomson found particles that had a charge that did not change regardless of the metal they came from…

He called these charged particles electrons (negatively charged particles)

Later he proposed a model for the composition of the atom called the ‘raisin bun’ or ‘plum pudding model’ although he was proved to be incorrect

Adding Electrons to the Model

Materials, when rubbed, can develop a charge difference. This electricity is called ‘cathode rays’ when passed through an evacuated tube

These rays have small mass and are negative

Thomson noted that these negative subatomic particles were a fundamental part of all atoms

1) Dalton’s ‘Billiard Ball’ model (1800-1900): atoms are solid and indivisible

2) Thomson’s ‘Plum Pudding’ Model (1900): negative electrons in a positive

framework

3) The Rutherford Model (around 1910): atoms are mostly empty space negative electrons orbit a positive

nucleus

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

1800

Dalton

1903

Rutherford

1900Thomson

Rutherford Used radiation to unlock clues to the atom;

alpha particle (α) -positive charge and beta particle(β) -negative charge

Ernest Rutherford short alpha (α) particles at gold foil

Most particles passed through; therefore, atoms are mostly empty

Some positive alpha deflected or bounced back!

Thus, a ‘nucleus’ is positive and holds most of an atom’s mass

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

1800

Dalton

1907

Millikan

1903

Rutherford

1900Thomson

Millikan In 1907 Robert Millikan found that all electrons

are identical

Millikan was able to find the charge of a single electron

Atomic Theory Timeline

400 B.C.

Democritus

350 B.CAristotle

0 1743-1794Lavoisier

1800

Dalton

1907

Millikan

1903

Rutherford

1900Thomson

1913

Bohr

Bohr Around 1913 Niels Bohr discovered that

electrons orbit the nucleus in ‘shells’

Electrons can be bumped up to a higher sell if hit by an electron or a photon of light

It is when electrons fall back down energy level(s) that they release a photon

There are two types of spectra : continuous spectra line spectra

These jumps down from shell to shell account for the line spectra seen in gas discharge tubes (through spectroscopes)