Atoms!Have you ever tried to observe or study something that you could not see?Models: tools constructed based on all the known experimental evidence about a particular thing. *As time goes by, more and more experiments are performed, and models evolve and change to account for new understanding.

Sizes of atoms Graphite contains the element Carbon. Even if you grind your

pencil lead to dust, each dust particle would contain all the physical and chemical properties of the carbon atom.

If you could keep dividing each speck of dust, you would eventually reach a size that was THE smallest it could possibly be.

Atom: the smallest particle of an element that retains the properties of that element.

Can we see atoms?

New instruments have been invented to help scientists visualize the atom: pictured are atoms as seen through the lens of a scanning tunneling microscope.

Models

Scientists work with models because reality is complex and difficult. An atom is an example of a system that is both difficult and complex.

Models help us understand what we cannot see with our own eyes.

GA 13:13; 30:01

Since models are simpler than real objects or systems, they have limitations; even a great model has limitations.

A History of Atomic Theory Democritus: 450 B.C.E.

He started the ideas of “atomos”, but Aristotle (who lived 100 years after Democritus) rejected this idea, and his influence caused Democritus to be more or less forgotten for more than 2000 years. Thanks, Aristotle.

Dalton: 1800

He revived Democritus’ ideas. His research and experiments helped develop rudimentary atomic theory. He used solid wooden balls to model atoms. Scientists later disproved Dalton’s theories when they discovered subatomic particles.

Thomson:1897

He discovered electrons, which was the first anyone had heard of something smaller than an atom. He was able to get further with the research because rather than thinking of atoms in terms of light, using positive energy, he preferred to think of them in terms of electricity, with negatively-charged ions. (electrons) Which turned out to be correct. He fathered the “plum-pudding” atomic model.

Rutherford: 1899

He discovered the nucleus. He did an experiment in 1911 that proved his theory, and additionally discovered “protons” which were positively charged ions that existed within the atom. He fathered the “planetary” atomic model, disproving the “plum-pudding” model.

Neils Bohr: 1913

He primarily researched electrons, which led to his discovery of energy levels. Within an atom, at fixed distances from the nucleus, electrons would orbit at random. The energy levels would go up the further the electrons were from the nucleus.

Dmitri Mendeleev: 1869

He created the current table of periodic elements, including a way to predict properties that have not yet been discovered.

Physicists discovered that electrons do not travel in fixed paths. In fact, they found that electrons only have a certain chance of being in any particular place, and that the only way to predict the location was through complex mathematical formulas. This led to the electron cloud-model, which we use today.

Models of Atomic Theory and its Progress

Atoms, Molecules, and the Periodic Table

There are 92 naturally occurring types of atoms. These atoms are called the “elements.”

The Periodic Table is a table that gives important information about each element.

A molecule is the smallest unit of a chemical compound.

A compound is a substance made of two or more elements.

The elements in a chemical compound are always present in a certain ratio. (zB: Water molecules are all made of the same ratio: two hydrogen atoms and one oxygen atom.)