The Atom

Early Greek Theories Democritus 440 B.C. - Democritus thought matter could not be divided indefinitely. He called nature’s basic particle the “atom”, from the Greek word meaning “indivisible”. Aristotle 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years.

Foundations of the Atom In the late 1700s chemists believed the modern definition of an element as a substance that cannot be further broken down by ordinary chemical means. It was also clear that elements combine to form compounds that have different physical and chemical properties than those of the elements that form them. Na + Cl → NaCl Different properties

Chemists at this time could not agree whether elements always combined in the same ratio when forming a particular compound. When a substance or substances are transformed into one or more new substances, it is caused by a chemical reaction. Forming lead iodide

In the 1790s, scientists began to analyze chemical reactions In the 1790s, scientists began to analyze chemical reactions. This lead to the discovery of several basic laws. The law of conservation of mass (matter (mass) is neither created nor destroyed during ordinary physical or chemical changes). Antoine Lavoisier, a French chemist, verified this law by experimentation in 1789.

The Law of Conservation Of Mass

In 1799, Joseph Proust, a French chemist, showed that a given compound always contains exactly the same proportion of elements by mass. The principle of the constant composition of compounds, originally called “Proust’s law”, is now known as the law of definite proportions.

The law of definite proportions (a chemical compound contains the same proportions by mass regardless of the size of the sample or source of the compound. For example, sodium chloride (table salt) always consists of 39.34% by mass of the element sodium, Na, and 60.66% by mass of the element chlorine, Cl.

Proust’s discovery stimulated John Dalton, an English schoolteacher, to think about atoms as the particles that might compose elements. Dalton discovered another principle that convinced him of the existence of atoms. He noted, for example, that carbon and oxygen form two different compounds that contain different relative amounts of carbon and oxygen (CO and CO2). This led to the law of multiple proportions in 1803.

The law of multiple proportions (if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. For example, the elements carbon and oxygen form two compounds, carbon dioxide and carbon monoxide. CO molecules are always composed of one C atom and one O atom. CO2 molecules are always one C atom and two O atoms.

Law of Multiple Proportions

Dalton’s Atomic Theory In 1808, John Dalton proposed an explanation for the three laws (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.

Modern Atomic Theory Aspects of Dalton’s theory have been proven incorrect (we know now that atoms are divisible and a given element can have atoms with different masses).