The Atom- Are the building blocks of matter that make up everything that we encounter every day. - The basic unit of matter- Are the smallest individual part of an

element and it consists of subatomic particles such as protons, electrons and neutrons.

TIMELINE OF ATOMIC NATURE

He proposed that all substances are composed of four elements such as air, earth, fire and water in different proportion.

fire

air

water

earth

Aristotle

322 – 384 BC

Empedocles divided matter into four elements such as air, earth, fire and water.

Approximately 450 BC

• Democritus and Leucippus – proposed that everything is composed of small bit of matter that is indivisible and call it atom (atomos – “indivisible”)

500 BC

• Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis, and he devised a theory of the formation of chemical compounds from elements.

1777

• Coulomb explained that like fluids repel and unlike attract. This was important in the development of the theory of absolute measurement, and had a great impact on electrical potential.

1780

(The Solid Sphere Model)– Billiard Ball Model - small

solid sphere– Developed notion of

conservation of mass and that atoms combine in specific ratios

Dalton’s Atomic Theory 1803

The postulates of this theory are the ff;

1. All matter is composed of extremely small, indivisible, indestructible particles called atoms.

2. All atoms of each element are exactly alike but they differ from atoms of the other elements. The atoms of different elements have different mass and properties.

The postulates of this theory are the ff;

3. When atoms of different elements combine to form a compound, the constituent atoms are always present in the same fixed numerical ratio.

The postulates of this theory are the ff;

4. A chemical reaction involves a rearrangement of atoms. No atom is created or destroyed.

Sir William Crookes

- constructed the forerunner of the modern television picture tube in the 1870s to investigate the properties of cathode rays.

- Crookes tube – a vacuum discharge tube

1870

Wilhelm Roentgen

- discovered X-ray by observing the fluorescence they produced.

1896

• J.J. Thomson– Plum Pudding Model - positive

and negative particles dispersed throughout the atom

– Used Cathode Ray tubes to discover the electron - first subatomic particle discovered!

1898

Cathode Ray Tube

• A tube that contains a stream of electrons going from a negative disk (cathode) to a positive disk.

• Deflected stream showed that electrons are negative.

Rutherford– Nuclear Model of an atom– Discovered dense positively charged

nucleus of the atom while working with alpha particles• Gold Foil Experiment

• Neils Bohr - – Planetary Model of an Atom– Electrons travel in specific,

circular orbits

Neils Bohr and Arnold Sommerfeld- Arnold Sommerfeld improved Bohr’s model and introduced the concept of elliptical orbit.