Atomic Theory ScientistsThe history of the atom.

HISTORY OF THE ATOM- Democritus

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

•ATOMOS

•(greek for indivisible)

•This is where we get the word “atoms”

History of the Atom- Aristotle

• Aristotle said there were four elements:

– Earth

– Air

– Water

– Fire

Boyle

• Boyle rejected the idea that there were 4 elements

– Impossible to combine the 4 elements to form a substance

– Impossible to extract these elements from a substance

• Boyle defined an element as any substance that cannot

be decomposed into a simpler substance

HISTORY OF THE ATOM- Dalton

1808

John DaltonDalton's Atomic Theory

1) All matter is made of atoms. Atoms are indivisible

and indestructible.

2) All atoms of a given element are identical in mass

and properties

3) Compounds are formed by a combination of two or

more different kinds of atoms.

4) A chemical reaction is a rearrangement of atoms.

Dalton’s “Billiard Ball Model”:

Atoms are the smallest type of matter and they

are entirely solid like a billiard ball

HISTORY OF THE ATOM- Thomson

Joseph John Thomson

• Found the electron using the cathode ray tube because of

its negative charge

• He found that atoms could sometimes eject a far smaller

negative particle which he called an ELECTRON .

• Also developed the Plum Pudding Model of the atom

Thomson’s Plum Pudding Model

Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere surrounded

by a soup of positive charge to balance the electron's charge

like plums surrounded by pudding.

PLUM PUDDING

MODEL

History of the Atom- Millikan

• Discovered the charge and mass of an electron by his famous oil drop experiment

– The experiment entailed observing tiny charged droplets of oil between two horizontal metal electrodes. This enabled him to measure the charge of an electron

– calculated the mass of an electron, which is 9.1 × 10-

31 kg

Robert Millikan

What’s next?

• Two inferences:

– Something had to account for the mass of the atom since the electron doesn’t contribute to it

– Since the charge on an atom is neutral, there must be a positively charged area to balance out the negative charge of the electron

HISTORY OF THE ATOM- Rutherford

Ernest Rutherford

– He used his Gold Foil experiment to prove Thomson’s plum

pudding model wrong, discovered protons, and developed the

idea of a nucleus

– He passed particles through gold foil, and realized not

everything passed through, so there must be something other

than just a negative charge

– He developed the Nuclear Atom model or the Planetary model

of the atom

– Most of the mass of the atom is in the nucleus and electrons

orbit the nucleus like planets orbit the sun

HISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the helium nuclei passed

through the foil, a small number were deflected and, to their

surprise, some helium nuclei bounced straight back.

helium nuclei

History of the Atom- Chadwick

• Chadwick took Rutherford’s research further and found the neutron

• The problem was the atomic number was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2.

Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron.

James Chadwick

Niels Bohr

1913

Niels BohrBohr refined Rutherford's idea by adding

that the electrons were in orbits. Like

planets orbiting the sun. With each orbit

only able to contain a set number of

electrons. This is called Bohr model or

Quantum model of the atom.

Schrodinger

Bohr’s model only fits hydrogen and was proven

incorrect by Schrodinger.

Schrodinger suggested that orbits were not

always circular, but in a pattern. He called them

orbitals.

Schrodinger’s model is called the Quantum

Mechanical Model of the atom

Heisenberg Werner Heisenberg

• The Heisenberg uncertainty principle states that it

is impossible to know simultaneously the exact

position and momentum of a particle.

• That is, the more exactly the position is determined,

the less known the momentum, and vice versa.