1. SCH4C UNIT 1: Matter and qualitative Analysis Atomic Theory
Development of the Rutherford Model of the Atom

2. Early Atomic Theory
Aristotle and other Greek philosophers thought that matter was composed of 4 classical elements was accepted until the late Middle Ages.
Democritis proposed that if matter is divided into smaller and smaller pieces, a finite limit known as the atom is ultimately reached.

3. Dalton’s Atomic Theory (1803)
Dalton revived the atomic theory to explain several observations and chemical laws:
Elements cannot be broken down
Compounds can be broken down into elements
Law of Conservation of Mass
This led to the “Billiard Ball” model of indivisible atoms of elements. These combine to form all known compounds.

4. Dalton’s Atomic Theory
All matter is made of tiny indivisible particles called atoms.
Atoms cannot be created or destroyed.
All atoms of a particular element are identical.
Compounds are formed through the combination of elements.
Chemical reactions occur when atoms in compounds join together or separate to form new compounds.

5. Dalton’s Billiard Ball Model

6. Thomson’s Model of the Atom (1897)
The study of electricity and batteries (electrochemistry) suggested that matter contains electric charges.
Thomson detected and measured the mass of a beam of particles in a vacuum tube (or cathode ray tube, CRT) .
He called the very tiny particles that emerged from a metal cathode electrons.

8. Thomson’s Plum Pudding Model
Thomson explained electrical conduction in metals by the movement of subatomic electrons in a solid.
Ions (charged atoms) are formed by losing or gaining electrons.

9. Rutherford’s Atomic Model
The discovery of radioactivity (,  and  rays) allowed Ernest Rutherford to probe inside the atom.
Based on Thomson’s model, Rutherford proposed that alpha rays (high speed helium ions, He2+) should pass through the positive pudding in a very thin sheet of gold foil.
The results were slightly different…

10. Gold Foil Experiment
Expected result:
Actual result:

11. Rutherford’s Planetary Model (1911)
Rutherford concluded that the atom consists of a very tiny, dense nucleus composed of the positive charge.
>99.99% of the atom consists of empty space.
Tiny electrons are found orbiting the nucleus.
A teaspoon on the atomic nuclei would weigh six billion tonnes (6 x 1012 kg)!!!!
900 x

12. In 1914, Rutherford proposed that there existed a positively charged particle called a proton.
In 1932, Chadwick & Rutherford found that the nucleus also contained a neutral particle called a neutron.

13. Revisiting Dalton’s Theory…
All matter is made of tiny indivisible particles called atoms.
FALSE [Atoms consist of p, n and e]
Atoms cannot be created or destroyed.
FALSE [Nuclear reactions create or destroy atom]
All atoms of a particular element are identical.
FALSE [Elements consist of several forms called isotopes]

14. Compounds are formed through the combination of elements.
TRUE!
Chemical reactions involve atoms recombining to form new substances.

15. The Modern Atomic Theory
All matter is made of atoms. Atoms are made of protons, neutrons and electrons.
Atoms cannot be created or destroyed in chemical reactions.
Atoms of one element have the same average properties.
Atoms combine in specific proportions to form compounds.

16. Summary of the Subatomic Particles
Symbol
Location
Charge
Mass (amu)
Proton p
Nucleus +1
1.0073
Neutron n
1.0087
Electron e-
Outside Nucleus -1
amu = atomic mass unit = 1.66 x g