1. Our view of the atom has evolved over time.
Modern Atomic Theory
Our view of the atom has evolved over time.

2. Origin of Atomic Theory
The concept of the atom (Greek: atomos, "indivisible"), an indivisible particle of matter, goes back to ancient Greece and a man named Democritus, a rival of Aristotle. Democritus held that all matter could be subdivided only until some finite particle was reached.

3. Democritus (460 – 370 BC)
“The concept of the infinite divisibility of matter was flatly contradicted by his atomic theory.”

4. John Dalton (1766-1844) Father of Modern Atomic Theory
In 1808 the first statement of a modern atomic theory was published by John Dalton, a Quaker schoolmaster from Manchester, England.
His theory was used to explain two of the major laws in chemistry: the Law of Conservation of Mass and the Law of Constant (definite) Composition.

5. Dalton’s View of Matter
Everything is made of atoms
the atoms of an element all weigh the same and can not be divided
atoms of different elements have different masses
atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

6. J.J. Thomson
Identified that atoms can be subdivided into charged particles.
Credited with discovering the electron.

7. Cathode Ray Tube

8. Cathode Ray Tube

9. Ernest Rutherford (1871-1937) Gold-Foil Experiment.
Theorized the atom was mostly empty space with a dense, positively-charged central core.
Discovered nucleus.

10. Views of the Atom
Plum Pudding Model
Rutherford’s View

11. Basic Atomic Structure
Nucleus is composed of protons (+) and neutrons (0). Extremely small and dense.
Number of protons determines the type of atom and is called the atomic number (Z).
Sum of protons and neutrons determines the mass of the atom and is called the mass number (A).

12. Basic Atomic Structure
Atoms of the same element (same number of protons) that contain different numbers of neutrons (and therefore, different masses) are called isotopes of one another.
Electron cloud (-) determines the diameter of the atom.
Neutral atoms have equal numbers of protons and electrons.

13. Atoms vs. Ions
Atoms that do not have equal numbers of protons and electrons carry an overall charge and are called ions.
Generally, ions form when electrons are gained or lost by an atom.
Anion = negative ion (gained electrons).
Cation = positive ion (lost electrons).

14. Metals vs. Nonmetals
Generally speaking, most elements on the Periodic Table are classified as metals and they tend to lose electrons when forming ions.
The upper right hand corner of the Table contains nonmetal elements which tend to gain electrons when forming ions.

15. Electron Arrangement
The exact location and movement of electrons relative to the nucleus has been a source of great debate throughout the development of Modern Atomic Theory. The following slides present an abbreviated history of significant discoveries that shape our current view of electron arrangement.

16. Niels Bohr ( )
“We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.” – Neils Bohr

17. Bohr Model (Planetary Model)

18. Spectral Lines of Hydrogen Gas

19. Max Planck (1858-1947) Helped develop Quantum Theory
Basically states that moving electrons act much like electromagnetic waves.
The energy of an electron is related to the frequency of its wave pattern.

20. Plank’s Constant
In further refinement of his formula, he found a fundamental constant of nature, which is known as Planck's Constant.
This says:
Energy = Planck’s Constant x Frequency
E = h
(h = x J.s)

21. Wave Mechanical Model Assumptions
Electrons sometimes behave like waves.
Electrons do not orbit the nucleus in circular paths. They are found in orbitals – regions of high probability of finding electrons.
Electrons are confined to specific distances from the nucleus each possessing a specific energy and differing slightly from element to element.
When electrons move to or from higher energy levels energy is absorbed or released.
Energy is transferred to or from electrons in definite amounts (quanta) in the form of photons. (E=hv)

22. Arranging Electrons
Electrons always occupy lowest energy orbitals first.
The periodic table ranks orbitals from lowest to highest energy.
A maximum of two electrons may occupy a given orbital, provided they spin in opposite directions.
The number of electrons in the highest energy level determines chemical reactivity.