By convention there is sweet and by convention there is bitter,

By convention, hot and by convention, cold.

By convention there is color; but in reality there are only atoms and space.  -Democritus (c. 400 BCE)

Microsoft Word Version

The Atom: A History of a Scientific Concept

What we now know as ____________ began with the ancient Greek philosophers who worked hard to ____________ society and nature without resorting to superstition or ____________ thinking. Democritus is ____________ as being one of the first to ____________ that everything we know is really made of tiny basic ____________ called atoms. The Greek word for atom, “atomos” means “not ____________. Democritus ____________ that atoms form ____________ we can see or know – including us! To Democritus reality came down to atoms and ________________. He taught that what we think of as reality is the result of ____________ combinations of atoms – and those combinations might be very impermanent. Democritus’ atoms, on the other hand, could neither be ____________ or destroyed.

magical science taught propose everything created explain systematic space divisible particles credited

Today’s scientists have much more ____________ ideas about atoms. Many of these ideas are based on ____________ and imagination the way Democritus worked. But these modern ideas are also built into ____________ that are supported by a huge number of carefully ____________ observations and experiments. They also use extremely ____________ mathematical and logical calculations that continue to be ____________ and rechecked. Modern science has also developed all kinds of ____________ machines and techniques that allow them to “see” things much too small for the ancient Greeks to have ever ________________.

If Democritus were alive today, he would question whether ____________ science had really found ____________ for “his” atoms. By ____________ the ________________ in his theory were indivisible – meaning they had no subparts. Scientists have known for about 100 __________ that what we call “atoms” actually have __________ parts.

recorded years intuition evidence sophisticated smaller atoms checked complicated theories abstract imagined modern definition

Thompson and Rutherford showed that the ________________ called atoms have a positively charged nucleus ________________ by tiny, negatively charged, ________________. Others figured out that an atomic ________________ was made up of particles called protons. Later, they found another kind of subatomic nuclear ____________ Since that particle was ________________, they called it a neutron.

When scientists first ________________ that tiny electrons were ____________ around a central atomic nucleus, they imagined an atom much like a ________________ solar system. They drew ________________ of atoms that depicted the nucleus like a sun and the electrons like orbiting ________________. This is still a very common ________________ used to represent atoms to the ________________.

recognized public uncharged electrons miniature nucleus image surrounded particles flying planets models particle

Neils Bohr ________________ all kinds of experiments and calculations done by ________________ and came to many surprising conclusions. One of his more basic ________________ was that electrons are ________________ into specific energy levels - sometimes called “shells”. High school ________________ students use this idea to understand how and why certain atoms can combine together to form ________________. It turns out that atoms connect into molecules by exchanging or ________________ electrons with other atoms. The number of electrons in an atom’s ________________ energy level is what ________________ how atoms do - and do not - combine. Atoms with full outer shells are inert – they do not ________________ with other atoms. Atoms without full outer shells are ________________ to the kinds of atoms whose electrons are best organized to ________________ their outer shells.

The more scientists ____________ about atoms, the more ________________ they appear – meaning they look less and less like Democritus’ basic, ______________, particles.

molecules learn conclusions studied attracted organized sharing others elemental complete determines complicated react chemistry outermost