Atomic Timeline

650 BC - Greek Philosophers

Earth

Water

Fire

Wind

450 BC - Democritus

Greek: atomus – means indivisible

Coined the term “atom”

Deep Thinker - No evidence was provided

The Dark Ages

Religious and scientific persecutionBUT only in

EUROPE

Meanwhile…

Trade routes between the Middle East and Asia kept Science

going.

Far Eastern alchemists designed modern glassware and apparatus

1808 - Dalton

►First model based on experiments.

1. All matter is made up of tiny particles called atoms. They cannot be created, destroyed or divided into smaller particles.

2. The atoms of one element cannot be converted into the atoms of any other element.

3. All the atoms of one element have the same properties, such as mass and size.

4. Atoms of different elements combine in specific proportions to form compounds.

7

1817 - Johann Dobereiner

●Johann Dobereiner 1829 “Law of Triads”○Regularity in atomic mass of chemically and physically

similar elements○Example: Li, Na, K

• All soft, very reactive metals• Difference in atomic masses -> 16.1

○BUT: regularity did not generally apply

When ordering the elements by atomicweight he noticed the properties of the8th element were like the 1st

1863 - John Newlands

Li Be B C N O F Na Mg

And the properties of the 2nd element were similar to the9th and so on.

Hence he named this the Law of Octaves

(Noble gases were not then known)

○BUT: only worked for first 20 elements

Mendeleev prepared a card for each knownelement listing :

o Atomic symbolo Atomic masso Chemical properties

1869 – Dimitri Mendeleev

He arranged the cards in order of increasing atomic mass and noticeda pattern.

The Periodic Law“ When elements are arranged in order of Increasingatomic mass their physical and Chemical properties show a predictable periodic pattern”

The PERIODIC LAW forms the basis for the organization of the Periodic Table.

i.e. the properties of yet undiscovered elements can be predicted based on their apparent location in the Periodic table.

1903 – J.J. Thomson

►Introduction of atomic charge.

Sphere of positive mass with negative charges interspersed.

Raisin Bun model

1911 – Ernest Rutherford

►Gold Foil Experiment

Anticipated Results

Actual Results

1911 - Rutherford

►1. most of the atom is empty space ►2. there must be a central component

of the atom containing all of the positive charge.

He called this the Nucleus

1913 – Neils Bohr

►Explained the path of electrons (orbits) around the positive nucleus

►These orbits are specific distances from the nucleus

►Electron energy level model.

1913 - Henry Moseley

In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements*. He rearranged the elements in order of increasing atomic number.

Currently accepted periodic table

“ When elements are arranged in order of increasing atomic number their physical and Chemical properties show a predictable periodic pattern”

Henry MoseleyHis research was halted when the British government sent him to serve as a foot soldier in WWI. He was killed in the fighting in Gallipoli by a sniper’s bullet, at the age of 28. Because of this loss, the British government later restricted its scientists to noncombatant duties during WWII."You see actually the Rutherford work [the concept of the nuclear atom] was not taken seriously. We cannot understand today, but it was not taken seriously at all. There was no mention of it any place. The great change came from Moseley." ~Neils Bohr

Modern Atomic Theory

1. All matter is made up of atoms. Each atom consists of subatomic particles: electrons, protons & neutrons (an atom is divisible, it is the smallest part of an element)

2. Atoms of on element cannot be converted into atoms of another element by a chemical reaction (nuclear reactions, alter the composition of the nucleus, so convert atoms of one element into another)

3. All atoms have the same properties such as size and mass. (exception is isotopes)

4. Atoms of different elements combine in fixed proportions to form compounds. (no changes yet)