Atoms: The Building Blocks of Matter From Philosophical Idea to Scientific Theory

Particle theory of matter Democritus (400 BC) Called nature’s basic particle an atom – based on the Greek word meaning indivisible

Aristotle – Didn’t believe in atoms Thought all matter was continuous His opinion was accepted for nearly 2000 years

There was no experimental evidence to back up the views of either Democritus or Aristotle During the 18th century, scientists began to perform experiments and gather evidence that favored the atomic theory of matter.

Foundations of Atomic Theory By the late 1700s, it was accepted that elements were substances that couldn’t be broken down by ordinary chemical means.

Scientists also recognized that elements combine to form compounds that have different physical and chemical properties than those of the elements that form them.

In the 1790s, chemists began looking at quantitative analysis of chemical reactions, leading to the discovery of several basic laws

Law of Conservation of Mass Mass is neither created or destroyed during ordinary chemical reactions or physical changes. C + O2  CO2

Law of Definite Proportions A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. H2O Always 11.2% hydrogen and 88.8% oxygen

While a compound ALWAYS has the same proportions, more than one compound can be formed from the same elements. NO, NO2, N2O5 NO is 47% N and 53% O NO2 is 30% N and 70% O N2O5 is 26% N and 74% O

Law of Multiple Proportions The Law of Multiple Proportions says that if two or more different compounds are composed of the same elements, then the ratio of the masses of those elements is always a ratio of small whole numbers.

Dalton John Dalton, an English schoolteacher, proposed an explanation for these laws.

He deduced that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds. His theory can be summed up in 5 statements.

Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged.

By relating atoms to mass, Dalton turned Democritus’ idea into a hypothesis that could be experimentally tested, and in turn into a scientific theory that was backed up by those experiments. Further experimentation with modern equipment has shown that not all aspects of Dalton’s theory are correct.

Modern Atomic Theory Atoms are divisible into even smaller particles by processes other than normal chemical reactions BUT, an atom is still the smallest unit of an element to retain the properties of that element A given element can have atoms with different masses isotopes

Modern Atomic Theory The Modern Atomic Theory is a modification of Dalton’s Atomic Theory Several facts remain unchanged: All matter is composed of atoms Atoms of any one element differ in properties from atoms of other elements Atoms can be combined in simple whole number ratios to form compounds