Models of the Atom a Historical Perspective

In order to understand our modern concept of the atom… …we must first have a basic understanding of how that concept evolved. It took 2500 years to get to where we are now, but the pace has really heated up in the last century!

Early Greek Theories Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely – there must be a smallest piece. This led to the idea of atoms in a void. He called his theoretical smallest possible piece atomos, meaning “uncuttable” or “can’t be divided.

Early Greek Theories Aristotle fire air water earth , 350 B.C. – (50 years later) - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong! However, his theory persisted for over 2000 years!

John Dalton 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

Adding Electrons to the Model Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. Thompson “Plum pudding” model (1900) Negative electrons in a positive framework. The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus.

Millikan’s Oil Drop Experiment 1909 – Robert Millikan devised an experiment that measured the electric charge on tiny oil droplets. Based on the mass of the droplets he was able to calculate the charge on individual electrons.

Ernest Rutherford’s Gold Foil Experiment Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Rutherford shot alpha () particles at gold foil. Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

Neils Bohr and The Bohr Model In 1913 , Danish physicist Neils Bohr, working for Rutherford, came up with the theory that the electrons orbit the nucleus in “shells”, or very specific energy levels. This turned out to be a great leap forward in our understanding of atomic structure. This is the Bohr Diagram of Beryllium 4 p+ 5 n° Be

Lewis “electron dot” Diagrams In 1916, Gilbert Lewis used a simplified version of the Bohr diagram to draw and describe the ways that atoms bond to form molecules. This is the Lewis diagram of Beryllium Be

Good News! That’s all the history that we’re going to cover right now. For the next few days, we’ll learn and practice how to draw Bohr diagrams, and then Lewis (electron dot) diagrams using the information found on the periodic table of the elements. After we’ve mastered that, we’ll be ready to explore chemical bonding.